Contrary to expectations, proline and trimethylamine oxide inhibited both the initial time course and the extent of reactivation of lactate dehydrogenase from bovine heart following dena
Trang 1Perturbation of folding and reassociation of lactate dehydrogenase
by proline and trimethylamine oxide
Oscar P Chilson and Anne E Chilson
Department of Biology, Washington University, St Louis, MO, USA
Investigations of protein–solute interactions typically show
that osmolytes favor native conformations In this study,
the effects of representative compatible and counteracting
osmolytes on the reactivation of lactate dehydrogenase from
two different conformational states were explored Contrary
to expectations, proline and trimethylamine oxide inhibited
both the initial time course and the extent of reactivation of
lactate dehydrogenase from bovine heart following
dena-turation in guanidine hydrochloride, as well as following
inactivation at pH 2.3 Reactivation of acid-dissociated
porcine heart lactate dehydrogenase was inhibited by both
proline and trimethylamine oxide (2M) In all instances,
trimethylamine oxide was the more effective inhibitor of
reactivation Analysis of the catalytic properties of the
reactivating enzyme provided evidence that the molecular
species that was enzymatically active during the initial stages
of reactivation of acid-inactivated porcine heart lactate
dehydrogenase reflects a non-native conformation Proline
and trimethylamine oxide stabilize polypeptides through exclusion from the polypeptide backbone; the inhibition of renaturation/reassociation described here is probably due to attenuation of this stabilizing influence through favorable interactions of the osmolytes with sidechains of residues that lie at the interfaces of the monomers and dimers that asso-ciate to form the active tetramer In addition, these osmo-lytes may stabilize non-native intermediates in the folding pathway The high viscosity of solutions containing more than 3M proline was a major factor in the inhibition of reassociation of acid-dissociated porcine heart lactate dehydrogenase as well as other viscosity-dependent trans-formations that may occur during reactivation following unfolding in guanidine hydrochloride
Keywords: renaturation; osmolytes; proline; trimethylamine oxide; lactate dehydrogenase
In order to accommodate environmental water stress (e.g
salinity, desiccation, freezing), many organisms accumulate
one or more osmotically active solutes (osmolytes) [1] Two
classes of osmolytes are recognized Those that stabilize
proteins in vitro without significantly perturbing protein
function are defined as compatible osmolytes [2]
Counter-acting osmolytes, such as trimethylamine oxide (TMAO),
tend to buffer proteins and other cellular constituents
against elevated concentrations of chaotropic agents such
as urea [1,3,4]
Efforts to delineate the molecular basis of osmolyte
action have generated large amounts of literature on
protein–solute interactions Virtually all studies of the
effects of osmolytes on protein stability have demonstrated
that these chemical chaperones strongly favor the native conformation For example, TMAO protects ribonuclease T1 against thermal denaturation [4] There are also several reports by Bolen and coworkers showing that both proline and TMAO, as well as other osmolytes, have a propensity for forcing intrinsically unstable polypeptides to fold into more compact, native-like, conformations (e.g [5]) Interest in protein–osmolyte interactions arises from several considerations Inasmuch as unfolded polypeptides would be expected to be particularly sensitive to environ-mental stress and protease action, it is reasonable to ask whether osmolytes may facilitate the folding of nascent polypeptides; i.e perhaps one of the functions of osmolytes
is to act as chemical chaperones during the terminal stages
of protein synthesis Observations showing the effects of osmolytes on protein conformation in vivo provide support for this hypothesis [6–10]
Several investigators have noted that some osmolytes are
of potential practical use in the rescue of inclusion bodies [e.g 7,11] It is also conceivable that coexpression of appro-priate osmolytes may retard or prevent the formation of such aggregates in expression systems
With apparently few exceptions [e.g 1,7,11–14], previous investigations did not include testing of the possible effects
of osmolytes on the kinetics of reactivation of denatured polypeptides Also, study of the effects of osmolytes on the reactivation of oligomeric, cytosolic proteins seems to have been somewhat limited (see Discussion)
In the light of these considerations, we chose to explore the possible effects of osmolytes on renaturation/
Correspondence to O P Chilson, Department of Biology, Box1137,
One Brookings Drive, Washington University, St Louis,
MO, 63130–4899, USA.
Fax: + 314 9354432, Tel.: + 314 9356859,
E-mail: Chilson@biology.wustl.edu or Chilsonoa@sbcglobal.net
Abbreviations: BHLDH, lactate dehydrogenase from bovine heart;
EDTA, ethylenediaminetetraacetic acid; GdnHCl, guanidine
hydro-chloride; LDH, lactate dehydrogenase; NADH, nicotinamide adenine
dinucleotide (reduced); PHLDH, lactate dehydrogenase from porcine
heart; TMAO, trimethylamine oxide.
Enzyme: lactate dehydrogenase (EC 1.1.1.27).
(Received 27 May 2003, revised 18 October 2003,
accepted 20 October 2003)
Trang 2reassociation of lactate dehydrogenase (LDH; EC 1.1.1.27).
