Leucine incorporation and its potential as a measure of protein synthesis by bacteria in natural aquatic systems

Leucine incorporation was examined as a method for estimating rates of protein synthesis by bacterial assemblages in natural aquatic systems. The proportion of the total bacterial population that took up leucine in three marine environments was high (greater than 50%). Most of the leucine (greater than 90%) taken up was incorporated into protein, and little (less than 20%) was degraded to other amino acids, except in two oligotrophic marine environments. In samples from these two environments, ca. 50% of the leucine incorporated had been degraded to other amino acids, which were subsequently incorporated into protein. The degree of leucine degradation appears to depend on the organic carbon supply, as the proportion of 3H-radioactivity incorporated into protein that was recovered as [3H]leucine after acid hydrolysis increased with the addition of pyruvate to oligotrophic water samples. The addition of extracellular leucine inhibited total incorporation of [14C]pyruvate (a precursor for leucine biosynthesis) into protein. Furthermore, the proportion of [14C]pyruvate incorporation into protein that was recovered as [14C]leucine decreased with the addition of extracellular leucine. These results show that the addition of extracellular leucine inhibits leucine biosynthesis by marine bacterial assemblages. The molar fraction of leucine in a wide variety of proteins is constant, indicating that changes in leucine incorporation rates reflect changes in rates of protein synthesis rather than changes in the leucine content of proteins. The results demonstrate that the incorporation rate of [3H]leucine into a hot trichloroacetic acid-insoluble cell fraction can serve as an index of protein synthesis by bacterial assemblages in aquatic systems.     

Leucine incorporation and its potential as a measure of protein synthesis by bacteria in natural aquatic systems.

Leucine incorporation was examined as a method for estimating rates of protein synthesis by bacterial assemblages in natural aquatic systems. The proportion of the total bacterial population that took up leucine in three marine environments was high (greater than 50%). Most of the leucine (greater than 90%) taken up was incorporated into protein, and little (less than 20%) was degraded to other amino acids, except in two oligotrophic marine environments. In samples from these two environments, ca. 50% of the leucine incorporated had been degraded to other amino acids, which were subsequently incorporated into protein. The degree of leucine degradation appears to depend on the organic carbon supply, as the proportion of 3H-radioactivity incorporated into protein that was recovered as [3H]leucine after acid hydrolysis increased with the addition of pyruvate to oligotrophic water samples. The addition of extracellular leucine inhibited total incorporation of [14C]pyruvate (a precursor for leucine biosynthesis) into protein. Furthermore, the proportion of [14C]pyruvate incorporation into protein that was recovered as [14C]leucine decreased with the addition of extracellular leucine. These results show that the addition of extracellular leucine inhibits leucine biosynthesis by marine bacterial assemblages. The molar fraction of leucine in a wide variety of proteins is constant, indicating that changes in leucine incorporation rates reflect changes in rates of protein synthesis rather than changes in the leucine content of proteins. The results demonstrate that the incorporation rate of [3H]leucine into a hot trichloroacetic acid-insoluble cell fraction can serve as an index of protein synthesis by bacterial assemblages in aquatic systems.     

AMPK activation suppresses mTOR/S6K1 phosphorylation and induces leucine resistance in rats with sepsis

Although it has been known that protein synthesis is suppressed in sepsis, which cannot be corrected by leucine supplement (also known as leucine resistance), the molecular signaling mechanism remains unclear. This study aimed to investigate the AMP‐activated protein kinase/mammalian target of rapamycin (AMPK/mTOR) pathway in sepsis‐induced leucine resistance and its upstream signals, and to seek a way to correct leucine resistance in sepsis. Sepsis was produced by cecal ligation and puncture (CLP) model in rat. Both septic rats and sham operation rat received total parenteral nutrition (TPN) with or without leucine for 24 h, and then protein synthesis and AMPK/mTOR and protein kinase B (PKB) were tested. In vitro C2C12 cells were treated with or without leucine, and we tested the AMPK/mTOR pathway and protein synthesis. We blocked AMPK by compound C and stimulated it by 5‐aminoimidazole‐4‐carboxamide ribonucleoside (AICAR) individually. The results showed that AMPK was highly phosphorylated and suppressed mTOR/S6K1 activation in CLP rats. In vitro when AMPK was activated by AICAR, protein synthesis was suppressed and leucine resistance was observed. High phosphorylation of AMPK was accompanied by PKB inactivation in CLP rats. When PKB was blocked, both AMPK activation and leucine resistance were observed. In CLP rats, nutrition support with intensive insulin therapy reversed leucine resistance by activating PKB and suppressing AMPK phosphorylation. These findings suggest that high phosphorylation of AMPK induced by PKB inactivation in sepsis suppresses mTOR, S6K1 phosphorylation, and protein synthesis and leads to leucine resistance. Intensive insulin treatment can reverse leucine resistance by suppressing AMPK activation through activation of PKB.     

