Protein synthesis in rat liver: influence of amino acids in diet on microsomes and polysomes

Polysome profiling in liver identifies dynamic regulation of endoplasmic reticulum translatome by obesity and fasting

Obesity-associated metabolic complications are generally considered to emerge from abnormalities in carbohydrate and lipid metabolism, whereas the status of protein metabolism is not well studied. Here, we performed comparative polysome and associated transcriptional profiling analyses to study the dynamics and functional implications of endoplasmic reticulum (ER)–associated protein synthesis in the mouse liver under conditions of obesity and nutrient deprivation. We discovered that ER from livers of obese mice exhibits a general reduction in protein synthesis, and comprehensive analysis of polysome-bound transcripts revealed extensive down-regulation of protein synthesis machinery, mitochondrial components, and bile acid metabolism in the obese translatome. Nutrient availability also plays an important but distinct role in remodeling the hepatic ER translatome in lean and obese mice. Fasting in obese mice partially reversed the overall translatomic differences between lean and obese nonfasted controls, whereas fasting of the lean mice mimicked many of the translatomic changes induced by the development of obesity. The strongest examples of such regulations were the reduction in Cyp7b1 and Slco1a1, molecules involved in bile acid metabolism. Exogenous expression of either gene significantly lowered plasma glucose levels, improved hepatic steatosis, but also caused cholestasis, indicating the fine balance bile acids play in regulating metabolism and health. Together, our work defines dynamic regulation of the liver translatome by obesity and nutrient availability, and it identifies a novel role for bile acid metabolism in the pathogenesis of metabolic abnormalities associated with obesity.

Chronic diseases including obesity and associated metabolic abnormalities have become the greatest threat to human health worldwide. How metabolic organs and organelles adapt to nutritional fluctuations, or fail to do so, remains incompletely understood. To explore these issues, we developed a new platform to explore translational responses in the liver, a critical organ for metabolic homeostasis. In this translatomic platform, we integrated polysome profiling and global analysis of polysome-associated mRNAs to systematically quantify protein synthesis on each transcript in obesity and during fasting. Our analysis demonstrated for the first time that protein synthesis is progressively suppressed in the obese liver and that the overall translatome profile of obese liver markedly resembles that of fasting lean mice, particularly in mitochondrial function and bile metabolism. We also examined the physiological impact of some of these alterations and concluded that aberrant bile acid metabolism in the obese liver represents a novel mechanism contributing to hyperglycemia and continuous weight gain. Together, our work reveals abnormal translational regulation as a novel aspect of obesity that could impact future directions in metabolic disease treatment, and we believe translatome profiling represents a new approach to unravel complex mechanisms regulating cellular function and disease pathology.

The distribution of ribosomes between different functional states in livers of fed and starved mice

1. To investigate the role of ribosome function in regulating protein synthesis, the activity, distribution and functional states of ribosomal particles were investigated in livers of mice fed ad libitum or starved overnight. 2. The distribution of protein-synthesizing activity between polyribosomes of different sizes was analysed after incorporation of radioactive leucine, and the quantitative distribution of ribosomes as native subunits, monomers and polyribosomes was analysed after incorporation of orotic acid. Precursors labelled with (3)H or (14)C were given separately to fed and starved mice, so that livers from the two groups of animals were processed together. 3. The former experiments showed that starvation has little effect on the distribution of protein-synthesizing activity across polyribosome sedimentation patterns, though the latter experiments showed that the proportion of ribosomes existing as monomers increased from 9.5% to 15.2%, whereas the proportion existing as polyribosomes decreased from 81.4% to 75.6%. Starvation had a negligible effect on the proportion of native subunits, which accounted for 9.1% and 9.2% of the ribosomes in fed and starved mice respectively. 4. The monomeric ribosome fraction was isolated and subjected to ionic conditions which selectively dissociate single ribosomes. Starvation increased the proportion of monomers that dissociated from 59% to 72%, so the monomers that accumulate in livers of starved animals are single ribosomes and not monoribosomes resulting from degradation of polyribosomes. 5. The fate of newly formed ribosomal particles was studied by measuring the specific radioactivity of native subunits, monomers and polyribosomes at different times after injection of radioactively labelled orotic acid. Starvation did not appear to affect equilibration between newly formed particles and polyribosomes, and the radioactivity of polyribosomes in both groups of mice reached about 90% of that in native subunits after 4h. The radioactive labelling of monomers proceeded at a slower rate, especially after starvation. At 4h, the radioactivity of monomers was 64% and 55% that of native subunits in fed and starved mice respectively.