The choice of experimental system was based on the
following considerations LDH is an oligomer, comprised of
four polypeptides of identical size, and its refolding and
reactivation following denaturation/dissociation in various
solvent media have been extensively investigated Thus, the
pathway for refolding and reassociation is generally well
established [15] Reactivation of LDH following
denatura-tion in 6Mguanidine hydrochloride (GdnHCl) begins with
the fully unfolded polypeptide subunits and the time course
reflects a complex series of molecular events that include
folding, dimerization of monomers and association of the
dimers to form the active tetramer (see Discussion)
However, when acid-dissociated monomers are stabilized
by sodium sulfate, the rate limiting step is restricted to
association of the dimer to produce the active tetrameric
species [15] Thus, study of renaturation (following
unfold-ing in GdnHCl), as well as reassociation (followunfold-ing
inacti-vation at low pH), allowed exploration of the effect of
osmolytes on the reactivation of the enzyme from two very
different conformational states
In this investigation we explored the effects of
represen-tative compatible and counteracting osmolytes on the
kinetics and extent of reactivation of LDH from beef heart
following denaturation in 6MGdnHCl, as well as following
dissociation at pH 2.3in the presence of sodium sulfate In
contrast with expectation, based on results obtained with
other proteins [5,11,16], TMAO and proline were found to
inhibit both the time course and extent of renaturation of
LDH from bovine heart (BHLDH) following unfolding by
the chaotropic agent, as well as reactivation of the enzyme
following inactivation at low pH Reactivation of
acid-dissociated LDH from porcine heart (PHLDH) was also
sensitive to both osmolytes Evidence was obtained that the
molecular species that is enzymatically active during the
initial stages of reactivation of acid-inactivated PHLDH
reflects an altered conformation and that this
non-native species is kinetically stabilized by interaction with
osmolytes
Materials and methods
Dithiothreitol, ethylenediaminetetraacetic acid (EDTA),
GdnHCl, LDH from bovine heart (type III), nicotinamide
adenine dinucleotide (reduced, NADH), sodium pyruvate
and Tris base were obtained from Sigma-Aldrich (St
Louis, MO, USA) Lactate dehydrogenase from porcine
heart was from Roche Molecular Biochemical
(Indiana-polis, IN, USA) L-Proline was from Sigma-Aldrich
(Sigma Ultra) or Fluka (MicroSelect; Milwaukee, WI,
USA) Trimethylamine N-oxide dihydrate (> 99%) was
from Fluka N,N-Bis(hydroxyethyl)-2-aminoethane
sulfon-ic acid (Bes) was from Research Organsulfon-ics (Cleveland, OH,
USA)
Enzyme stock solutions
Stock solutions of enzyme ( 5–9 mgÆmL)1) were prepared
by dialysis ( 5 C) against 100 mM Tris/HCl, 1 mM
EDTA (pH 7.4; prior to denaturation in GdnHCl) or
100 mMsodium phosphate, 100 mMEDTA, 1 mM
dithio-threitol (pH 7.6; prior to dissociation at low pH)
Enzyme concentration Enzyme concentration (mg proteinÆmL)1) was calculated from A2800:1% ¼ 1.5 for BHLDH [17] and 1.4 for PHLDH [18] The preparation of LDH from beef heart was composed of approximately 70% H4and 30% H3M [17]; i.e > 92% H subunits The preparation from pig heart was composed of approximately 95% H4 [19] and a small fraction of H3M; it contained 98% H subunits
Assay of enzymatic activity This was performed at room temperature (21–24C) by measurement of the rate of decrease in absorbance at
340 nm with a Shimadzu 1601PC spectrophotometer Reaction mixtures (1.021 mL, in polystyrene cuvettes) contained 128 lMNADH, 350 lMsodium pyruvate (unless stated otherwise) and approximately 1 pmol enzyme (added last) The buffer for the assay was 100 mMpotassium Bes (or Tris/HCl, pH 7.0, for enzyme denatured in GdnHCl)
or 100 mMsodium phosphate, 1 mM EDTA (pH 7.6, for enzyme inactivated at acid pH) The specific activities of the BHLDH and PHLDH were 317 and 310 UÆmg)1, respect-ively, at 22C Molar concentration of enzyme was based
on a tetramer molecular mass of 140 000 Da
None of the osmolytes tested inhibited enzymatic activity
of the untreated enzyme at the concentrations that were present during enzyme assays (£ 60 mMTMAO,£ 150 mM proline; data not shown)
Denaturation and acid-induced dissociation Apart from where indicated, unfolding in 6MGdnHCl was initiated by the addition of a 10 lL aliquot of stock enzyme (containing 76–81 lg LDH) to 90 lL 6.7M GdnHCl in
100 mMTris/HCl , 1 mMEDTA (pH 7.4) The inactivation mixtures were incubated for 10 min at room temperature Inactivation at pH 2.3was initiated by addition of 8–9 lL LDH (containing 52–54 lg protein) to 91–92 lL cold
100 mM sodium phosphate, 800 mM sodium sulfate (pH 2.3); samples were incubated on ice for 60 min All incubations, for both inactivation and reactivation, were performed in polypropylene tubes
Reactivation Renaturation following unfolding in GdnHCl was initiated
by 50-fold dilution in 100 mM Tris/HCl, 1 mM EDTA,
2 mMdithiothreitol (pH 7.4), with or without the indicated concentration of the specified osmolytes (proline or TMAO); stock solutions of osmolytes were adjusted to
pH 7.4 All reactivations were performed at room tempera-ture Protein concentration in these reactivation mixtures was typically 15–16 lgÆmL)1
Reactivation of acid-inactivated enzyme was initiated by 50- or 100-fold dilution in 100 mMsodium phosphate, 1 mM EDTA, 1 mM dithiothreitol (pH 7.6) plus or minus the indicated osmolytes (TMAO or proline) at room tempera-ture; stock solutions of osmolytes were adjusted to pH 7.6 The protein concentrations in reactivation mixtures are specified in the relevant figure legends Aliquots were removed from reactivation mixtures at the indicated times
Trang 3after initiation of reactivation and assayed for enzymatic
activity as described above
Molecular graphics analysis
Two models of the structure of the H4isoform of LDH are
found in the RCSB Protein Data Bank (PDB) The PDB
code for porcine H4is 5LDH Analysis of both this model
and the one for the major isoform from human cardiac
muscle (PDB code 1I0Z) byDEEP VIEW[20,21] shows that
the latter is the superior model This assessment was based
on the fact that opening up the model for 5LDH inDEEP
VIEW, reveals a lengthy list of missing amino acid sidechains;
this reflects uninterpretable electron density in those areas
There are no such uncertainties in the model for 1I0Z As
the primary structures for LDH H4from pig and human
heart are 95% identical (97% similar) we chose to base our
analysis of buried and surface residues on the human
enzyme The model for 1I0Z is for the dimer We obtained
a model for the tetramer from 1I0Z through PROTEIN
EXPLORER[22], using the link to protein quaternary analysis
(PQS [23])
Results
Effect of osmolytes on the reactivation of bovine LDH
following denaturation in GdnHCl
Proline inhibits the rate of reactivation.The equilibrium
level of reactivation of BHLDH in the absence of osmolytes,
following denaturation in 6M GdnHCl (65 ± 2.8%,
relative to the activity of the untreated enzyme; data not
shown), was several-fold greater than that reported for
similar studies of PHLDH under similar conditions [24]
For each experiment, the activity for the control (no
