Fasting and insulin regulation of the utlization of acetoacetate for fatty acid synthesis.

The effects of fasting and of insulin on the incorporation of acetoacetate, β-OH-butyrate, or acetate into fatty acids of liver, adipose tissue, and carcass were studied in mice. Fasting decreases the incorporation of the three precursors, more so in liver than in the other tissues. Insulin totally restores lipogenesis in adipose tissue when the precursor is acetate or acetoacetate. Its effect is less marked on the incorporation of β-OH-butyrate and in the liver. The incorporation of acetate or acetoacetate into fatty acids by 100,000g supernatant protein of mouse liver was also studied. Fasting strongly decreases the incorporation of both compounds and insulin partially restores it. The activities of cytoplasmic acetyl-CoA and acetoacetyl-CoA synthetases were measured in the liver supernatant solution of fed or fasted mice. Fasting strongly decreases the activities of both enzymes; refeeding restores the activities; refeeding and insulin increase the activities above normal levels. Actinomycin suppresses the effect of insulin. The results strongly suggest that insulin is an inducer of the synthesis of cytoplasmic acetyl-CoA and acetoacetyl-CoA synthetases, and that both these synthetases are adaptative enzymes.     

The effects of food deprivation and re-feeding on bovine adipose-tissue glycogen synthase.

Bovine adipose-tissue glycogen metabolism was studied during food deprivation and re-feeding. Changes in the specific activity of adipose-tissue glycogen synthase paralleled changes in tissue glycogen content: both parameters increased during food deprivation and remained so during the first 10 days of re-feeding. The values for the A0.5 (activation constant) for glucose 6-phosphate of the freshly isolated enzyme from adipose tissue from fed and starved steers were 2.9 +/- 0.1 mM and 0.90 +/- 0.05 mM respectively. Additionally, whereas incubation of adipose-tissue extracts from fed steers did not activate endogenous glycogen synthase (through a presumed phosphoprotein phosphatase mechanism), the enzyme from starved or re-fed (up to 3 days re-feeding) steers was reversibly activated as measured by changes in the value for the A0.5 for glucose 6-phosphate. Thus activation of bovine adipose-tissue glycogen synthase during food deprivation appears to be related to expression of glycogen synthase phosphatase activity. These effects of food deprivation on bovine glycogen metabolism contrast markedly with the effects observed in rat adipose tissue.     

Modulation of the activity of insulin-dependent enzymes of lipogenesis by glucocorticoids.

Administration of triamcinolone or dexamethasone to rats led to a prompt, marked and persistent rise in liver acetyl-CoA carboxylase activity. The activity of fatty acid synthetase increased to a lesser extent and after a more prolonged glucocorticoid treatment, whereas the changes in that of NADP-malate dehydrogenase and ATP-citrate lyase were not appreciable. The overall channeling of [1-14-C]acetyl-CoA to fatty acids was enhanced. The triamcinolone effect on acetyl-CoA carboxylase activity appeared to be dependent on the coincident hyperinsulinemia since it was not obtained in alloxan-diabetic rats, whereas the alanine-aminotransferase-inducing effect of this hormone was additive to that of insulin deficiency. In adipose tissue triamcinolone treatment caused a reduction in the activity of all lipogenesis enzymes and blunted their response to insulin administration. The antagonism of glucocorticoids toward insulin, selectively modulating the responses of the insulin-sensitive enzymes in liver and adipose tissue is discussed. The rise in hepatic lipogenic capacity, through the retention of the ability of insulin to induce acetyl-CoA carboxylase, may be physiologically important in restraining the ketogenesis from acetyl-CoA despite the increased fat utilization during glucocorticoid excess.     

Regulation of fatty acid synthesis and malonyl-CoA content in mouse brown adipose tissue in response to cold-exposure, starvation or re-feeding.

