Tài liệu Quantitative aspects of ruminant digestion and metabolism - Phần 11 ppt

For example, a largeproportion of free amino acids arising from protein breakdown is reutilized forprotein synthesis, so that the rate of whole-body protein synthesis is muchgreater than

14 Protein Metabolism and Turnover

D Attaix,1 D Re´mond1 and I.C Savary-Auzeloux2

1Institut National de la Recherche Agronomique, Unite´ de Nutrition etMe´tabolisme Prote´ique, Theix, 63122 Ceyrat, France;2Institut National

de la Recherche Agronomique, Unite´ de Recherches sur les Herbivores,Theix, 63122 Ceyrat, France

Introduction

All cellular proteins are in a continuous state of turnover in which they aresynthesized and degraded (Waterlow et al., 1978) Thus, the intracellularconcentration of any protein, and the tissue, organ or whole-body proteinmass, are determined by the relative synthetic and degradation rates It should

be pointed out that a change in the size of a given protein pool only depends onthe imbalance between both processes of protein turnover In other words, anincrease or a decrease in such a protein pool does not necessarily correlate withonly an enhanced rate of either protein synthesis or protein breakdown,respectively For example, the anabolic agent trenbolone acetate decreasedrates of both protein synthesis and breakdown and resulted in net muscleprotein gain (Vernon and Buttery, 1976)

The cyclical nature of protein turnover also implies that rates of proteinsynthesis and degradation are considerably greater than the net flux (proteindeposition or loss) through the protein turnover cycle For example, a largeproportion of free amino acids arising from protein breakdown is reutilized forprotein synthesis, so that the rate of whole-body protein synthesis is muchgreater than the rate of dietary influx of amino acids

Both protein synthesis and breakdown require energy (see below) ever, the process of protein turnover provides the organism with severaladaptive mechanisms that clearly outweigh the metabolic costs:

How-1 Growth and mobilization of tissue/organ and whole-body protein mass iseasily achieved, depending on the physiological status

2 Large amounts of free amino acids can be mobilized from skeletal muscleand used to provide energy and precursors for protein synthesis in vital organs(brain, heart, etc.) and synthesis of specific sets of proteins (e.g acute phase

ß CAB International 2005 Quantitative Aspects of Ruminant Digestion

and Metabolism, 2nd edition (eds J Dijkstra, J.M Forbes and J France) 373

proteins by the liver) in stress situations, even when dietary amino acid supply isdeficient.

3 Abnormal (e.g miscoded or misfolded) proteins can be broken-down and

do not accumulate in cells

4 Both endogenous and exogenous proteins, including bacterial and viralproteins, are hydrolysed into peptides and presented on major histocompat-ibility complexes to eventually activate the immune system

5 The intracellular abundance of key proteins (e.g enzymes, cyclins ortranscription factors) is tightly regulated so that major biological processes areprecisely controlled

A major challenge is to understand both general and tissue/organ-specificmechanisms, which are responsible for these adaptations In vitro studies haveprovided detailed information on the regulatory mechanisms of protein turn-over In vivo studies are inevitably more descriptive, and experiments in animalproduction are mostly designed to optimize protein deposition efficiency inskeletal muscle (meat) or milk production Furthermore, the cost of research inlarge animal species has clearly impeded our understanding of protein metab-olism in ruminants, so that most available information remains fragmentary

Mechanisms of Protein Turnover

The precise mechanisms of protein synthesis, which include transcription,translation and post-translational modifications, have been extensively studiedand are detailed in many textbooks of biochemistry The mechanisms thatregulate protein breakdown are much more obscure First, there are severalproteolytic pathways within cells (e.g lysosomal, Ca2þ-dependent, ubiquitin–proteasome-dependent (see Fig 14.1), etc.), and many proteases remain to

be discovered or characterized In addition, the relative contribution of lytic pathways to the rate of overall proteolysis is tissue specific The lysosomalpathway plays a prominent role in liver (Attaix et al., 1999), while the ubiqui-tin–proteasome system has a major importance in skeletal muscle (Attaix andTaillandier, 1998; Jagoe and Goldberg, 2001) Second, there are many alter-native routes within a given proteolytic process (Attaix et al., 1999) Third,

proteo-in vivo, different proteolytic systems may either proteo-independently degrade a givenprotein substrate (Attaix et al., 1999), or sequentially participate to its com-plete hydrolysis into free amino acids (Attaix et al., 2002)