osmolyte in the reactivation mixture), determined 24 h after
initiation of reactivation, was taken as representing the
equilibrium level of reactivation under the experimental
conditions employed and was assigned a value of 1.0 The
enzymatic activity observed at intermediate times (with or
without osmolyte) was expressed as a fraction of this
equilibrium control value and was defined as relative
reactivation The initial time course for reactivation of
controls was routinely hyperbolic (Fig 1)
The kinetic profile for reactivation in the presence of 1M
proline was virtually indistinguishable from that of controls,
but in the presence of increasing concentrations of proline,
the time course became increasingly sigmoidal (Fig 1) Due
to this sigmoidicity, the effects of intermediate levels of
proline (2M and 3M) were most pronounced during the
early phase of reactivation; e.g while inhibition by 2M
proline was significant during the first hour, by 5 h the
activity approached that of controls Relative reactivation in
the presence of 4 and 5Mproline remained at less than 0.1
throughout the indicated time period
Inhibition of the extent of reactivation of LDH by proline
is correlated with the unusual solution properties of
proline Relative reactivation, based on activity determined
at presumed equilibrium (24 h after initiation of
reactiva-tion), was taken as a measure of the extent of reactivation
The effect of proline concentration on the extent of
reactivation of LDH is summarized in Fig 2A At 1M proline, reactivation was unaffected, and 2M proline diminished the extent of reactivation only slightly, but inhibition was progressively more significant above 3M; at
5Mproline, inhibition was virtually complete
The concentration dependence of the viscosity of aqueous solutions of proline is somewhat unusual relative to that of compounds of similar molecular weight [26]; values for the viscosities of proline solutions over the concentration range from 1 to 6 M are included in Fig 2 Inhibition of reactivation is most pronounced in solutions of proline that exhibit the greatest viscosity This relationship is illustrated more clearly in Fig 2B
Trimethylamine oxide is a potent inhibitor of the extent of reactivation of GdnHCl-denatured LDH The effect of TMAO on the relative reactivation at equilibrium is summarized in Fig 3 The data for proline are included for comparison The counteracting osmolyte, TMAO, was the more potent inhibitor of reactivation The concentration
of TMAO required to reduce the relative reactivation at equilibrium to 0.5 was approximately 700 mM, while the concentration of proline that was required to elicit a similar level of inhibition was 3.2M
Perturbation of the reactivation of acid-dissociated LDH by proline and TMAO
Reactivation of acid-dissociated BHLDH was inhibited
by proline Both the initial rate and extent of reactivation
of bovine LDH, following dissociation at pH 2.3, were significantly greater than for enzyme denatured in GdnHCl
Fig 1 Effect of proline on the kinetics of reactivation of LDH from beef heart after denaturation in GdnHCl Enzyme was denatured in 6 M
GdnHCl as described in Materials and methods Reactivation was initiated by 50-fold dilution (to 15 lgÆmL)1) in 100 m M Tris/HCl,
1 m M EDTA, 2 m M dithiothreitol (pH 7.4) in the absence (control) or presence of the indicated concentrations of proline The time course over the first 5 h after initiation of reactivation is illustrated For each experiment, the enzymatic activity at each time point was expressed relative to the activity of the corresponding control, determined 24 h after initiation of reactivation; the relative reactivation of controls at
24 h was assigned a value of 1.0 Each of the lines shown for proline represents a single experiment, the points for the line for controls reflect five independent determinations.
Trang 4Similar differences have been reported by Jaenicke and
coworkers in studies of the porcine LDH [27] The time
required for relative reactivation in the absence of osmolytes
to reach 0.5 during reactivation from GdnHCl was
30 min (Fig 1), but 5 min for enzyme inactivated at
acid pH in the absence of the chaotropic agent (Fig 4A)
Activity at apparent equilibrium in the absence of osmolytes
following acid dissociation approached 90% of the activity
of the untreated enzyme (data not shown) The time courses
for reactivation of acid-dissociated enzyme in the absence
of osmolytes, as well as in the presence of 1 or 2Mproline,
were hyperbolic and virtually indistinguishable (Fig 4A);
3Mproline, however, was inhibitory and the time required
to attain a relative reactivation of 0.5 was increased to
20 min At 4M proline the time course became slightly
sigmoidal and the time required to reach a relative
reactivation of 0.5 was 90 min (Fig 4A); the initial rate
of reactivation in the presence of 5Mproline was virtually zero and relative reactivation rose only slightly (to 0.1) over the 5 h incubation period (Fig 4A)
Proline concentrations up to 3M did not inhibit the extent of reactivation (Fig 4B); at 4 and 5M proline, relative reactivation was reduced to approximately 0.85 and 0.3, respectively A limited analysis of the effect of high proline concentrations on the reactivation of acid-inacti-vated PHLDH yielded similar results (data not shown)
Trimethylamine oxide inhibits the rate of reactivation of bovine LDH following inactivation at acid pH The time required to reach a relative reactivation of 0.5 was increased from 5 to 25 min by 1M TMAO; at 5 h, relative reactivation with the osmolyte approached that of controls (Fig 5A) When reactivation was performed in the presence
of 2MTMAO, relative reactivation was less than 0.1 at 5 h after initiation of reactivation
TMAO (1M) enhances the initial rate of reactivation of porcine LDH following inactivation at pH 2.3, while 2M TMAO inhibits reactivation The time course for reacti-vation of acid-dissociated PHLDH in the absence of osmolyte was hyperbolic (Fig 5B), and reactivation reached apparent equilibrium at approximately 90% of the activity of the untreated enzyme (data not shown) The initial rate of reactivation was slower that that of the beef enzyme; the time required to reach a relative reactivation of 0.5 was increased from 5 min (Fig 5A, BHLDH) to 20 min (Fig 5B, PHLDH) In marked contrast to the inhibitory effect of 1M TMAO on reactivation of the bovine enzyme (Fig 5A), this concen-tration of the osmolyte enhanced the rate of reactivation
of the pig dehydrogenase; the time required to reach a relative reactivation of 0.5 was reduced from 20 min to
7 min (Fig 5B) At 5 h after initiation of reactivation the activities of controls and the mixture containing 1
Fig 3 Comparison of the effects of TMAO, and proline on the extent of reactivation of GdnHCl-denatured BHLDH Enzyme was denatured and reactivated as described in the legend for Fig 1 in the absence and presence of the indicated concentrations of the osmolytes Relative reactivation at presumed equilibrium was assessed as described in the legend for Fig 2 Each point represents the mean of two independent determinations; error bars are shown where the magnitude of the error exceeds the size of the symbol.