1. The effect of nutritional status on fatty acid synthesis in brown adipose tissue was compared with the effect of cold-exposure. Fatty acid synthesis was measured in vivo by 3H2O incorporation into tissue lipids. The activities of acetyl-CoA carboxylase and fatty acid synthetase and the tissue concentrations of malonyl-CoA and citrate were assayed. 2. In brown adipose tissue of control mice, the tissue content of malonyl-CoA was 13 nmol/g wet wt., higher than values reported in other tissues. From the total tissue water content, the minimum possible concentration was estimated to be 30 microM 3. There were parallel changes in fatty acid synthesis, malonyl-CoA content and acetyl-CoA carboxylase activity in response to starvation and re-feeding. 4. There was no correlation between measured rates of fatty acid synthesis and malonyl-CoA content and acetyl-CoA carboxylase activity in acute cold-exposure. The results suggest there is simultaneous fatty acid synthesis and oxidation in brown adipose tissue of cold-exposed mice. This is probably effected not by decreases in the malonyl-CoA content, but by increases in the concentration of free long-chain fatty acyl-CoA or enhanced peroxisomal oxidation, allowing shorter-chain fatty acids to enter the mitochondria independent of carnitine acyltransferase (overt form) activity.     

Fatty acid synthetase activity in the liver and adipose tissue of rats fed with various carbohydrates

1. The inclusion of sucrose in the diet of rats led to an increase in hepatic fatty acid synthetase activity compared with that of rats fed with starch as the sole carbohydrate. The higher activity occurred within 18h of the introduction of sucrose and persisted with fluctuations for the 30 days of the experiment. Reversal of the diets in some rats after 21 days led to changes in the enzyme activity to values appropriate to the second diet. The plasma triglyceride concentration followed a similar pattern. 2. A comparison of the effects of diets with starch, glucose, maltose, sucrose or fructose showed that fructose gave the highest values of triglyceride content and of fatty acid synthetase activity in liver, but the lowest values of the synthetase activity in adipose tissue and the lowest values of plasma insulin concentration. These effects may perhaps be attributed to the low insulin response to fructose and to the high affinity of the liver for this sugar. 3. When the diet contained fructose or sucrose there was a correlation between hepatic synthetase activity and plasma triglyceride concentration. Neither of these, however, was related to plasma insulin concentration. On the other hand, there was a correlation between plasma insulin concentration and fatty acid synthetase activity in adipose tissue. 4. When rats were starved and then re-fed the differences in enzyme activities induced by fructose or glucose were minimized. This, together with the varying degree of difference during the course of the experiments, may explain why other workers, using the starvation-re-feeding technique and making measurements on one day only, have failed to observe differences in the activities of lipogenic enzymes in animals fed with either fructose or glucose.     

Hormonal regulation of fatty acid synthetase, acetyl-CoA carboxylase and fatty acid synthesis in mammalian adipose tissue and liver.

The major objectives of this study were to define the roles of adrenal glucocorticoids and glucagon in the long-term regulation of fatty acid synthetase and acetyl-CoA carboxylase of mammalian adipose tissue and liver. Particular emphasis was given to elucidation of the mechanisms whereby these hormones produce their regulatory effects on enzymatic activity. To dissociate mental manipulation, nutritional conditions were ridgidly controlled in the experiments described. Administration of glucocorticoids to adult rats led to a marked reductionin activities of fatty acid synthetase and carboxylase in adipose in adipose tissue but no change occurred in liver. Adrenalectomy produced an increase in activities of these lipogenic enzymes in adipose tissure, but, again, no change was noted in liver. The decrease in enzymatic activities in adipose tissue with glucocorticoid administration correlated well with a decrease in fatty acid synthesis, determined in vivo by the 3-H2O method. The mechanisms whereby glucocorticoids led to a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the decrease in fatty acid synthetase activity observed in adipose tissue was shown to reflect a decrease in content of enzyme, and not a change in catalytic efficiency. The mechanism underlying the decrease in enzyme content is a decrease in synthesis of the enzyme. The relation of the effects of glucocorticoids to the effects of certain other hormones involved in regulation of lipogenesis was investigated in hypophysectomized and in diabetic animals. Thus, the observation that the glucocorticoid effect on synthetase and carboxylase occurred in adipose tissue of hypophysectomized rats indicated that alterations in levels of other pituitary-regulated hormones were not necessary for the effect. That glucocorticoids play some role in regulation of synthetase and carboxylase in liver, at lease in the diabetic state, was shown by the observation that the low activities of these enzymes in diabetic animals could be restored to normal by adrenalectomy. An even more pronounced restorative effect was apparent in adipose tissue of adrenalectomized, diabetic animals. Administration of glucagon during the refeeding of starved rats resulted in a marked reduction in the induction of fatty acid synthetase, acetyl-CoA carboxylase and in the rate of incorporation of 3-H from 3-H2O into fatty acids in liver, but no change in these parameters occurred in adipose tissue. Administration of theophylline resulted in intermediate reduction in liver. The mechanisms whereby glucagon led tto a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the changes in fatty acid synthetase activity were shown to reflect reductions in content of enzyme. The mechanism underlying these reductions in content is reduced synthesis of enzyme.     