Protein synthesis requires the hydrolysis of both ATP and GTP However,the actual cost of protein synthesis is much higher than the theoretical cost ofpeptide bond formation, presumably because many proteins involved in trans-lational control are G-proteins, which are activated in the presence of GTP.Direct measurements of oxygen consumption in the presence of cycloheximidehave yielded values of 5.4 and 7.5 kJ/g protein synthesis when measured

in vivo in chickens, and in vitro in sheep muscle, respectively (see Lobley,1994) Protein breakdown also requires energy For example, ATP hydrolysis

is required in many steps of ubiquitin–proteasome-dependent proteolysis

(Attaix et al., 2002) It has been suggested that 10% of the cellular energyrequirements are linked to proteolysis (Lobley, 1994) This estimation must betaken with caution The amount of energy required to degrade 1 g of protein isunknown, cannot be assessed experimentally, and presumably largely depends

on numerous factors, which include the nature of the substrate, the proteolyticsystem(s) involved in its breakdown, the site of proteolysis, etc

Measurement of Protein Synthesis and Degradation

Whole-body protein turnover

The constant infusion technique has been widely used to estimate both ponents of whole-body protein turnover A labelled amino acid is infusedintravenously until the plasma specific radioactivity or enrichment (for aradio- or a stable isotope, respectively) of the free amino acid used as a markerreaches a plateau This is achieved within a few hours (Fig 14.2a) The ratio,rate of isotope infusion/isotopic activity at the plateau, gives the flux or irre-versible loss rate (ILR) of the amino acid from the plasma If the labelled aminoacid infused into the blood/plasma free amino acid pool is an essential aminoacid, and if this pool has a constant size (steady state) the total input throughthis pool is equal to the total output, so that:

Fig 14.1 Schematic representation of the ubiquitin (Ub)–proteasome-dependent proteolyticpathway Polyubiquitination of the substrate is achieved in sequential steps (1) to (3) (1) TheUb-activating enzyme, E1, forms a thiol–ester bond with Ub (2) The activated Ub is thentransferred to an Ub-conjugating enzyme, E2, which also forms a thiol–ester linkage with Ub.(3) In the presence of an Ub–protein ligase, E3, that specifically recognizes the substrate, the E2

deubiquitinating enzymes (DUB) can remove the polyUb degradation signal, so that the substrate isnot degraded and free ubiquitin is recycled (5) More generally, the polyUb degradation signal isrecognized by the 26S proteasome, and the substrate is cut into peptides with recycling of free Ub.(6) The peptides generated by the proteasome are finally hydrolysed into free amino acids (AA) bythe tri-peptidyl peptidase II (TPP II) and several associated aminopeptidases (AP) (see Attaix et al.,

2002 for more detailed information)

ILR¼ Synthesis(S) þ Oxidation(O) ¼ Breakdown(B) þ Intake(I)