Fig 2 Effect of proline concentration on the extent of reactivation of
LDH from bovine heart and on the viscosity of proline solutions Enzyme
was denatured in GdnHCl and reactivated as described in the legend
for Fig 1 (A) Relative reactivation at presumed equilibrium (j) was
taken as a measure of the extent of reactivation at each proline
con-centration These values were based on measurements of enzymatic
activities of reactivation mixtures (with and without added osmolyte)
24 h after initiation of reactivation Each point represents the mean of
two independent determinations; error bars are shown where the
magnitude of the error exceeds the size of the symbol Values of
intrinsic viscosity for proline solutions (.,g) were taken from a
pre-vious report by Schobert and Tschesche [25], with permission from
Elsevier Science (B) A plot of the values for the extent of reactivation
vs the viscosity of proline solutions, both taken from (A).
Trang 5TMAO reached similar levels In the presence of 2M
TMAO, however, the initial rate of reactivation of
PHLDH was strongly inhibited, but slightly less so than
with BHLDH (Fig 5A,B)
TMAO (2M) significantly reduced the extent of
reacti-vation of both bovine and porcine LDH.In the presence of
1MTMAO, the relative reactivation at presumed
equilib-rium for the porcine enzyme was slightly greater than that of
controls, while the value for the bovine enzyme was reduced
to 0.9 (Fig 6) In 2MTMAO, the relative reactivation at
equilibrium was reduced to 0.4 and 0.1 for PHLDH
and BHLDH, respectively
As with reactivation of enzyme denatured in GdnHCl,
TMAO was a more potent inhibitor of reactivation of
acid-denatured BHLDH than proline The concentration of TMAO required to reduce relative reactivation at equilib-rium to 0.5 for enzyme dissociated at pH 2.3was approxi-mately 1.5M (Fig 6), whereas for a similar level of inhibition by proline, the concentration required was approximately threefold greater (compare Figs 4B and 6)
The time course of reactivation of acid-dissociated LDH
in the presence of proline is dependent on protein concentration The initial time course for the reactivation
of PHLDH in the absence of osmolytes was dependent on the concentration of LDH protein in the reactivation mixture (Fig 7A), consistent with a pathway involving a rate-determining association step (see Discussion) If attenu-ation, by osmolytes, of the reactivation of enzyme inacti-vated at low pH involves modulation of a rate-determining
Fig 5 Effect of TMAO on the kinetics of reactivation of LDH from bovine and porcine heart after inactivation at pH 2.3 Inactivation at
pH 2.3was performed as described in Materials and methods Reac-tivation was initiated by 100-fold dilution (to 5.4 lg proteinÆmL)1)
in 100 m M sodium phosphate, 1 m M EDTA, 1 m M dithiothreitol (pH 7.6) in the absence (control) and presence of the indicated con-centrations of TMAO Relative reactivation was assessed as described
in the legend for Fig 1 (A) Data obtained with LDH from bovine heart (B) Data obtained with LDH from porcine heart Each point for the controls represents the mean of four independent determinations; error bars are shown where the magnitude of the error exceeds the size
of the symbol The points for the lines for the experiments with TMAO represent the means of two independent experiments; error bars are shown where the magnitude of the error exceeds the size of the symbol.
Fig 4 Effect of proline concentration on the kinetics and extent of
reactivation of LDH from bovine heart after inactivation at pH 2.3.
Inactivation was in 100 m M sodium phosphate, 800 m M sodium
sul-fate (pH 2.3) as described in Materials and methods Reactivation was
initiated by 100-fold dilution (to 5.4 lg proteinÆmL)1) in 100 m M
sodium phosphate, 1 m M EDTA, 1 m M dithiothreitol (pH 7.6) in the
absence (control) and presence of the indicated concentrations of
proline (A) The time course of reactivation during the first 5 h after
initiation of reactivation Each of the lines shown for proline represents
a single experiment, the points for the line for controls reflect five
independent determinations (B) Effect of proline on the extent of
reactivation The extent of reactivation was assessed as described in the
legend for Fig 2 Each point represents the mean of two independent
determinations; error bars are shown where the magnitude of the error
exceeds the size of the symbol.