Estrogen induces hyperlipidemia in fasted chicks

To determine whether the estrogen-induced hyperlipidemia is affected by fasting, male growing chicks were administered subcutaneously a single dose of 17 beta-estradiol (25 mg/kg body wt), and the hormone treatment lasted for 2 days with or without feed (Experiment 1). In the second experiment, chicks were initially fasted for 1 or 3 days, and then treated with the same dosage of 17 beta-estradiol as in Experiment 1 for 2 days without feed. Plasma and liver lipids, and the activities of hepatic malic enzyme, glucose-6-phosphate dehydrogenase, and hormone-sensitive lipase in the adipose tissue were determined. Compared with fed control chicks, estrogen treatment in fed birds resulted in a marked elevation of plasma lipids, especially triglyceride during the 2-day period (137 vs 2263 mg/dl). In fasted chicks, the present finding that estrogen also induced a marked hyperlipidemia is noteworthy. Upon estrogen treatment (Experiment 1), the level of plasma triglyceride in fasted birds increased about 16 times over that of the fasted control group (133 vs 2093 mg/dl). Even in chicks fasted for 5 days (Experiment 2), estrogen treatment resulted in a persistent hypertriglyceridemia (75 vs 1369 mg/dl). In fed chicks, estrogen treatment also induced a fatty liver with massive accumulation of triglyceride, but the liver of estrogen-treated/fasted chicks appeared to be normal. In both fed and fasted chicks, malic enzyme was found to be the major NADPH producing enzyme in the liver. Upon fasting, both malic enzyme and glucose-6-phosphate dehydrogenase activities decreased significantly (P less than 0.05). In fed chicks, the total activities of both enzymes increased with estrogen treatment, whereas the effect of hormone on these enzymes was less obvious in fasted chicks. The hormone-sensitive lipase activity in the adipose tissue was much lower in fed chicks compared with that of fasted birds (0.15 vs 0.33 nmol of oleic acid released/min/mg protein). Estrogen treatment in fed chicks had no effect on the hormone-sensitive lipase activity, but its activity was enhanced by the hormone treatment in fasted chicks. The present finding that hyperlipidemia persisted in estrogenized chicks during the fasting seems to indicate the complex nature of this hormonal influence on lipid metabolism.     

Utilization of methylmalonate for the synthesis of branched-chain fatty acids by preparations of chicken liver and sheep adipose tissue.

1. The utilization of methyl[2-14C]malonyl-CoA for fatty acid synthesis was investigated using synthetase preparations from chicken liver and sheep adipose tissue. 2. The rate of fatty acid synthesis from acetyl-CoA and malonyl-CoA was greatly diminished in the presence of methylmalonyl-CoA. 3. In the absence of malonyl-CoA, methylmalonyl-CoA was utilized for fatty acid synthesis only very slowly by the synthetase from sheep adipose tissue and not at all by that from chicken liver. 4. Despite the inhibitory effect of methylmalonyl-CoA on fatty acid synthesis from malonyl-CoA, it was utilized by the synthetase preparations from both species to produce a complex mixture of methyl-branched fatty acids.     

Ornithine decarboxylase and polyamines in liver and kidneys of rats on cyclical regimen of protein-free and protein-containing diets. Relationship to deoxyribonucleic acid synthesis in liver

1. The activity of ornithine decarboxylase in the liver and kidneys of rats maintained on a cyclical regimen of protein-free and protein-containing diets was investigated. There was a daily activation of the enzyme in response to the feeding of protein after 3 days feeding of protein-free diet. 2. The activation of ornithine decarboxylase in the liver and kidneys of rats re-fed on protein was demonstrable throughout 16 cycles of alternating 3-day periods of protein-free and protein-containing diets. The magnitude of the activation in the kidneys diminished from 20-fold stimulation in the first cycle to 5-fold stimulation (compared with animals fed with protein-free diet) in the later cycles of protein re-feeding. The activation of the enzyme in liver was decreased from 20-fold stimulation in the first cycle to approx. 10-fold stimulation in later cycles. 3. The concentration of spermidine was increased by approx. 50% in the liver of animals during cycling from protein-free to protein-containing diets. Spermine was unchanged, and putrescine was maintained at a low concentration approx. one-fifth to one-tenth that of spermidine after protein re-feeding. 4. The incorporation of [(3)H]thymidine into liver DNA was increased 10-fold in animals re-fed with protein compared with animals receiving protein-free diets. 5. The activation of ornithine decarboxylase by re-feeding of protein was inhibited 90% by the injection of propane-1,3-diamine during re-feeding. The stimulation of DNA synthesis was inhibited 60% by multiple injections of propane-1,3-diamine during the re-feeding of protein.     