Effect of temperature on protein synthesis and leucine transport by yeast mitochondria

Protein synthesis in yeast mitochondria shows biphasic Arrhenius plots both in vivo and in vitro, with a twofold increase in the activation energy below the transition temperature suggesting a functional association between mitochondrial protein synthesis and the inner membrane. Analysis by gel electrophoresis of mitochondrial translation products labeled in vivo showed that the same proteins are synthesized and then inserted into the membrane above and below the transition temperature of the membrane. The rate of leucine uptake into mitochondria was decreased at least fivefold in the presence of chloramphenicol, suggesting that leucine is used mainly for protein synthesis. In the absence of chloramphenicol, the rate of leucine uptake was always slightly higher but comparable to the incorporation rate of leucine into protein at all temperatures studied, suggesting that the transport of leucine into mitochondria is not rate-limiting for protein synthesis. The ionophore valinomycin or the uncoupler carbonyl phenylhydrazone (CCCP) inhibited 75-80% of the leucine uptake in the presence of chloramphenicol. In addition, the omission of respiratory chain substrates and the ATP-regenerating system led to a 93% inhibition of uptake, suggesting that leucine uptake may occur by an active transport mechanism.     

Uncoupling of Protein and Ribonucleic Acid Synthesis by 5′,5′,5′-Trifluoroleucine in Salmonella typhimurium

The addition of 5',5',5'-trifluoroleucine (fluoroleucine) to leucine auxotrophs of Salmonella typhimurium permitted protein but not ribonucleic acid (RNA) synthesis to continue after leucine depletion. The uncoupling of the formation of these macromolecules by fluoroleucine was apparent if RNA and protein synthesis was measured either by the uptake of radioactive precursors or by direct chemical determinations. The analogue did not appear to be an inhibitor of RNA formation, since it was as effective as leucine in permitting RNA synthesis in a leucine auxotroph upon the addition of small amounts of chloramphenicol. In contrast to these data, fluoroleucine allowed continued protein and RNA formation in a leucine auxotroph of Escherichia coli strain W. In addition, contrary to the results obtained with S. typhimurium, the analogue replaced leucine for repression of the leucine bio-synthetic enzymes as well as the isoleucine-valine enzymes. We propose that these ambivalent effects of fluoroleucine on repression and RNA and protein synthesis in the two strains are due to differences in the ability of the analogue to attach to the various species of leucine transfer RNA.     

Effect of inhibition of protein synthesis on lipid metabolism in Lactobacillus plantarum.

In Lactobacillus plantarum 17-5, lipid synthesis appears to be correlated with protein synthesis. Inhibition of protein synthesis by chloramphenicol (50 mug/ml) caused the nearly simultaneous inhibition of incorporation of radioactive oleic acid into polar lipids before the cessation of growth. In addition, de novo fatty acid synthesis, as determined by the incorporation of radioactive acetate into cellular lipids, was also inhibited. Removal of the antibiotic resulted in the resumption of growth, protein synthesis, and polar lipid synthesis. Inhibition of protein synthesis by leucine deprivation also produced a marked reduction in the incorporation of radioactive oleic acid into the total polar lipids at about the same time that growth stopped (30 to 60 min after the removal of leucine). However, the different classes of lipids behaved differently. For example, the incorporation of oleic acid into cardiolipin was inhibited immediately upon removal of leucine from the cultures, whereas incorporation into phosphatidyl-glycerol was maintained at near normal rates for 60 min after the removal of leucine and then ceased. In contrast, the accumulation of radioactive oleic acid in a neutral lipid identified as diglyceride occurred to a much greater extent in leucine-deprived cultures than in control (+ leucine) cultures. Upon addition of leucine to leucine-deprived cultures, the rates of synthesis of phosphatidyl-glycerol and cardiolipin returned to normal; the amount of radioactivity in the diglyceride fraction decreased to normal levels concomitantly with increased phospholipid synthesis.     