Trang 6association step, the kinetics of reactivation in the presence
of an inhibitory concentration of osmolyte should also be
dependent on protein concentration Proline-inhibited
reac-tivation of acid-dissociated LDH was also protein
concen-tration-dependent (Fig 7B) TMAO-inhibited reactivation
of acid-dissociated enzyme, however, was independent of
protein concentration (Fig 7C)
Analysis of interfacial contacts in lactate dehydrogenase
from cardiac muscle The program MS [28] was used to
calculate the surface area buried in each subunit upon
formation of the tetramer, based on the coordinates
provided in PDB file 1I0Z, as modified as described in
Materials and methods Approximately 55% of these
buried sidechains are nonpolar in nature (Table 1) When
the model for the native tetramer was analyzed for groups
on the surface that are exposed to solvent usingDEEP VIEW
[20,21], 43% were found to be nonpolar (data not shown)
Effect of osmolytes on the kinetic properties of PHLDH
during reactivation following acid-induced
dissoci-ation The H4isoform of LDH is particularly sensitive to
pyruvic acid [29] An early study of the reactivation of LDH
from avian cardiac muscle, following unfolding in GdnHCl,
showed that during the initial stage of reactivation there
were one or more enzymatically active species that exhibited
diminished thermal stability and reduced inhibition by
pyruvic acid [30] It was of interest therefore to determine
whether during reactivation of acid-inactivated PHLDH
there were enzymatically active species that exhibited altered
pyruvate sensitivity and whether concentrations of proline
and/or TMAO that inhibited reactivation kinetically
stabil-ized these non-native molecular species
Two identical aliquots of PHLDH were inactivated at
pH 2.3; during reactivation, one reactivation mixture was
assayed at 350 lM pyruvate and the other at 10 mM
pyruvate The ratio of the rate observed at the lower
pyruvate concentration to that at the higher substrate
concentration for the untreated enzyme was typically 2.6 (data not shown) For reactivating enzyme, however, the activity observed at 10 m pyruvate during the initial stages
Fig.7 Effect of protein concentration on reactivation following dissoci-ation at pH 2.3 Enzyme was inactivated by addition of 7 lL PHLDH (8.36 mgÆmL)1; in 100 m M sodium phosphate, 1 m M EDTA, 1 m M
dithiothreitol, pH 7.6) to 93 lL 100 m M sodium phosphate, 800 m M
sodium sulfate (pH 2.3), followed by incubation on ice for 60 min Reactivation was initiated at room temperature by dilution to 5.85 lg proteinÆmL)1(j) or 11.7 lg proteinÆmL)1(m) in buffer alone (A, 100 m M sodium phosphate, 1 m M EDTA, 1 m M dithiothreitol,
pH 7.6), or in buffer containing 3.4 M proline (B) or in buffer con-taining 1.6 M TMAO (C) Each line represents a single experiment.
Fig 6 Effect of TMAO on the extent of reactivation of LDH after
inactivation at pH 2.3 Acid-induced inactivation and reactivation were
performed as described in the legend for Fig 5 The extent of
reacti-vation was assessed as described in the legend for Fig 2 For 1 and 2 M
TMAO, each point represents an individual determination; for 2 M
osmolyte and BHLDH, the two points were superimposed Points for
[TMAO] at < 1 M reflect single determinations.
Trang 7of reactivation was slightly higher than that with 350 lM
pyruvate; in the absence of osmolyte, at approximately
2 min after initiation of reactivation, the enzymatic rates
became equivalent (Fig 8A) Subsequently, the rate
observed at the lower substrate concentration became
increasingly greater than that at the higher substrate
concentration The addition of the osmolytes to the
reactivation mixtures markedly increased the period during
which the enzymatically active species was less sensitive to
substrate inhibition In the presence of proline (Fig 8B;
3.4M) or TMAO (Fig 8C; 1.6M), the activity at the higher
substrate concentration remained greater than that at the
lower substrate concentration until approximately 20 and
10 min in the presence of proline and TMAO, respectively
At presumed equilibrium (24 h after initiation of
reactiva-tion) the ratio of the rate observed at the lower substrate
concentration to that at the higher substrate concentration
was the same (within 5%) as for the untreated enzyme (data
not shown)
The results that are summarized in Fig 8 represent a
typical experiment Three such experiments were performed
While there was significant variation in absolute values for
points that determine the time courses, all the patterns were
similar to those shown in Fig 8 This experimental variation
probably reflects the complexity of the molecular events
associated with the generation of the putative non-native
intermediate and its conversion to the native conformation,
together with interaction with the osmolytes It is significant,
however, that the large differences between the controls (no
osmolyte in the reactivation mixture) and experimental
(with proline or TMAO in the reactivation mixtures)
samples in the time required for the rates observed at the
lower and higher substrate concentrations to become
equivalent were similar among experiments The results
for these experiments are summarized in Table 2
Fig 8 Effect of osmolytes on the kinetic properties of PHLDH during reactivation following acid-induced dissociation Two aliquots of enzyme were inactivated by addition of 7 lL PHLDH (8.27 mgÆmL)1; in
100 m M sodium phosphate, 1 m M EDTA, 1 m M dithiothreitol, pH 7.6 buffer) to 93 lL 100 m M sodium phosphate, 800 m M sodium sulfate (pH 2.3), followed by incubation on ice for 60 min Reactivation was initiated at room temperature by dilution to 5.8 lgÆmL)1in buffer alone (A, 100 m M sodium phosphate, 1 m M EDTA, 1 m M dithio-threitol, pH 7.6) One reactivation mixture was assayed at 350 l M (j) and the other at 10 m M (m) pyruvic acid Two similar experiments were performed in which reactivation mixtures were composed of buffer containing 3.4 M proline (B) or 1.6 M TMAO (C) Each line represents a single experiment.