Amino acid oxidation (O) can be determined by using a 1-14C or 1-13C traceramino acid, and collecting expired14CO2 or13CO2 that should be correctedfor an apparent CO2 fixation in the body The whole-body protein synthesisrate (S) is then deduced from S¼ ILR
O Alternatively, the whole-body rate

of protein breakdown (B) is equal to B¼ ILR
I in the fed state, or to B ¼ ILR

Fig 14.2 Schematic representation of

the specific activity of the tracer following

the administration of a constant infusion

(a) or of a flooding-dose (b) of a labelled

amino acid In (a) the ratio of the isotopic

activity of the label at the end of the

infusion crucially depends on the rate of

protein turnover in the tissue (e.g the

tissue homogenate/plasma isotopic activity

is high (0.9 to 0.7) in skeletal muscle,

where the intensity of protein turnover is

low, and is low (0.6 to 0.3) in tissues where

protein turnover is rapid (liver, gut)) In

(b), this problem is minimized over a short

period of time, and this ratio is usually over

0.7, including when protein turnover is a

Plasma

Plasma Muscle Liver

Time after label administration

(a)

Muscle Liver

Fig 14.3 Two-pool model used for the estimation of the whole-body irreversible loss rate (ILR)and tissue protein fractional synthesis rate (FSR) in vivo, see text Amino acid (AA) fluxes, whichare inputs into the free amino acid pool (e.g intake (I) and protein breakdown (B)), and outputsfrom this pool (e.g protein synthesis (S) and amino acid oxidation (O)) are shown The tracer,usually an essential amino acid, is infused or injected into the blood/plasma free amino acid pool,which is assumed to be the precursor pool for protein synthesis A third pool (e.g the intracellularfree amino acid pool in equilibrium with the blood/plasma free amino acid pool and the proteinpool) is often used to calculate the fractional rate of protein synthesis in a given tissue or organ(see Waterlow et al., 1978 for detailed explanations)

in the fasted state In ruminants I (absorption) is particularly difficult to estimate,and fasting is not easily achieved.

The technique is simple, non-destructive, allows different measurements inthe same animal, but has some major flaws, which have been extensivelydiscussed elsewhere (Waterlow et al., 1978; Lobley, 1994) First, whole-bodydata are difficult to interpret and the ILR technique totally obscures changes

in both rates of protein synthesis and breakdown in various tissues Second, thetechnique provides only a minimum estimate of the rates of protein turnover and

of amino acid oxidation since the isotopic activity is much higher in the plasmathan in tissues, where the tracer is diluted by unlabelled free amino acid fromprotein degradation (Fig 14.2a) Third, there is some recycling of the tracerfrom tissues where protein turnover is rapid (e.g liver, gastrointestinal tract(GIT), see below), and this also causes underestimation of the ILR

Regional estimations of protein turnover

Another closely related technique involves selective catheterization of an arteryand a vein draining a hind limb bed An index of both the rates of proteinbreakdown and synthesis is calculated by measuring the concentration of thelabel and its isotopic activity in arterial and venous blood, and the blood flow.Labelled phenylalanine (Barrett and Gelfand, 1989) and other amino acids can

be used (Hoskin et al., 2001) Amino acid oxidation can also be determined byfollowing the fate of the C-1 moiety of essential amino acids The arteriovenousapproach has the same limitations as the ILR technique, and there is somecontamination from the other tissues within the hind limb, e.g skin and bone.Amino acid mass transfers have been also quantified by arteriovenous proced-ures across the portal-drained viscera (PDV) and liver in sheep (Lobley et al.,1996) Such procedures require extensive surgery, but they allow repeatedmeasurements within the same animal

Tissue and organ protein turnover

Protein synthesis

To measure fractional rates of protein synthesis (FSR, usually expressed in %per day) in vivo the specific radioactivity (or enrichment) of the labelled aminoacid must be measured in both the precursor and the protein pools (Waterlow

et al., 1978) Except for skeletal muscle and skin, in which biopsies can beeasily performed, slaughter is usually required to collect internal samples Twotechniques have provided most of the data available in ruminants

The most commonly used is the constant tracer infusion analysis, as in theILR technique (see above and Fig 14.2a) The difficulty is to estimate the activity

of the precursor pool for protein synthesis The activity of the actual pool, thecharged aminoacyl-tRNAs, is technically very difficult to determine Based onexperiments performed in vitro and in vivo, it is generally assumed that ami-noacyl-tRNAs are charged from both extracellular (plasma) and intracellular