The regulation of protein synthesis in the liver of rats. Mechanisms of dietary amino acid control in the immature animal

Weanling (23-day-old) rats were fed either on an amino acid-deficient diet (6% of casein, which in effect represents an ;amino acid-deficient' diet) or on a diet containing an adequate amount of protein (18% of casein) for 28 days. The hepatic cells from the animals fed on the low-protein diet were characterized by low amino acid content, almost complete inhibition of cell proliferation and a marked decrease in cell volume, protein content and concentration of cytoplasmic RNA compared with cells from control rats. The lower concentration of cytoplasmic RNA was correlated with a decreased ribosomal-RNA content, of which a larger proportion was in the form of free ribosomes. The protein-synthetic competence and messenger-RNA content of isolated ribosomes from liver cells of protein-deprived animals were 40-50% of those noted in controls. At 1hr. after an injection of radioactive uridine, the specific radioactivity of liver total RNA was greater in the group fed on the low-protein diet, but the amount of label that was associated with cytoplasmic RNA or ribosomes was significantly less than that noted in control animals. From these data it was concluded that dietary amino acids regulate hepatic protein synthesis (1) by affecting the ability of polyribosomes to synthesize protein and (2) by influencing the concentration of cytoplasmic ribosomes. It is also tentatively hypothesized that the former process may be directly related to the concentration of cellular free amino acids, whereas the latter could be correlated with the ability of newly synthesized ribosomal sub-units to leave the nucleus.

4E-BP1 and S6K1: translational integration sites for nutritional and hormonal information in muscle

Maintenance of cellular protein stores in skeletal muscle depends on a tightly regulated synthesis-degradation equilibrium that is conditionally modulated under an extensive range of physiological and pathophysiological circumstances. Recent studies have established the initiation phase of mRNA translation as a pivotal site of regulation for global rates of protein synthesis, as well as a site through which the synthesis of specific proteins is controlled. The protein synthetic pathway is exquisitely sensitive to the availability of hormones and nutrients and employs a comprehensive integrative strategy to interpret the information provided by hormonal and nutritional cues. The translational repressor, eukaryotic initiation factor 4E binding protein 1 (4E-BP1), and the 70-kDa ribosomal protein S6 kinase (S6K1) have emerged as important components of this strategy, and together they coordinate the behavior of both eukaryotic initiation factors and the ribosome. This review discusses the role of 4E-BP1 and S6K1 in translational control and outlines the mechanisms through which hormones and nutrients effect changes in mRNA translation through the influence of these translational effectors.

Hepatic protein and amino-acid metabolism in poultry

Two subjects are discussed: first, the regulation of hepatic protein synthesis; and second, the intermediary metabolism of methionine, particularly with respect to the role of 2-hydroxy-4-(methylthio)-butanoic acid (HMB, Alimet). In the first section, the regulation of albumin synthesis is reviewed in terms of molecular events associated with the changes in albumin synthesis during fasting and refeeding. The effect of infection or of inflammatory stress on both albumin and total protein synthesis in the liver is also discussed. In the second part, research results are presented which indicate that HMB is a naturally occurring compound in methionine intermediary metabolism. The HMB synthesis by chick liver enzymes is demonstrated, and its role in normal avian methionine metabolism is discussed.

Effect of nutritional status on acute phase protein response to elective surgery

The aim of this study was to assess the effect of nutritional status on the acute phase protein response to elective surgery. Matched pairs of patients with gastric or colorectal cancer were studied. Undernourished patients (body-weight less than 80 per cent of ideal and triceps skinfold thickness less than 65 per cent of standard) were matched with controls (body-weight greater than 95 per cent of ideal and triceps skinfold thickness greater than 80 per cent of standard) for age, sex, type of pathology and surgery. Blood samples taken preoperatively and on days 1-5 after operation were analysed for C-reactive protein, alpha 1-antitrypsin, alpha 1-acid glycoprotein, albumin, prealbumin and transferrin. There was no significant difference in any of the preoperative protein concentrations between the two groups although undernourished patients tended to have lower transferrin concentrations (mean +/- s.e.m., 2.2 +/- 0.3 g/l) than the control group (2.9 +/- 0.1 g/l). C-reactive protein response was significantly smaller in the undernourished group than in the control group (P = 0.05, Mann-Whitney U test). The responses of the other acute phase proteins did not differ between the two groups. There was no difference in the length of hospital stay after operation when undernourished patients (mean +/- s.e.m., 17.7 +/- 3.0 days) were compared with controls (14.3 +/- 3.6 days). Undernourished patients therefore have an attenuated C-reactive protein response to elective surgery. The significance of this in relation to morbidity and mortality in severely ill patients requires further investigation.