Table 1 Analysis of interfacial contacts in lactate dehydrogenase from
cardiac muscle The program MS [28] was used to calculate the surface
buried in each subunit upon formation of the tetramer, based on the
coordinates provided in PDB file 1I0Z, as modified as described in
Materials and methods For these calculations, a probe radius of 1.7 A˚
was used.
Residue type
Surface area (A˚ 2 ) by residue class Main chain Sidechain Acidic: D,E 65.668 340.564
Basic: H,K,R 176.551 704.390
Polar: N,Q,S,T 192.795 480.940
Small: A,G 234.482 87.306
Hydrophobic: C,I,L,M,P,V 291.975 1460.52
Aromatic (nonpolar): F 5.249 79.957
Aromatic (polar): W,Y 26.231 311.659
Total area represented by
sidechains
3378 A˚2 Area represented by
hydrophobic sidechains
1852 A˚ 2 (54.8%) Area represented by polar
sidechains
1526 A˚2(45.2%)
Trang 8Early in the development of concepts regarding the interplay
between the effects on protein structure and function of
perturbants, such as urea, and counteracting osmolytes
(such as TMAO and alkyl amines), it was recognized that
alone, the latter might be harmful [31] Studies of the levels
and distribution of counteracting osmolytes among various
organisms support this hypothesis The concentrations of
alkyl amines (mostly TMAO) in muscles of several deep-sea
organisms approach 300 mmolÆkg tissue)1 [32], but are
elevated only in species in which a perturbant is also present
[33]; TMAO is high in deep-sea animals where pressure is a
perturbant, as well as in all cartilaginous fishes where urea is
a perturbant It was also demonstrated that several
stabil-izing solutes enhance the formation of abnormal amyloid
structures in vitro [34]
The data summarized in Fig 3seem to be consistent with
this hypothesis While the reduction in relative reactivation
at equilibrium (following denaturation in GdnHCl) by
250 mM TMAO was modest, it was significant This
concentration of the osmolyte approaches the physiological
range for some organisms Thus, to the extent that refolding
and reassociation of the polypeptides of LDH following
denaturation in GdnHCl mimic the folding of the nascent
protein, TMAO may be a physiologically significant
regulator of protein folding in some deep-sea organisms
Perhaps shallow-water organisms accumulate less TMAO
because it would interfere with protein folding Possible
further support for this hypothesis is provided by the
observations indicating that osmolytes may sometimes
stabilize altered protein conformations during folding
(Fig 8, and see below)
The effects of proline on folding and reassociation of LDH
described here occur over a concentration range that is much
higher than estimates of the level of accumulation of proline
in various organisms under physiological conditions
It is likely that molecular chaperones are involved in the
folding of LDH in vivo, but results of such investigations
have not been reported Studies of the interplay among
nascent (or unfolded) polypeptides, molecular chaperones
and osmolytes seem to be limited An investigation of the
effects of salt and heat stresses on aggregation and
disaggregation of malate dehydrogenase showed that
sev-eral osmolytes modulate the effects of complex chaperone
networks on protein folding [35] In vitro studies showed
that physiological levels of trehalose stabilized an inactive, partially folded, conformation of luciferase and inhibited chaperone-assisted reactivation of luciferase that had been unfolded in GdnHCl [7,8]
We are aware of only two prior reports of the effect of osmolytes on the reactivation of denatured LDH An early study by Yancey and Somero [1] showed that following inactivation at low pH, TMAO (200 mM) enhanced both the rate and extent of reactivation of the somewhat unstable isoform of LDH from rabbit muscle In addition to the species and isoform differences, those experiments differed
in two significant respects from the current study; dissoci-ation was performed in the absence of sodium sulfate to stabilize the monomers, and reactivation mixtures contained 1.5 mMNAD+ The results were somewhat similar to the enhanced rate of reactivation of acid-dissociated PHLDH
by 1M TMAO (Fig 5B) There are apparently no other reports of the effects of either of the osmolytes employed in this investigation on the reactivation of lactate dehydro-genase; however, glycerol was shown to retard the rate of reactivation of acid-dissociated porcine LDH isoforms [12] These reports appear to be the first recorded instances of the effects of osmolytes on the renaturation/reactivation of an oligomeric, cytosolic protein
In assessing possible molecular bases of the observations described in this communication, it is useful to consider some
of what is known about (a) the kinetics and mechanism of refolding and reactivation of LDH following denaturation/ dissociation in various media; (b) the energetics of differential interactions of solvent and osmolytes with sidechains and the polypeptide backbone; (c) the anomalous colligative prop-erties of proline in aqueous solution; and (d) the effects of proline and TMAO on the stability, folding and biological activity of LDH, as well as a few other proteins
Studies of the time course of folding of several tetrameric enzymes, following denaturation in various media have led
to the following general pathway for folding and association [15,27]:
where m represents the fully unfolded monomeric polypep-tide and M* represents the partially folded monomer having significant secondary structure, while M designates the monomeric polypeptide having assumed its tertiary struc-ture; D and T indicate the dimer and tetramer, respectively Thus, the model includes the major molecular species in the transition from random coil to native tetramer Inasmuch as the investigations by Jaenicke and coworkers have provided strong support for the proposition that only the tetramer is enzymatically active, appearance of activity parallels the formation of native structure (see below, however) For the H4and M4isoforms of LDH from porcine heart and muscle, the equilibrium constant for the 4M to 2D conversion is of the order of 108LÆmol)1, and the rate approaches that for a diffusion controlled reaction; the slow, first order 4M* to 4M conversion is preceded by a very fast 4m to 4M* transition that occurs before the initial measurement is performed [27]
Table 2 Effect of proline and TMAO on kinetic properties of PHLDH
following inactivation at pH 2.3 Enzyme was inactivated and
reacti-vated with and without the indicated concentration of osmolytes, and
assays of enzymatic activity were performed at 350 l M and 10 m M
pyruvic acid as described in the legend for Fig 8 The rates determined
at 350 l M and 10 m M were designated as L and H, respectively.