Requirement for the enzymes acetoacetyl coenzyme A synthetase and poly-3-hydroxybutyrate (PHB) synthase for growth of Sinorhizobium meliloti on PHB cycle intermediates

We have identified two Sinorhizobium meliloti chromosomal loci affecting the poly-3-hydroxybutyrate degradation pathway. One locus was identified as the gene acsA, encoding acetoacetyl coenzyme A (acetoacetyl-CoA) synthetase. Analysis of the acsA nucleotide sequence revealed that this gene encodes a putative protein with a molecular weight of 72,000 that shows similarity to acetyl-CoA synthetase in other organisms. Acetyl-CoA synthetase activity was not affected in cell extracts of glucose-grown acsA::Tn5 mutants; instead, acetoacetyl-CoA synthetase activity was drastically reduced. These findings suggest that acetoacetyl-CoA synthetase, rather than CoA transferase, activates acetoacetate to acetoacetyl-CoA in the S. meliloti poly-3-hydroxybutyrate cycle. The second locus was identified as phbC, encoding poly-3-hydroxybutyrate synthase, and was found to be required for synthesis of poly-3-hydroxybutyrate deposits.     

Evidence for a role of somatostatin in lipid metabolism of liver and adipose tissue

We have studied the effects of somatostatin on lipid metabolism in liver and adipose tissue of fasted mice. The animals were injected subcutaneously with 8 micrograms somatostatin and killed 5 min after injection. In vivo incorporation of [14C]acetate into triglycerides in both tissues and into hepatic cholesterol was significantly enhanced by somatostatin. Concomitantly, a decrease of triglyceride lipase activity was observed, which corresponds well with the variation undergone by cyclic AMP-protein kinase system. In addition, a marked increase of serum cholesterol levels was observed. Additionally, in vitro experiments were also performed by employing 2.4 X 10(-6) M somatostatin. The results showed that the direct effect of somatostatin on liver seems to be a decrease in acetate uptake. The results obtained with the adipose tissue were similar to those obtained in in vivo conditions. On the other hand, when somatostatin was administered in vivo, the ability to incorporate ortho[32P]phosphate into phospholipids was enhanced in both tissues. Likewise in the in vitro experiments with [14C]acetate, the somatostatin seems to act by decreasing the ortho[32 P]phosphate uptake in liver. While in adipose tissue the somatostatin only caused a strong increase in the specific activity of phosphatidylcholine. These data demonstrate in fasted mice that somatostatin is able to counteract the lipolytic manifestations of the fasted state.     

The relative significance of acetate and glucose as precursors for lipid synthesis in liver and adipose tissue from ruminants

1. The incorporation of labelled glucose into lipid by liver slices from sheep and cows is considerably less than that by liver slices from the rat, although oxidation to carbon dioxide occurs to a similar extent. ATP citrate lyase and NADP malate dehydrogenase are inactive in both sheep and cow liver but active in rat liver. The absence of the citrate-cleavage pathway of lipogenesis in ruminant liver has been confirmed by the negligible amounts of C-3 of aspartate incorporated into fatty acids. 2. Considerable amounts of [(14)C]acetate are incorporated into fatty acids and non-saponifiable lipid in rat and ruminant liver. Acetyl-CoA synthetase, the initial enzyme in the metabolism of acetate, has a high activity in liver from rat and ruminants. 3. In adipose tissue from ruminants more acetate than glucose is converted into lipids, whereas the converse is true in rat adipose tissue. The greater incorporation of [(14)C]acetate into fatty acids in adipose tissue from the ruminant as compared with the non-ruminant may be caused, in part, by the higher activity of acetyl-CoA synthetase activity in the ruminant. 4. The results suggest that, in both liver and adipose tissue from ruminants, acetate is a more important source of lipid than glucose. 5. Two enzymes of the hexose monophosphate shunt, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, are active in both tissues and from the three species.     