(tissue homogenate) free amino acid pools (Waterlow et al., 1978) However, asthe label is diluted by the unlabelled amino acid used as a marker, which arisesfrom protein breakdown, there are large differences between the isotopic activ-ities in these pools (Fig 14.2a) This is especially true when protein turnover ishigh (liver, GIT) Consequently there are also large differences between FSRcalculated by using the isotopic activity of the free label in the plasma and thetissue homogenates In addition, since the label is infused during several hours,secreted or export proteins, which are for example synthesized in the liver andthe intestines, are not taken into account in the measurements.

To overcome all these problems, the label can be injected with a large orflooding dose of the same unlabelled amino acid This results in nearly constantand close isotopic activity of the tracer, both in the plasma and in tissue hom-ogenates within a short period of time (Fig 14.2b) To meet these goals the largedose of unlabelled amino acid should ideally represent several times the whole-body free amino acid content For example, when [3H]valine was used as a tracer

in 1-week-old lambs the flooding dose was very efficient with an unlabelledamount of valine that represented about ten times the whole-body free valinecontent (Attaix, 1988) In such conditions, FSR calculated from the isotopicactivity of the free label either in the plasma or the tissue homogenates are quitesimilar Although the technique is potentially interesting for measuring proteinsynthesis in tissues where FSR are high, there are some potential problems First,the injection of a large amount of amino acid may affect amino acid transportand/or hormonal secretions (e.g insulin) Second, the procedure is rather ex-pensive Consequently, there are very few measurements in adult ruminants, andall published data have been obtained for only the ovine species Finally, theprocedure may favour the measurement of FSR in short-lived proteins

Protein breakdown

Methodological problems associated with reliable measurements of in vivoproteolysis impede the understanding of its regulation In addition, all tech-niques that can be used in vivo do not provide any information on proteolyticsystems that are responsible for changes in proteolysis

In tissues and organs from growing animals, the fractional rate of proteinbreakdown (FBR) can be calculated as the difference between FSR and thefractional rate of protein deposition (FGR) (Waterlow et al., 1978) Such es-timations are very imprecise because FGR must be estimated over several days,FSR being measured over a few minutes or hours However, FSR and FGR arenot necessarily constant over the period of measurements For example, theymay fluctuate largely with the feeding pattern In addition the technique requiresslaughter and cannot be used in tissues that secrete or export proteins

3-Methylhistidine is formed by a post-translational methylation of histidineresidues in actin and in myosin heavy chains of fast-twitch glycolytic skeletalmuscles In the rat and cattle, but not all species (see below), the urinary excretion

of 3-methylhistidine provides an index of myofibrillar protein breakdown.Unfortunately, the visceral smooth muscles of the GIT and other tissues such

as skin contain significant amounts of actin These tissues contribute tionately for their size to 3-methylhistidine urinary excretion, because of

dispropor-their high rates of protein turnover In addition, changes in renal clearance of methylhistidine may affect the interpretation of the data (see Attaix and Taillan-dier, 1998) Finally, in some species (e.g in pigs and to a lesser extent in sheep),

3-a high proportion of 3-methylhistidine is ret3-ained in muscle 3-as 3-a dipeptide,balenine (Harris and Milne, 1987) A compartmental model of 3-methylhistidinemetabolism has been developed, which involves the assessment of muscleproteolysis and 3-methylhistidine kinetics without the collection of urine (Rath-macher and Nissen, 1998) However, due to the numerous limitations of the 3-methylhistidine approach, caution must be exercised