Numbers in parentheses indicate the number of independent
deter-minations.
Osmolyte added Time at L/H ¼ 1.0
None 2.3± 0.5 min (4)
3.4 M Proline 16.7 ± 3.1 min (3)
1.6 M TMAO 12.5 ± 2.3min (3)
Trang 9The time course of reactivation following acid-induced
dissociation in the presence of sodium sulfate reflects a
somewhat simpler sequence of molecular events than that
following unfolding in the presence of a chaotropic agent
In this instance, the first and second order events in the
mechanism of renaturation are uncoupled; reactivation
begins with structured monomers Following the rapid
equilibrium of the diffusion-controlled association of
monomers to form the dimer, the rate determining step is
the association of dimers to form the active tetramer; under
these conditions the kinetic profile is second order and
hyperbolic Experimental support for this reassociation
pathway was provided by studies of the porcine LDH
isoforms by Jaenicke and coworkers [15,27,36]
There have been no similar dissociation/reactivation
studies of LDH from bovine tissues, but given the structural
and functional similarities among the major isoforms of
LDH from heart tissue of various species [37], and the
similarity in kinetic profiles for reactivation, in the absence
of osmolyte, of acid-inactivated PHLDH and BHLDH
observed in this study (Fig 5), it is probable that the
mechanism proposed for reassociation of the porcine
dehydrogenase also applies to the bovine enzyme There is
a clear difference, however, in the effect of TMAO
concentration on reactivation While 2Mosmolyte inhibits
reactivation of both enzymes, 1M TMAO enhances the
initial rate of reactivation of the porcine dehydrogenase but
inhibits initial stages in the reactivation of the bovine
enzyme (Fig 5) This most likely reflects species differences
in sensitivity of exposed residues to interaction with the
solute (see below), due to conformational variations arising
from differences in primary structure
Useful insights regarding differential interactions of
sidechains and the polypeptide backbone with osmolytes
were provided in a recent review by Bolen and Baskakov
[5] Analysis of the free energy of transfer of the sidechains
and polypeptide backbone of ribonuclease T1from water to
osmolyte showed that interactions between osmolyte and
sidechains were uniformly favorable (negative DG) but
interactions between osmolyte and the polypeptide
back-bone were unfavorable (positive DG) For both the native
and denatured conformations, the magnitude of the
unfavorable interaction with the polypeptide backbone
was much greater than the favorable interaction with the
sidechains The principal difference for the two
conforma-tions was that the magnitude of the free energy change for
transfer of the backbone of the denatured conformation
from water to osmolyte solution was much greater than
that for the native conformer The net result of this
solvophobic effect, which they term osmophobic, is the
stabilization of the native conformation Their analysis
further showed that although proline is similar to TMAO
as a stabilizing solute, on a molar basis, it is significantly
less effective Interaction of both proline and TMAO with
sidechains of amino acids is uniformly favorable, and while
both osmolytes interact more strongly with polar residues,
interaction of these residues with proline is significantly
stronger than with TMAO [38]
The protein concentration dependence of the effect of
proline on reactivation of PHLDH, following inactivation
at pH 2.3(Fig 7B), is consistent with inhibition of an
association process, and with the hypothesis that
reactiva-tion in the presence of the osmolyte follows a path similar to that in buffer alone The perturbation of reactivation of acid-dissociated LDH by this osmolyte may be partially mediated by interactions between proline and sidechains of amino acid residues Such interactions could arise from clustering of sidechains that lie at the interfaces of folded monomers or dimers that are involved in the stabilization of quaternary structure, as in the formation of dimers and/or the enzymatically active tetramer To the extent that osmolytes bind preferentially to interfacial domains of monomers or dimers, and/or a non-native conformation of the presumed tetramer (see below), formation of the fully native LDH tetramer would be retarded
As noted above, analysis of the buried surface area for each subunit in the LDH tetramer showed that these interfacial regions are approximately 55% nonpolar (Table 1), while approximately 57% of those on the surface
of the fully native tetramer that are exposed to solvent were found to be polar (see Results) Thus, there is not a differential clustering of polar residues (with which proline and TMAO interact preferentially [38]) in the regions that interact to form the tetramer It is conceivable that the strength of the interaction of the osmolytes with sidechains
of certain residues (or some combinations of them) is greater than the interaction with others and that these residues are distributed preferentially in the interfacial regions
Efforts to explain the effects of high concentrations of proline on refolding and/or reassociation of LDH must also include consideration of the unusual colligative properties of this osmolyte [11,16,25,26,39] It is unusually soluble, and unlike most low molecular weight compounds, the relative viscosity of aqueous proline solutions increases exponenti-ally with increasing concentration; the rise is particularly dramatic above 3.5M([25] and Fig 2)
The rates of second order processes, such as the rate-determining association of dimers to generate active tetra-mers in the reactivation of acid-dissociated LDH (see above), are inversely proportional to the viscosity of the medium It should also be noted that if there are motions on the surface of a monomer, which are large enough to affect the monomer–monomer (or dimer–dimer) interface, then they could be viscosity- dependent, irrespective of the diffusion of the monomer (or dimer) per se The correlation between the effect of increasing proline concentration on viscosity and on reactivation of acid-dissociated enzyme (Figs 2 and 4B) supports the proposition that much of the effect of proline on reactivation following dissociation at low pH is due to the high viscosity of the medium The viscosity of glycerol solutions undoubtedly contributed to the inhibition of reactivation of acid-inactivated LDH that was previously reported [12]