Stimulation of liver tryptophan pyrrolase during heat exposure

Exposure of rats to heat (39 +/- 1 degree C) stimulated liver tryptophan pyrrolase 2-fold between 3 and 48 h. Plasma corticosterone increased 2-fold after 1 h of heat exposure and decreased to a low value of 50% by 16 h. The effect of heat exposure on the enzyme was obtained in adrenalectomized animals. Stimulation by cortisol and tryptophan of the enzyme was also obtained in heat exposure, and the effects seemed to be additive. The concentration of tryptophan in the liver remained unchanged, and that in the plasma decreased to about 50% at 8 h exposure to heat and reverted to normal by 46 h. Simultaneous administration of noradrenaline to heat-exposed rats had no effect, whereas that of thyroxine partly prevented the stimulation of the enzyme activity. Hypothyroid conditions obtained by thyroidectomy or treatment with propylthiouracil significantly stimulated the enzyme activity. Cycloheximide treatment of heat-exposed rats did not prevent the stimulation of the enzyme activity. The results indicate that the effect of heat exposure on liver tryptophan pyrrolase is obtained, due to the accompanying hypothyroid condition, by increasing the activity of the existing protein by a mechanism possibly different from those known at present.

The influence of glucocorticoid on albumin synthesis and its messenger RNA in rat in vivo and in hepatocyte suspension culture

Corticosteroids are known to stimulate the synthesis of a number of liver-specific proteins. The reports regarding the effect of glucocorticoid on albumin synthesis in vivo and in vitro are controversial. In an attempt to determine the mechanism by which glucocorticoid exerts its influence on hepatic albumin synthesis and to find an explanation for the conflicting data, we have studied the effect of dexamethasone disodium phosphate on albumin synthesis and albumin messenger RNA as determined by the molecular hybridization technique in hepatocytes in rat in vivo and in suspension culture. In hepatocyte suspension culture, addition of 0.48 microM dexamethasone in medium at zero time led to a significant increase (20%) in incorporation of labeled precursor into albumin as compared to control experiments; this was accompanied by a maintainance of the initial level of full-length albumin mRNA for a 9 h period. In hepatocytes cultured without dexamethasone in the medium there was a progressive loss of albumin mRNA content. Despite this finding, dexamethasone was not able to increase the albumin mRNA content in hepatocyte to a level higher than the initial value. Moreover, administration of this hormone either intraperitoneally or intravenously into rats did not lead to enhanced cell-free albumin synthesis or to an increased level of albumin mRNA. These findings suggest that glucocorticoid does not play an essential role in the regulation of albumin synthesis in vivo. In vitro, however, glucocorticoid leads to a preservation of the initial level of albumin mRNA and thus plays a role in the control of spontaneous dedifferentiation of liver cells in culture.

Effects of chronic renal failure on protein synthesis and albumin messenger ribonucleic acid in rat liver

Previously we reported that chronic renal failure in rats leads to preferential disaggregation of liver membrane-bound polysomes associated with a decrease in albumin synthesis. To determine whether reduced albumin synthesis results from reduced cellular levels of albumin messenger RNA (mRNA) or some other molecular mechanism, we have employed mRNA-DNA hybridization in conjunction with cell-free protein synthesis to determine albumin mRNA sequence content and biological activity in subcellular fractions from control and uremic rat liver. Using high specific activity albumin [3H]-complementary DNA prepared from purified-albumin mRNA, we found that total liver polysomes and albumin mRNA sequence content are increased in uremic animals. The extra polysomes are located within the membrane-bound subcellular fraction. These polysomes, however, have reduced ability to synthesize albumin in the cell-free system, and mRNA isolated from membrane-bound polysomes of uremic liver showed reduced albumin synthesis. Evaluation of albumin mRNA size by hybridization analysis revealed a reduced content of intact albumin mRNA molecules per microgram of RNA in the liver of uremic animals. This was associated with increased ribonuclease activity in uremic cytosol. The diminished albumin synthesis by membrane-bound polysomes of uremic rat liver can, therefore, be explained by enhanced degradation of albumin mRNA.