Regulation of protein synthesis in lactating rat mammary tissue by cell volume

The effect of changing cell volume on rat mammary protein synthesis has been examined. Cell swelling, induced by a hyposmotic challenge, markedly increased the incorporation of radiolabelled amino acids (leucine and methionine) into trichloroacetic acid (TCA)-precipitable material: reducing the osmolality by 47% increased leucine and methionine incorporation into mammary protein by 147 and 126% respectively. Conversely, cell shrinking, induced by a hyperosmotic shock, almost abolished the incorporation of radiolabelled amino acids into mammary protein: increasing the osmolality by 70% reduced leucine and methionine incorporation into mammary protein by 86 and 93% respectively. The effects of cell swelling and shrinking were fully reversible. Volume-sensitive mammary tissue protein synthesis was dependent upon the extent of the osmotic challenge. Isosmotic swelling of mammary tissue, using a buffer containing urea (160 mM), increased the incorporation of radiolabelled leucine into TCA-precipitable material by 106%. Swelling-induced mammary protein synthesis was dependent upon calcium: removing extracellular calcium together with the addition of EGTA markedly reduced volume-activated protein synthesis. Cell swelling-induced protein synthesis was inhibited by the Ca(2+) ATPase blocker thapsigargin suggesting that volume-sensitive protein synthesis is dependent upon luminal calcium.     

Leucine and insulin activate p70 S6 kinase through different pathways in human skeletal muscle

Amino acids and insulin have anabolic effects in skeletal muscle, but the mechanisms are poorly understood. To test the hypothesis that leucine and insulin stimulate translation initiation in human skeletal muscle by phosphorylating 70-kDa ribosomal protein S6 kinase (p70(S6k)), we infused healthy adults with leucine alone (n = 6), insulin alone (n = 6), or both leucine and insulin (n = 6) for 2 h. p70(S6k) and protein kinase B (PKB) serine(473) phosphorylation were measured in vastus lateralis muscles. Plasma leucine increased from approximately 116 to 343 micromol/l during the leucine-alone and leucine + insulin infusions. Plasma insulin increased to approximately 400 pmol/l during the insulin-alone and leucine + insulin infusions and was unchanged during the leucine-alone infusion. Phosphorylation of p70(S6k) increased 4-fold in response to leucine alone, 8-fold in response to insulin alone, and 18-fold after the leucine + insulin infusion. Insulin-alone and leucine + insulin infusions increased PKB phosphorylation, but leucine alone had no effect. These results show that physiological concentrations of leucine and insulin activate a key mediator of protein synthesis in human skeletal muscle. They suggest that leucine stimulates protein synthesis through a nutrient signaling mechanism independent of insulin, raising the possibility that administration of branched-chain amino acids may improve protein synthesis in insulin-resistant states.     

Tissue-specific effects of chronic dietary leucine and norleucine supplementation on protein synthesis in rats

Acute administration of leucine and norleucine activates the mammalian target of rapamycin (mTOR) cell-signaling pathway and increases rates of protein synthesis in a number of tissues in fasted rats. Although persistent stimulation of mTOR signaling is thought to increase protein synthetic capacity, little information is available concerning the effects of chronic administration of these agonists on protein synthesis, mTOR signal transduction, or leucine metabolism. Hence, we developed a model of chronic leucine/norleucine supplementation via drinking water and examined the effects of chronic (12 days) supplementation on protein synthesis in adipose tissue, kidney, heart, liver, and skeletal muscle from ad libitum-fed rats. The relative concentration of proteins involved in mTOR signaling and the two initial steps in leucine oxidation were also examined. Leucine or norleucine supplementation was accompanied by increased rates of protein synthesis in adipose tissue, liver, and skeletal muscle, but not in heart or kidney. Supplementation was not associated with increases in the anabolic hormones insulin or insulin-like growth factor I. Chronic supplementation did not cause apparent adaptation in either components of the mTOR cell-signaling pathway that respond to leucine (mTOR, ribosomal protein S6 kinase, and eukaryotic initiation factor 4E-binding protein-1) or the first two steps in leucine metabolism (the mitochondrial isoform of branched-chain amino acid transaminase, branched-chain keto acid dehydrogenase, and branched-chain keto acid dehydrogenase kinase), which may be involved in terminating the signal from leucine. These results suggest that provision of leucine or norleucine supplementation via the drinking water results in stimulation of postprandial protein synthesis in adipose tissue, skeletal muscle, and liver without notable adaptive changes in signaling proteins or metabolic enzymes.     