Non-quantitative approaches

Non-quantitative approaches may be of special interest in ruminant tissues, due

to the costs of experiments with isotopic amino acids As a very crude rule, thecontrol of protein synthesis occurs mainly at the transcriptional level Thereforethe quantification of the mRNA(s) of a given protein by molecular biologytechniques is often used as an index of protein synthesis However, manymRNAs are also subject to translational control, and the relative amount ofany mRNA depends on both rates of transcription and of mRNA breakdown.Finally, there are frequent discrepancies between mRNA levels and the corre-sponding protein levels and/or activities Similarly, changes in mRNA levels formany proteolytic genes, in particular within the muscle ubiquitin–proteasome-dependent pathway, closely mimic variations of proteolytic rates measured withincubated rodent muscles (see Attaix and Taillandier, 1998) These observa-tions, together with the use of specific inhibitors of lysosomal and Ca2þ-dependent proteases and of the proteasome, lead to the concept that mostmuscle proteins, and in particular myofibrillar proteins, are degraded in anubiquitin–proteasome-dependent fashion (Attaix and Taillandier, 1998; Jagoeand Goldberg, 2001) However and again, elevated mRNA levels for proteo-lytic genes only reflect increased transcription in a few instances (see Attaix andTaillandier, 1998), and do not always strictly correlate with rates of proteolysis(see Combaret et al., 2002) Measuring proteolytic gene expression may be ofinterest in small muscle biopsies from ruminants, with complementary ap-proaches (e.g measurements of protein levels for some enzymes of the ubiqui-tination machinery and proteasomal subunits, of the rate of ubiquitination ofprotein substrates, and of proteasome activities)

Whole-body Protein Metabolism

The age of animals and the level of nutrition are the best described factors thatregulate whole-body protein metabolism in ruminants When expressed on ametabolic liveweight basis, whole-body protein synthesis in lambs increasesduring the first days following birth, declines very rapidly within 6 months(without any major effect of weaning), and thereafter remains stable withincreasing age (Fig 14.4)

Whole-body protein synthesis (g/day/kg BW0:75) increases with able energy (ME) intake (kJ/day/kg BW0:75) (Fig 14.5) This increase is linear insheep (Harris et al., 1992; Yu et al., 2000; Savary et al., 2001), but not in steers(Dawson et al., 1998; Lapierre et al., 1999) In both species fed above mainten-ance (based on an energy maintenance requirement of 400 and 500 kJ/day/kg

metaboliz-BW0:75 for sheep and steers, respectively) the slope of the relationship is verysimilar (e.g 13–14 g of protein synthesized per MJ ME) However, below main-tenance, protein synthesis decreases in sheep but is not altered in steers (Lapierre

et al., 1999) Above maintenance requirements, the calculated whole-body tein degradation rate (protein synthesis minus deposition) increases in both sheepand steers (Harris et al., 1992; Lapierre et al., 1999) Below maintenanceprotein breakdown decreases in sheep (Harris et al., 1992), but increases in steers(Lapierre et al., 1999) Besides species differences, the duration of the under-feeding period, the composition of the diet and the age of animals may account forthese discrepancies Nevertheless, whole-body protein loss was similar (about

pro-1 g/day/kg BW0:75) in both underfed (0.6 maintenance) steers and sheep

Tissue Protein Metabolism

Yu et al., 2000; Savary et al., 2001.)

et al., 1997b, Berthiaume et al., 2001) Moreover, the sequestration of vidual essential amino acids in the PDV may account from one- to two-thirds oftheir whole-body flux, and the majority (80%) of the amino acids sequestratedarose from the arterial supply (MacRae et al., 1997a) Thus, first-pass PDV

Metabolizable energy (kJ/day/kg BW0.75)

Fig 14.5 Effect of metabolizable energy intake on whole-body (WB) protein synthesis in cattle(a) and sheep (b)

metabolism of dietary amino acids as well as PDV use of systemic amino acidssignificantly impact the quantitative and qualitative supply of amino acids toother tissues or organs The portal vein drains heterogeneous tissues (GIT,pancreas, spleen, omentum), but the GIT is by far the major contributor toPDV protein synthesis For this reason, only GIT protein metabolism isreviewed below.