The role of acetyl-CoA carboxylase phosphorylation in the control of mammary gland fatty acid synthesis during the starvation and re-feeding of lactating rats.

Activation of acetyl-CoA carboxylase during incubation of crude extracts of lactating rat mammary gland with Mg2+ and citrate can be blocked by NaF, suggesting that it represents a dephosphorylation of the enzyme. The greater extent of activation in extracts from 24 h-starved rats (200%) compared with fed controls (70%) implies that the decrease in acetyl-CoA carboxylase activity in response to 24 h starvation may involve increased phosphorylation of the enzyme. Acetyl-CoA carboxylase was purified from the mammary glands of lactating rats in the presence of protein phosphatase inhibitors by avidin-Sepharose chromatography. Starvation of the rats for 24 h increased the concentration of citrate giving half-maximal activation by 75%, and decreased the Vmax. of the purified enzyme by 73%. This was associated with an increase in the alkali-labile phosphate content from 3.3 +/- 0.2 to 4.5 +/- 0.4 mol/mol of enzyme subunit. Starvation of lactating rats for 6 h, or short-term insulin deficiency induced by streptozotocin injection, did not effect the kinetic parameters or the phosphate content of acetyl-CoA carboxylase purified from mammary glands. The effects of 24 h starvation on the kinetic parameters and phosphate content of the purified enzyme were completely reversed by re-feeding for only 2.5 h. This effect was blocked if the animals were injected with streptozotocin before re-feeding, suggesting that the increase in plasma insulin that occurs on re-feeding was responsible for the activation of the enzyme. The effects of re-feeding 24 h-starved rats on the kinetic parameters and phosphate content of acetyl-CoA carboxylase could be mimicked by treating enzyme purified from 24 h-starved rats with protein phosphatase-2A in vitro. Our results suggest that, in mammary glands of 24 h-starved lactating rats, insulin brings about a dephosphorylation of acetyl-CoA carboxylase in vivo, which may be at least partly responsible for the reactivation of mammary lipogenesis in response to re-feeding.     

Acute effects in vivo of anti-insulin serum on rates of fatty acid synthesis and activities of acetyl-coenzyme A carboxylase and pyruvate dehydrogenase in liver and epididymal adipose tissue of fed rats.

Plasma insulin concentrations in fed rats were altered acutely by administration of glucose or anti-insulin serum. Rates of fatty acid synthesis in adipose tissue and liver were estimated from the incorporation of 3H from 3H2O. In the adipose tissue dehydrogenase and acetyl-CoA carboxylase were evident. In liver, although changes in rates of fatty acid synthesis were found, the initial activity of pyruvate dehydrogenase did not alter, but small parallel changes in acetyl-CoA carboxylase activity were observed.     

Increased transcription and decreased degradation control and recovery of liver ribosomes after a period of protein starvation.

In the livers of 5-days-protein-depleted mice there is a decrease of 47% of the ribosome mass. When these animals are fed with an adequate diet, ribosome content is restored to the normal value after 1 day of re-feeding. The mechanisms underlying this phenomenon were studied. It was found that: (1) the activity of RNA polymerase I in the nuclei of livers from re-fed animals showed an enhancement of about 2-fold compared with the activity in normal and protein-depleted liver nuclei; (2) ribosome degradation, measured by the disappearance of radioactivity from ribosomal proteins previously labelled by the administration of NaH14CO3 to the mice, stopped during the first day after re-feeding.     

Studies on lipogenesis in vivo. Lipogenesis during extended periods of re-feeding after starvation