An in vitro amino acid incorporation method for assessing the status of in vivo protein synthesis

A quantitative in vitro amino acid incorporation assay is described which can be used to assess the status of in vivo protein synthesis. The preparation and incubation conditions employed result in constant precursor specific activity and limit amino acid incorporation to completion of nascent peptide chains. Results obtained with this method correlate well with measurements of polyribosome profiles using sucrose gradient centrifugation. The assay is easily applied to a large number of samples, and requires only a fraction of the time and tissue necessary for conventional measures of polysome aggregation. The method has been found suitable for studies of protein synthesis in mouse brain and liver, and in gerbil brain, but not in mouse kidney. Products of in vitro protein synthesis can be separated by standard electrophoretic techniques, allowing a characterization of proteins whose mRNAs are actively translated in vivo.

Tryptophan overload in the pregnant rat: effect on brain amino acid levels in in vitro protein synthesis

The concentration of most amino acids was higher in the brains of 19- and 21-day rat fetuses than in their respective mothers. After an intraperitoneal load of tryptophan to the mother, the intracerebral concentration of several amino acids (including leucine) decreased not only in the mothers, but also in their fetuses. The in vitro incorporation of [3H]leucine into proteins in brain postmitochondrial supernatant fractions was enhanced in both the mothers and fetuses after tryptophan administration, but this effect disappeared when protein synthesis was calculated by using specific activities corrected for the amount of unlabeled leucine in the preparation. By this criterion, protein synthesis activity appeared similar in the brains of 19- and 21-day pregnant rats but was higher in their fetuses, especially in the 21-day subjects. Thus, protein synthesis in the brain was not altered by marked changes in the amino acid pool and more profound and prolonged metabolic disturbances must occur to cause permanent damage in the developing brain.

Acute effects of ethanol intake on albumin and total protein synthesis in free and membrane-bound polyribosomes of rat liver

Controversial results have been reported in the last few years concerning the effects of ethanol on hepatic protein synthesis. In most of the studies no distinction has been made between the synthetic capabilities of the polyribosomes and the secretory product of labelled protein by the hepatocytes. In order to assess the influence of a single feeding of ethanol on the synthesis of albumin and total protein by the polyribosomes of rat liver, free and membrane-bound polyribosomes were isolated quantitatively from rats given 4--8 g ethanol per kg body weight 3--5 h before killing. The following results were obtained: (1) No difference was found in yield and size of free and membrane-bound polyribosomes isolated from control and ethanol-treated rats. The abilities to synthesize albumin and total protein were also equal for polyribosomes from both groups. (2) Addition of 1% ethanol to the incubation mixture of protein synthesis lowered albumin and total protein synthesis by 20%. No effect was observed with 0.5% ethanol. (3) Cell sap prepared from ethanol-treated rats contains a factor or factors which stimulate protein synthesis (10--15%). (4) The albumin mRNA sequence content was not changed in free and membrane-bound polyribosomal RNA fractions of ethanol-treated rats as compared to the control animals.

L-Tryptophan action on hepatic RNA synthesis and enzyme induction

L-Tryptophan increases the activity of hepatic amino acid metabolizing enzymes, affects gluconeogenesis and displays a modulatory effect on several enzymes connected with RNA synthesis. The underlying mechanism differ in individual cases and result in both an increase of enzyme synthesis de novo and a decrease of enzyme degradation. Tryptophan displays a unique effect causing aggregation of hepatic polyribosomes connected with enhanced protein synthesis and preceded by a higher transport of poly (A) messenger RNA from the nucleus to the cytoplasm. The variety of rather specific effects mediated by tryptophan brings to mind hormonal action and the existence of specific tryptophan receptors is predicted.