It was suggested that some of the unusual colligative properties of proline in aqueous solution are due to its association to form multimeric species, the size of which is concentration-dependent [26] The structure proposed for these supramolecular assemblies remains somewhat specu-lative [11,16], but it is plausible that some of the effects of proline on reassociation following acid dissociation (or renaturation from GdnHCl) involve association of poly-peptide intermediates in the reactivation pathway with these postulated multimeric proline species It is likely that the energetics of interaction between exposed sidechains on the
Trang 10surface of intermediates in the folding/reassociation
path-way and these proline assemblies differ significantly from
their interaction with proline monomers
Trimethylamine oxide is a more potent inhibitor of
reactivation of acid-dissociated enzyme than proline; e.g
while 2M proline had virtually no effect on the level of
reactivation at presumed equilibrium, 2MTMAO inhibited
the extent of reactivation > 50% ( 60% for PHLDH and
90% for BHLDH; Fig 6) As noted above, evidence
from studies by others supports the hypothesis that in the
presence of sodium sulfate, the acid-dissociated subunits are
stabilized in their native conformation [36] However, the
absence of protein concentration dependence on inhibition
of reactivation of acid-dissociated enzyme by TMAO
(Fig 7C) indicates that, unlike proline, inhibition of
reac-tivation by this compound is not due to attenuation of a
rate–determining association step It is also very unlikely
that the effect of TMAO includes a viscosity component,
but it is probable that this osmolyte inhibits reactivation by
stabilization of non-native conformation(s) of one or more
intermediates in the reactivation pathway, presumably by
favorable interaction between TMAO and exposed
clus-tered sidechains Perhaps the sodium sulfate-stabilized
monomers are in equilibrium with a partially folded
monomer (non-native) that is stabilized by binding of
exposed residues to TMAO
While the major isoforms of LDH in skeletal muscle (M4)
and cardiac tissue (H4) are very similar, there are very
significant differences For example, H4 is typically more
stable than M4, and is much more sensitive to inhibition by
pyruvic acid [17,29,37,40]
Analysis of substrate inhibition provided additional
insight regarding the basis of osmolyte effects on the time
course of reactivation of acid-inactivated PHLDH As
outlined above, one interpretation of the inhibitory effects
of osmolytes on the initial rate of reactivation of
acid-dissociated lactate dehydrogenase suggests that proline
and TMAO may stabilize one or more intermediates in
the reactivation pathway Although previous studies have
shown that the tetramer is the only enzymatically active
molecular species during the reactivation of lactate
dehydrogenase [15,27], in the course of the current studies,
it was found that during the early stages of the
reactivation of PHLDH following acid-induced
inactiva-tion, the enzymatically active species exhibits a kinetic
property (i.e diminished substrate inhibition) that differs
markedly from that of the untreated enzyme or
reactiva-ted enzyme at presumed equilibrium (see above) The
presence of inhibitory concentrations of osmolytes during
reactivation of acid-inactivated PHLDH prolonged the
lifetime of one or more enzymatically active (presumably
tetrameric) molecular species that was/were less sensitive
to pyruvate inhibition approximately five- to sevenfold
(Fig 8 and Table 2) These observations are consistent
with the proposition that the molecular species that is
(are) enzymatically active during the initial period of
reactivation has (have) an altered conformation(s) and
that concentrations of proline or TMAO that inhibit
reactivation tend to stabilize this (these) altered
confor-mation(s)
As noted above, the pathway for folding and association
presented above (Eqns 1–4, above), as formulated by
Jaenicke and coworkers [15,27], postulates that the final step in the pathway is:
where T, the tetramer, is the only enzymatically active species In light of the results presented in Fig 8 and Table 2, perhaps step four of the pathway should be revised, and an additional step should be added as follows:
where T* represents the non-native tetramer and T repre-sents the native enzyme
Compelling evidence for the existence of tetrameric species having altered conformations during the early stages
of the reassociation of bovine and porcine LDH polypep-tides was presented by King and Weber [41] The enzyme dissociates at high hydrostatic pressure, generating enzy-matically inactive subunits having diminished affinity for one another; on decompression the tetramer forms rapidly, but due to slow reversal of the conformational drift that occurs upon reassociation, recovery of activity occurs on a much slower time scale The results presented in Fig 8 and Table 2 are consistent with such a model
The effects of TMAO and proline on the rate and extent
of reactivation following denaturation in GdnHCl were qualitatively similar to those observed with acid-dissociated enzyme Reactivation following unfolding in the chaotropic agent, however, was far more sensitive to the osmolytes (compare Fig 3with Figs 4B and 6) For example, inhibition of the extent of reactivation of the GdnHCl-treated enzyme by 4Mproline was approximately 75%, but only 15% for the enzyme inactivated at low pH TMAO was the more potent inhibitor; concentrations of TMAO up
to 1M were virtually without effect on the extent of reactivation of the acid-dissociated enzyme (Fig 6), but inhibition of the extent of reactivation following unfolding
in GdnHCl by 500 mMTMAO was very significant and was almost complete in 1M TMAO; equivalent inhibition by proline required 5Mosmolyte (Fig 3)
As with acid-dissociated protein, it seems likely that inhibition of reactivation following unfolding in GdnHCl
by these osmolytes arises from stabilization of non-native intermediates in the reactivation pathway Due to extensive unfolding by the chaotropic agent, the potential for interaction with sidechains that are not exposed to solvent
in the sodium sulfate stabilized subunits of the acid-dissociated protein, as well as those that lie at the interfaces
of the subunits, may contribute to the greater osmolyte sensitivity of reactivation from GdnHCl Interaction among one or more of these molecular species and the postulated multimeric proline species may also contribute to the inhibitory effects of this osmolyte
Viscosity undoubtedly also plays a role in inhibition by proline of reactivation of LDH following denaturation in GdnHCl However, the greater complexity of the reactiva-tion pathway precludes identificareactiva-tion of the specific mole-cular transitions that may be sufficiently large to be affected
by the hydrodynamic properties of the solute; some of these are likely to be more viscosity-sensitive than others Thus, it
is perhaps not surprising that with enzyme unfolded by the chaotropic agent, inhibition becomes significant at