Leucine stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing mTORC1 activation

Skeletal muscle in the neonate grows at a rapid rate due in part to an enhanced sensitivity to the postprandial rise in amino acids, particularly leucine. To elucidate the molecular mechanism by which leucine stimulates protein synthesis in neonatal muscle, overnight-fasted 7-day-old piglets were treated with rapamycin [an inhibitor of mammalian target of rapamycin (mTOR) complex (mTORC)1] for 1 h and then infused with leucine for 1 h. Fractional rates of protein synthesis and activation of signaling components that lead to mRNA translation were determined in skeletal muscle. Rapamycin completely blocked leucine-induced muscle protein synthesis. Rapamycin markedly reduced raptor-mTOR association, an indicator of mTORC1 activation. Rapamycin blocked the leucine-induced phosphorylation of mTOR, S6 kinase 1 (S6K1), and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1) and formation of the eIF4E.eIF4G complex and increased eIF4E.4E-BP1 complex abundance. Rapamycin had no effect on the association of mTOR with rictor, a crucial component for mTORC2 activation, or G protein beta-subunit-like protein (GbetaL), a component of mTORC1 and mTORC2. Neither leucine nor rapamycin affected the phosphorylation of AMP-activated protein kinase (AMPK), PKB, or tuberous sclerosis complex (TSC)2, signaling components that reside upstream of mTOR. Eukaryotic elongation factor (eEF)2 phosphorylation was not affected by leucine or rapamycin, although current dogma indicates that eEF2 phosphorylation is mTOR dependent. Together, these in vivo data suggest that leucine stimulates muscle protein synthesis in neonates by enhancing mTORC1 activation and its downstream effectors.     

Effect of leucine and metabolites of branched chain amino acids on protein turnover in heart.

Leucine, but not isoleucine or valine, inhibited protein degradation and accelerated protein synthesis in hearts perfused with buffer that contained glucose (15 mM) and normal plasma levels of other amino acids, except for the branched chain compounds. Products of leucine, isoleucine, and valine metabolism also inhibited protein degradation and stimulated protein synthesis. These compounds included the transamination and decarboxylation products, as well as acetate, acetoacetate, and propionate. In some, but not all instances, inhibition of degradation and acceleration of synthesis were accompanied by an increase in intracellular leucine. When insulin was added to the perfusate, the rate of degradation was reduced by 40%, but addition of leucine was ineffective in the presence of the hormone. Insulin, leucine (2 mM) and a mixture of branched chain amino acids at normal plasma levels increased latency of cathepsin D in hearts that were perfused with buffer containing glucose. A combination of leucine and insulin increased latency more than either substance alone. These studies indicate that leucine as well as a variety of substrates that are oxidized in the citric acid cycle are involved in regulation of protein turnover in heart muscle.     

Effect of exercise on protein metabolism in humans as explored with stable isotopes

Exercising for 3.75 h on a treadmill at 50% VO2 max in the fed state induced an increased excretion of 71 mg nitrogen/kg over the 18 h after exercise. However, measurements of the time course of changes in 13CO2 excretion from ingested [1-13C]leucine indicated that all of this increased nitrogen production occurs during the exercise period. Because of the reduced renal clearance and slow turnover of the urea pool, urea excretion lags behind urea production. Measurements of nitrogen flux from the plateau labeling of urinary ammonia achieved by repeated oral doses of 15N-labeled glycine indicated that the nitrogen loss resulted from an increase in protein degradation and a decrease in protein synthesis. Further studies with [1-13C]leucine indicated that a 2-h treadmill exercise induced an increase in the nitrogen loss from 5.4 to 16 mg . kg-1 . h-1 measured with a primed constant infusion of [1-13C]leucine. This resulted from a fall in whole-body protein synthesis. Glucose given at the rate of 0.88 g . kg-1 . h-1 depressed the rate of whole-body protein degradation and appeared to suppress the exercise-induced increase in nitrogen excretion. When leucine oxidation rates were measured at increasing work rates, a linear relationship between percentage of VO2 max and leucine oxidation was observed up to 89% VO2 max when 54% of the flux of leucine was oxidized. These changes may involve nonmuscle as well as muscle tissue. Thus the source of the increased nitrogen losses is probably liver. In muscle, protein degradation is actually decreased judged by methylhistidine excretion, whereas in liver, protein degradation may be increased. Also the fall in whole-body protein synthesis may reflect changes in nonmuscle tissues because in running rats protein synthesis in muscle is maintained. As far as leucine metabolism is concerned, because the increase in leucine oxidation occurs when leucine and its keto acid concentration falls, exercise must specifically activate the 2-oxoacid dehydrogenase.     