1. Lipogenesis was studied in mice re-fed for up to 21 days after starvation. At appropriate times [U-(14)]glucose was given by stomach tube and incorporation of (14)C into various lipid fractions measured. 2. In mice starved for 48hr. and then re-fed for 4 days with a diet containing 1% of corn oil, incorporation of (14)C from [U-(14)C]glucose into liver fatty acids and cholesterol was respectively threefold and eightfold higher than in controls fed ad libitum. The percentages by weight of fatty acids and cholesterol in the liver also increased and reached peaks after 7 days. Both the radioactivity and weights of the fractions returned to control values after 10-14 days' re-feeding. These changes could be diminished by re-feeding the mice with a diet containing 20% of corn oil. Incorporation of (14)C from [U-(14)C]glucose into extrahepatic fatty acids (excluding those of the epididymal fat pads) was not elevated during re-feeding with a diet containing either 1% or 20% of corn oil. However, incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads was increased in mice re-fed with either diet, as compared with non-starved controls. 3. Lipogenesis was also studied in mice alternately fed and starved. Mice given a diet containing 1% of corn oil for 6hr./day for 4 weeks lost weight initially and never attained the weight or carcass fat content of controls fed ad libitum. Incorporation of (14)C from dietary [U-(14)C]-glucose into the fatty acids of the epididymal fat pads was elevated threefold in the mice allowed limited access to food, although the incorporation into the remainder of the extrahepatic fatty acids was not different from that found for controls. Mice given a diet containing 20% of corn oil for 6hr./day adapted to the limited feeding regimen quicker and in 4 weeks did attain the weight and carcass fat content of controls. Incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads and the remainder of the extrahepatic fatty acids was respectively fivefold and threefold higher than in controls fed ad libitum. 4. The elevation in liver lipogenesis during re-feeding was greatest on a diet containing 1% of corn oil, whereas in extrahepatic tissues the increase in lipogenesis was greater when the mice were re-fed or were allowed limited access to a diet containing 20% of corn oil. These results suggest that the causes of the increased rate of incorporation of (14)C from [U-(14)C]glucose into fatty acids during re-feeding may be different in liver from that in extrahepatic tissues.     

Fasting-induced changes of tyrosine transaminase activity in rat tissues

Tyrosine transaminase activity in liver, kidney, intestine, stomach, skin, adipose tissue, striated muscle and brain in fed and 24-hour fasted rats, has been studied. Maximal activity has been found in liver, with only fractional activity in the other tissues. 24 hour fasting induced significant decrease in liver and adipose tissue activity, while no changes have been detected in the other tissues. The possible implications of these facts are discussed.     

Pyruvate dehydrogenase activities and rates of lipogenesis during the fed-to-starved transition in liver and brown adipose tissue of the rat.

The percentages of pyruvate dehydrogenase complex (PDH) in the active form (PDHa) in two lipogenic tissues (liver and brown adipose tissue) in the fed state were 12.0% and 13.4% respectively. After acute (0.5 h) insulin treatment, PDHa activities had increased by 77% in liver and by 234% in brown fat. Significant decreases in PDHa activities were observed in both tissues by 5 h after the removal of food. The patterns of decline in PDHa activities in the two lipogenic tissues were similar in that the major decreases in activities were observed within the first 7 h of starvation. The significant decreases in PDHa activities observed after starvation for 6 h were accompanied by decreased rates of lipogenesis. Hepatic and brown-fat PDHa activities after acute (30 min) exposure to exogenous insulin were less in 6 h-starved than in fed rats, but the absolute increases in PDHa activities over the 30 min exposure period were similar in fed and 6 h-starved rats. Increases in PDHa activities were paralleled by increases in lipid synthesis in both tissues. Re-activation of PDH in response to insulin treatment or chow re-feeding after 48 h starvation occurred more rapidly in brown adipose tissue than in liver. The results are discussed in relation to the importance of the activity of the PDH complex as a determinant of the total rate of lipogenesis during the fed-to-starved transition and after insulin challenge or re-feeding.     

The effect of 3-mercaptopicolinic acid on phosphoenolpyruvate carboxykinase (GTP) in the rat and guinea pig.

In vitro, 3-mercaptopicolinic acid inhibited phosphoenolpyruvate carboxykinase activity in supernatant fractions of liver, kidney cortex, and adipose tissue obtained from fasted rats. 3-Mercaptopicolinic acid also inhibited enzymatic activity in the mitochondrial and supernatant fractions of liver obtained from fasted guinea pigs. In the fasted rat, the oral administration of 3-mercaptopicolinic acid increased liver carboxykinase activity even though the blood glucose concentrations decreased. Kidney cortex carboxykinase decreased while adipose tissue enzyme was unchanged. In the fasted guinea pig, the oral administration of 3-mercaptopicolinic acid lowered blood glucose concentrations but had no effect on liver mitochondrial or supernatant carboxykinase activity. The elevation in rat liver enzymatic activity appears to be due to protein synthesis, since the concurrent administration of cycloheximide prevents the increase in enzyme activity. 3-Mercaptopicolinic acid appears to be noncompetitive with respect to Mn²⁺.