Gastrointestinal tract

The mass of the GIT increases with intake, and the importance of its differentcompartments varies according to the composition of the diet Protein mass ofthe ruminant GIT accounts for 4–6% of whole-body proteins (Lobley et al.,1980; Attaix, 1988; MacRae et al., 1993) However, because of the high FSR

in these tissues, the GIT contributes25–35% of whole-body protein synthesis(Lobley et al., 1980, 1994; Attaix, 1988) compared to12% in pre-ruminantanimals (Attaix and Arnal, 1987) The large dose procedure is best suited formeasuring protein synthesis in the GIT (see above), and data reported in thissection are derived from studies using this technique (export proteins beingincluded in synthesis) Whatever the age, the pattern of FSR along the GIT isvery similar to the highest values in the small intestine (Fig 14.6)

FORESTOMACHS Rumen growth is rapid and stimulated by the initiation of solidfood intake and the concomitant establishment of microbial fermentation.Thus, the reticulorumen represents 7% and 30% of the GIT protein mass

in 1-week-old milk-fed and 8-week-old weaned lambs, respectively (Attaix,

1-week-old 8-week-old 8-month-oldFig 14.6 Protein fractional synthesis rates in the gastrointestinal tract from milk-fed

(1-week-old) and weaned lambs (Data from Attaix, 1988; Lobley et al., 1994.)

1988) After weaning, the rumen development results from a stimulated FSR,which reflects an increase in both ribosomal capacity (e.g total RNA-to-proteinratio) and protein synthetic efficiency (e.g the amount of protein synthesizedper unit RNA) (Attaix, 1988) In adult ruminants the mass of the rumen mucosaincreases with the amount of food ingested (Nozie`re et al., 1999) and isassociated with an increased FSR (Lobley et al., 1994; Adams et al., 2000).Whether the nature of the diet may affect ruminal protein turnover is notdocumented.

ABOMASUM Protein metabolism in the abomasum is dominated by thesecretion of digestive enzymes (pepsin, lysosyme, etc.) and mucins, and inyoung ruminants the FSR is eightfold greater in the mucosa than in theserosa (Attaix, 1988) Both the abomasum protein mass (Nozie`re et al.,1999) and FSR (Lobley et al., 1994) are poorly influenced by dietarytreatments However, weaning stimulates FSR in the abomasum, and thiseffect is more marked in the musculosa than in the mucosa (Attaix, 1988;Attaix et al., 1988)

SMALL INTESTINE FSR in the small intestine are higher than in any other part ofthe GIT (Fig 14.6) This high protein synthesis activity reflects epithelial cellturnover, synthesis of brush border enzymes and mucins and the presence ofimmune cells Accordingly, FSR in the mucosa is considerably higher than inthe serosa all along the small intestine (Lobley et al., 1994) FSR decrease fromthe duodenum to the ileum (Attaix et al., 1992; Southorn et al., 1992; Lobley

et al., 1994) This gradient, which correlates with reduced efficiency of proteinsynthesis, is seen in 8-week-old lambs, either milk-fed or weaned, but does notprevail in younger animals (Attaix et al., 1992) Thus, intrinsic developmentalfactors are presumably mainly responsible for the regional differences in smallintestinal FSR, which poorly reflect rates of cell renewal in lambs raised insimilar conditions (Attaix and Meslin, 1991)

The small intestine receives amino acids for protein synthesis from bothluminal and systemic routes It is difficult to demonstrate in vivo whether theprecursor pool for protein synthesis is preferentially charged from eithersource However, it is noteworthy that intestinal FSR are poorly reduced infasted rats (Samuels et al., 1996), and only slightly increased with the level ofintake in lambs fed above maintenance (Lobley et al., 1994; Adams et al.,2000) This suggests that luminal factors have no major influence on smallintestinal protein synthesis, except solid food ingestion at the time of weaning(Attaix et al., 1992) Observations in piglets showed that amino acids mayactually decrease mucosal FSR in jejunal segments isolated from systemicinfluence (Adegoke et al., 1999) In contrast, intestinal protein mass is highlysensitive to food intake and dramatically reduced in fasting, suggesting thatproteolysis plays a major role in this tissue Accordingly, mRNAs for severalproteolytic genes increased in the small intestine from fasted rats (Samuels