Increase of total protein synthesis during mouse oocyte growth

Quantitative changes of mouse oocyte protein synthesis during oogenesis have been studied by determining the rate of leucine incorporation into total cell protein. The leucine intracellular poor was artificially expanded by exposing the isolated oocytes to different concentrations of radiolabelled leucine, and leucine incorporation was followed as a function of the time. Protein synthesis by mouse oocytes increases linearly with the increase of the cell volume, and such increase is constant throughout the entire period of oocyte growth.     

Regulation of muscle protein degradation,not synthesis,by dietary leucine in rats fed a protein-deficient diet

The aim of this study was to elucidate the effects of long-term intake of leucine in dietary protein malnutrition on muscle protein synthesis and degradation. A reduction in muscle mass was suppressed by leucine-supplementation (1.5% leucine) in rats fed protein-free diet for 7 days. Furthermore, the rate of muscle protein degradation was decreased without an increase in muscle protein synthesis. In addition, to elucidate the mechanism involved in the suppressive effect of leucine, we measured the activities of degradation systems in muscle. Proteinase activity (calpain and proteasome) and ubiquitin ligase mRNA (Atrogin-1 and MuRF1) expression were not suppressed in animals fed a leucine-supplemented diet, whereas the autophagy marker, protein light chain 3 active form (LC3-II), expression was significantly decreased. These results suggest that the protein-free diet supplemented with leucine suppresses muscle protein degradation through inhibition of autophagy rather than protein synthesis.     

Specificity of the effects of leucine and its metabolites on protein degradation in skeletal muscle.

The effects of leucine, its metabolites, and the 2-oxo acids of valine and isoleucine on protein synthesis and degradation in incubated limb muscles of immature and adult rats were tested. Leucine stimulated protein synthesis but did not reduce proteolysis when leucine transamination was inhibited. 4-Methyl-2-oxopentanoate at concentrations as low as 0.25 mM inhibited protein degradation but did not change protein synthesis. The 2-oxo acids of valine and isoleucine did not change protein synthesis or degradation even at concentrations as high as 5 mM. 3-Methylvalerate, the irreversibly decarboxylated product of 4-methyl-2-oxopentanoate, decreased protein degradation at concentrations greater than or equal to 1 mM. This was not due to inhibition of 4-methyl-2-oxopentanoate catabolism, because 0.5 mM-3-methylvalerate did not suppress proteolysis, even though it inhibited leucine decarboxylation by 30%; higher concentrations of 3-methylvalerate decreased proteolysis progressively without inhibiting leucine decarboxylation further. During incubation with [1-14C]- and [U-14C]-leucine, it was found that products of leucine catabolism formed subsequent to the decarboxylation of 4-methyl-2-oxopentanoate accumulated intracellularly. This pattern was not seen during incubation with radiolabelled valine. Thus, the effect of leucine on muscle proteolysis requires transamination to 4-methyl-2-oxopentanoate. The inhibition of muscle protein degradation by leucine is most sensitive to, but not specific for, its 2-oxo acid, 4-methyl-2-oxopentanoate.     

Determination of rates of protein synthesis, gain and degradation in intact hind-limb muscle of lambs.

A model of leucine metabolism in the hind-limb muscles of the milk-fed lamb was developed which permitted simultaneous estimation of the rates of protein synthesis (Ks, days-1), degradation (Kd) and therefore gain (Kg) of muscle in vivo. The conclusions drawn from the model were: the rate of protein synthesis in muscle was related to uptake of leucine; the rate of degradation of protein was related to leucine output, as leucine, or its corresponding oxo acid, 4-methyl-2-oxopentanoic acid, or CO2. These findings support findings drawn from a wide range of studies in vitro. There was no correlation between rate of protein synthesis and rate of protein degradation, which suggests that the method can allow independent estimates of each. Estimates of protein synthesis obtained from the model (of leucine metabolism in muscle) were compared with those obtained simultaneously by constant infusion of radioisotope and analysis of incorporation into tissue. There were no significant differences between the mean values obtained for synthesis (Ks), gain (Kg) and degradation (Kd) by either method (Ks 0.051 +/- 0.002, 0.046 +/- 0.007; Kg 0.016 +/- 0.002, 0.004 +/- 0.008; Kd 0.035 +/- 0.004, 0.041 +/- 0.008 day-1, respectively, for tissue analysis and the model). However, Ks obtained from the model was significantly and positively correlated with uptake of leucine from plasma, whereas Ks obtained from tissue analysis was not.     