Effects of fasting and tryptophan force-feeding on the activity of hepatic nuclear RNA polymerases in rats

The effects of fasting, and subsequent force-feeding of L-tryptophan on the activity of hepatic nuclear DNA-dependent RNA polymerases were studied in adult (5-6 weeks old), and old (5-6 months) male Wistar rats. Liver nuclei, nucleoli, and nucleoplasmic fraction were isolated from rats following a single tube-feeding of tryptophan or water, and were assayed in vitro for the activity of different RNA polymerases. Whereas in adult rats 24 h of fasting caused a significant reduction in the activity of RNA polymerase I and II, in old rats the activity of only polymerase II was decreased after 24 h of fasting. In fasted adult rats administration of tryptophan promptly restored the activities of both polymerases to the respective normal fed levels, while in old rats none of the polymerases were affected by tryptophan. In fasted adult rats the pattern of response for both forms of polymerases to a single tube-feeding of tryptophan, over a period of 5 h, was found to be biphasic. When ribonuclease activity of nuclei was suppressed by performing incubations at low temperatures (17-30 degrees C) the difference between the two groups for polymerase I was greatly reduced, and for polymerase II the difference was fully abolished. Pre-treatment of fasted adult rats with cycloheximide (1.5 mg/kg) was found to abolish the 30 min tryptophan-mediated stimulation of both polymerase I and II activities. In cycloheximide pretreated rats the activity of polymerase II, but not polymerase I returned to its original level 5 h after tryptophan force-feeding.

Experimentally induced and natural recovery from the effects of phenylalanine on brain protein synthesis

The decrease in the neural polyribosomes produced during hyperphenylalaninemia could not be restored to normal levels by the injection of other single neutral amino acids. All of the neutral amino acids that are transported with phenylalanine were found to produce an alteration of neural polyribosomes similar to that measured with phenylalanine. However, the injection of a balanced mixture of 6 or 7 neutral amino acids could restore the brain polyribosomes to normal states. Although this experimentally induced recovery did not lower brain phenylalanine concentrations, it did restore the acylation levels of methionyl-tRNA, and in particular, the methionyl-tRNA initiator species. This also led to a concomitant stimulation of the elongation rate of brain polypeptide synthesis. A natural recovery of brain polyribosomal levels (occurring 2 h after 1 mg/g phenylalanine is injected) did not appear to represent a real recovery of neural protein metabolism. Phenylalanine concentrations were increased in the brain, the acylation levels of methionyl-tRNA, alanyl-tRNA and the initiator methionyl-tRNA remained altered, and the rate of ribosome translocation was decreased 28%.

Influence of chronic renal failure on protein synthesis and albumin metabolism in rat liver

Chronic renal failure in rats leads to changes in hepatic protein synthesis and albumin metabolism at both the cellular and molecular level. In rats with chronic uremia (blood urea nitrogen greater than 45 mg/100 ml 1 mo after surgical reduction in renal mass), cell-free protein synthesis is reduced 30--40% in liver membrane-bound polyribosomes. Albumin synthesis by membrane-bound polysomes in uremia is reduced even more than the reduction in total protein synthesis. Activity of free polysomes remains norma. There is also intracellular accumulation of albumin in liver of uremic rats and a concomitant decrease in serum albumin. In normal liver, most intracellular albumin is located in the microsomal fraction, whereas in liver from uremic animals the excess albumin is found in the free cytosol fraction. These results can be explained either by a defect in synthesis of albumin by membrane-bound polysomes with release of newly synthesized albumin into the cytosol or by a reduced ability of polysomes synthesizing albumin to associate with the membrane fraction in rats with chronic renal failure.

Albumin synthesis in cultured hempatoma cells. Regulation by essential amino acids

The effects of essential amino acids on albumin synthesis by a mouse hepatoma cell line have been investigated. The amino acids tested were tryptophan, phenylalanine, histidine, isoleucine and leucine. Cellular rates of synthesis (molecules albumin/cell per min) were determined from rates of [3H]leucine incorporation into immunoprecipitable albumin in the culture medium. The effects of amino acids on albumin synthesis fall into three distinct groups. The concentration of tryptophan producing half-maximal synthesis is 4 micronM. The corresponding concentration for leucine is 100 micronM. Histidine, phenylalanine and isoleucine were very similar, the half-maximal concentrations being approximately 15 micronM. The concentrations of amino acids producing half-maximal synthesis correlate directly with the amino acid composition of albumin. The levels of these essential amino acids necessary to saturate albumin synthesis have been compared with amino acid levels in normal plasma.