et al., 1996) Conversely, amino acids decreased the expression of proteolyticgenes in the intestinal mucosa from piglets (Adegoke et al., 1999) However,there was no change in the expression of proteolytic genes in the intestines

from chronically underfed ewes (Nozie`re et al., 1999), suggesting that suchchanges are only seen following acute manipulation of dietary intake.

LARGE INTESTINE In 8-week-old weaned lambs the large intestine accounts for13% of the protein mass of the GIT, and for 9% of its absolute rate of proteinsynthesis (Attaix, 1988) In mature sheep, corresponding values are 22%(Nozie`re et al., 1999) and 18% (Lobley et al., 1994) The mass of the largeintestine increases with the level of intake (Nozie`re et al., 1999), owing to atendency for enhanced FSR (Lobley et al., 1994) However, several lines ofevidence suggest a role of proteolysis in the control of large-intestinal proteinmass (Attaix et al., 1992; Samuels et al., 1996)

Liver

FSR in the liver follows the general pattern observed for whole-body proteinsynthesis (Fig 14.7) FSR increases during the first days following birth, andthereafter declines exponentially with increasing age This decline is linked to adecrease in both ribosomal capacity and protein synthesis efficiency In rumin-ants, FSR in the liver is not affected by the level of intake (Lobley et al., 1994;Adams et al., 2000) Absolute protein synthesis in the liver accounts for about35–40% of the PDV protein synthesis (Attaix, 1988; Lobley et al., 1994).Assuming that all plasma proteins are of hepatic origin, it has been estimatedthat export proteins accounted for 38–51% of total hepatic protein synthesis,

1-day 1-week 5-week 8-week 13-month 20-month

AgeFig 14.7 Effect of age on protein fractional synthesis rate in sheep liver (Data from PatureauMirand et al., 1985; Attaix, 1988; Lobley et al., 1992; Adams et al., 2000.)

and that albumin represented 15–22% of export protein production (Connell

et al., 1997) These proteins act as a mobile protein reservoir, and synthesis inthis fraction (but not in the constitutive protein fraction) is particularly sensitive

to acute change in nutritional status such as fasting (Connell et al., 1997;Lobley et al., 1998) In contrast, changes in liver protein mass in response tointake (Burrin et al., 1992; Nozie`re et al., 1999) seem mainly related toalterations in protein degradation (Lobley and Milano, 1997)

Peripheral tissues

Skeletal muscle

Proteins in skeletal muscle account for about 30–45% of whole-body proteinmass (Attaix, 1988; Lobley et al., 1994) Although this is the largest proteinreservoir in the body, muscle contributes only 15% to 22% to whole-bodyprotein synthesis because of its low FSR (Attaix, 1988; Adams et al., 2000;Lobley et al., 2000) In lambs, FSR declines exponentially between birth and 4months of age (Fig 14.8) This decline is fully related to a decrease in thecapacity for protein synthesis and is not confounded by nutritional effects(Attaix et al., 1988) In sheep, muscle FSR increases linearly with the level ofintake from 0.6 to 1.8 maintenance (Fig 14.9) Similar data were obtained

in the perfused hind limb between 0.5 and 2.5 maintenance (Boisclair et al.,1993; Thomson et al., 1997; Hoskin et al., 2001; Savary et al., 2001) Data

on protein degradation in the perfused hind limb are more confusing An

0 2 4 6 8 10

Fig 14.8 Effect of age on protein fractional synthesis rate in sheep skeletal muscle (Data fromPatureau Mirand et al., 1985; Attaix, 1988; Lobley et al., 1992; Adams et al., 2000.)