Effects of inhibitors of protein degradation on the rate of protein synthesis in Chinese hamster ovary cells

In the absence of serum and amino acids, cultured Chinese Hamster Ovary cells released to the medium two thirds of the leucine produced by protein degradation. Because protein synthesis requires all the amino acids, the loss of leucine implies incomplete reincorporation of the other amino acids as well. Leupeptin (0.45 mg/ml) and chloroquine (up to 40 microM) inhibited protein breakdown by 21 and up to 41%, respectively, and resulted in proportional decreases in protein synthesis. Chloroquine abolished the stimulation of protein breakdown by amino acid deprivation. From the values of protein synthesis and leucine output with and without chloroquine, it is estimated that the stimulation of protein degradation not only permitted continuing protein synthesis but also increased amino acid output. In the presence of serum or amino acids protein breakdown was slower than in their absence and less sensitive to inhibition by chloroquine, but proportional effects on synthesis and degradation were still observed. It is suggested that protein degradation may be necessary for the maintenance of optimum intracellular concentrations of amino acids even in the presence of extracellular amino acids.     

Amino acid availability and age affect the leucine stimulation of protein synthesis and eIF4F formation in muscle

We have previously shown that a physiological increase in plasma leucine for 60 and 120 min increases translation initiation factor activation in muscle of neonatal pigs. Although muscle protein synthesis is increased by leucine at 60 min, it is not maintained at 120 min, perhaps because of the decrease in plasma amino acids (AA). In the present study, 7- and 26-day-old pigs were fasted overnight and infused with leucine (0 or 400 micromol.kg(-1).h(-1)) for 120 min to raise leucine within the postprandial range. The leucine was infused in the presence or absence of a replacement AA mixture (without leucine) to maintain baseline plasma AA levels. AA administration prevented the leucine-induced reduction in plasma AA in both age groups. At 7 days, leucine infusion alone increased eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1) phosphorylation, decreased inactive 4E-BP1.eIF4E complex abundance, and increased active eIF4G.eIF4E complex formation in skeletal muscle; leucine infusion with replacement AA also stimulated these, as well as 70-kDa ribosomal protein S6 kinase, ribosomal protein S6, and eIF4G phosphorylation. At 26 days, leucine infusion alone increased 4E-BP1 phosphorylation and decreased the inactive 4E-BP1.eIF4E complex only; leucine with AA also stimulated these, as well as 70-kDa ribosomal protein S6 kinase and ribosomal protein S6 phosphorylation. Muscle protein synthesis was increased in 7-day-old (+60%) and 26-day-old (+40%) pigs infused with leucine and replacement AA but not with leucine alone. Thus the ability of leucine to stimulate eIF4F formation and protein synthesis in skeletal muscle is dependent on AA availability and age.     

Determination of Regional Rates of Cerebral Protein Synthesis Adjusted for Regional Differences in Recycling of Leucine Derived from Protein Degradation into the Precursor Pool in Conscious Adult Rats

The quantitative autoradiographic L-[1-14C]leucine method for the determination of regional rates of cerebral protein synthesis in vivo takes into account recycling of unlabeled leucine derived from protein degradation into the precursor pool for protein synthesis. We have evaluated the degree of recycling by measuring the ratio of the apparent steady-state leucine specific activity in the precursor amino acid pool (tRNA-bound leucine) to that in the arterial plasma. In the whole brain of the conscious rat this ratio (lambda WB) equals 0.58. The equivalent ratio for leucine in the acid-soluble pool in whole brain (psi WB) is 0.49. A first-degree polynomial equation for lambda WB as a function of psi WB was fitted from paired determinations. To determine the degree of recycling in local regions of the brain, we have measured in individual brain regions (i) psi i and calculated lambda i assuming that the fitted equation also applies to these localized regions. Our results indicate that the degree of recycling into the precursor pool does vary regionally; lambda i in the individual regions varies from 0.62 in the hypoglossal nucleus to 0.50 in the globus pallidus. Local rates of protein synthesis were then determined by the autoradiographic technique with regional corrections for recycling of unlabeled leucine. Rates of leucine incorporation into protein averaged 6.1 nmol/g of tissue/min in the brain as a whole, with the rates in gray matter about twice those in white matter.