The effect of tryptrophan on nucleocytoplasmic translocation of RNA in rat liver

The effect of the administration of tryptophan on the transport of nuclear poly (A)-containing mRNA to the cytoplasm in rat liver was investigated. Administration of tryptophan to fasted rats pretreated with cordycepin and actinomycin D led to decreased levels of nuclear poly (A)-mRNA and a concomitant increase in the levels of polyribosomal poly (A)-mRNA in the cytoplasm as determined by measuring in vivo incorporation of labeled precursors into hepatic RNA. Using isolated hepatic nuclei of rats prelabeled in vivo with [14C]orotic acid, there was greater release of labeled poly(A)-mRNA into the incubation medium from nuclei of tryptophan-treated rats than from nuclei of control animals. The increased release of RNA from hepatic nuclei of tryptophan-treated animals was not related to the cell sap present in the media since cell saps from livers of control and experimental rats gave similar results. These results support earlier findings which suggest that in the rat tryptophan increases the rate of translocation of hepatic poly(A)-mRNA from nucleus to cytoplasm.

Influence of tryptophan on the level of hepatic microsomal cytochrome P-450 in well-fed normal, adrenalectomized, and phenobarbital-treated rats

In well-fed normal male rats, either force-feeding of tryptophan or a single injection of phenobarbital produced significant increments in hepatic microsomal cytochrome P-450 and the associated aniline hydroxylase activity. Administration of both tryptophan and phenobarbital together resulted in even greater stimulation than when the compounds were given alone. Adrenalectomy lowered instead the cytochrome P-450 concentration in comparison with that of normal rats, and administration of tryptophan and phenobarbital in this condition produced no significant increment in cytochrome P-450 concentration. In addition, phenobarbital administered either singly or in combination with tryptophan resulted in 80% mortality, which was reduced to zero by pretreatment with cortisol. While in cortisol-treated adrenalectomized rats administration of phenobarbital caused a 56% increment in cytochrome P-450 as compared to controls, tryptophan produced only a minor (9%) increase. In normal, as well as in adrenalectomized rats, tryptophan and phenobarbital administered either idividually or together increased microsomal protein concentration. In normal rats actinomycin-D treatment reduced both cytochrome P-450 and microsomal protein concentrations below that of the non-treated control levels. Further administration of either tryptophan or phenobarbital slightly increased the level of cytochrome P-450, and the two compounds together caused 40 and 21% increments of the same compared to actinomycin-treated and non-treated controls, respectively.

[Resorption and incorporation of radioactive-labeled amino acids during administration of various protein carriers in rats. 3. Uptake of radioactivity by blood plasma, liver and muscular tissue and incorporation into tissue proteins after intragastric administration of 14C-leucine and 3H-glycine]

Growing male rats received diets of varying biological value (protein sources: powdered whole egg (V); fish meal (F); yeast (H); gelatine (G); protein-free diet (e)) for a 14-day feeding period. Subsequently, 14C leucine and 3H glycine were administered intragastrically. The level of uptake of 14C and 3H radioactivity into blood plasma, liver and muscular tissue and the rate of incorporation of the radioactive tracers into the proteins of these tissues was examined. A negative correlation was found to exist between the incorporation of radioactivity into liver proteins and the biological value of dietary proteins, the former being mainly dependent on the level of incorporation into the liver. For muscular proteins the rate of incorporation decreases with the decreasing biological value of the dietary proteins. This may be attributed to the fact that with poor protein nutrition the rate of protein synthesis in the skeletal muscles is also reduced. Comparative studies on the specific 14C radioactivity from free leucine made in the group on the protein-free diet and in the group receiving the whole egg diet showed that the leucine pool of the skeletal muscles was markedly redueced in animals fed a protein-deficient diet while the leucine pool in the liver remained comparatively constant.

Phosphopyruvate carboxylase induction by L-tryptophan. Effects on synthesis and degradation of the enzyme

1. The administration of l-tryptophan to fed rats produces a twofold increase in hepatic phosphopyruvate carboxylase activity that represents a comparable increase in enzyme protein. With specific antibody against the enzyme we have shown that the increase in phosphopyruvate carboxylase is partially mediated via an actinomycin D-sensitive increase in enzyme synthesis. 2. In starved animals tryptophan increases the enzyme activity without any change in the relative rate of phosphopyruvate carboxylase synthesis. In this condition degradation of the enzyme is retarded by tryptophan by a mechanism that is not prevented by cycloheximide.