Induction of Hepatic Enzyme Synthesis in Vivo by Adenosine 3',5'-Monophosphate

Hepatic adaptations to maintain metabolic homeostasis in response to fasting and refeeding in mice

Background

The increased incidence of obesity and associated metabolic diseases has driven research focused on genetically or pharmacologically alleviating metabolic dysfunction. These studies employ a range of fasting-refeeding models including 4–24 h fasts, “overnight” fasts, or meal feeding. Still, we lack literature that describes the physiologically relevant adaptations that accompany changes in the duration of fasting and re-feeding. Since the liver is central to whole body metabolic homeostasis, we investigated the timing of the fast-induced shift toward glycogenolysis, gluconeogenesis, and ketogenesis and the meal-induced switch toward glycogenesis and away from ketogenesis.

Methods

Twelve to fourteen week old male C57BL/6J mice were fasted for 0, 4, 8, 12, or 16 h and sacrificed 4 h after lights on. In a second study, designed to understand the response to a meal, we gave fasted mice access to feed for 1 or 2 h before sacrifice. We analyzed the data using mixed model analysis of variance.

Results

Fasting initiated robust metabolic shifts, evidenced by changes in serum glucose, non-esterified fatty acids (NEFAs), triacylglycerol, and β-OH butyrate, as well as, liver triacylglycerol, non-esterified fatty acid, and glycogen content. Glycogenolysis is the primary source to maintain serum glucose during the first 8 h of fasting, while de novo gluconeogenesis is the primary source thereafter. The increase in serum β-OH butyrate results from increased enzymatic capacity for fatty acid flux through β-oxidation and shunting of acetyl-CoA toward ketone body synthesis (increased CPT1 (Carnitine Palmitoyltransferase 1) and HMGCS2 (3-Hydroxy-3-Methylglutaryl-CoA Synthase 2) expression, respectively). In opposition to the relatively slow metabolic adaptation to fasting, feeding of a meal results in rapid metabolic changes including full depression of serum β-OH butyrate and NEFAs within an hour.

Conclusions

Herein, we provide a detailed description of timing of the metabolic adaptations in response to fasting and re-feeding to inform study design in experiments of metabolic homeostasis. Since fasting and obesity are both characterized by elevated adipose tissue lipolysis, hepatic lipid accumulation, ketogenesis, and gluconeogenesis, understanding the drivers behind the metabolic shift from the fasted to the fed state may provide targets to limit aberrant gluconeogenesis and ketogenesis in obesity.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Contrasting effects of acute and chronic ethanol administration on rat liver tyrosine aminotransferase

We compared the effects of acute and chronic ethanol administration on the activity and synthesis of tyrosine aminotransferase (TAT) in rat liver. In acute experiments, chow-fed rats received a single dose of either ethanol (6 g/kg body wt.) or saline. In chronic studies, rats were pair-fed liquid diets containing either ethanol (36 % of calories) or isocaloric maltose-dextrin for 6-8 weeks. In rats acutely fed ethanol, the relative rate of TAT synthesis was more than twofold higher than in saline-treated controls. In rats subjected to chronic ethanol administration, both the TAT activity and synthesis rate were the same as in pair-fed controls, but both these parameters in the two groups were equal to those in animals given acute ethanol acutely. These findings indicate that whereas acute ethanol administration was associated with a stimulation of TAT synthesis, long-term ethanol administration was not. The data suggest that ethanol itself does not directly induce TAT. Rather, enzyme synthesis is regulated by one or more endogenous secondary effector(s) whose production is influenced differently by acute or chronic ethanol feeding.

The increase in hepatic tyrosine aminotransferase activity by ethanol administration involves an acceleration of enzyme synthesis

The present study was conducted to examine the nature of the increase in tyrosine aminotransferase (TAT) activity by acute ethanol administration. A significant rise in aminotransferase activity was observed as early as 1 hr after intact rats were gavaged with ethanol. Ethanol administration also increased TAT activity in adrenalectomized rats. Inhibition of ethanol metabolism by pyrazole administration had no effect on the ethanol-induced increase in TAT activity. Immunochemical analyses revealed that the enhancement of TAT activity in ethanol-fed rats correlated with an increase in aminotransferase protein. Measurement of the rate of TAT synthesis showed that in ethanol-fed rats, [3H]leucine was incorporated into the aminotransferase protein at a higher rate than in controls by a factor which was similar to the enhancement in enzyme activity. Our findings indicate that an acceleration of TAT synthesis fully accounts for the increase in TAT activity during the early stage of enzyme induction. TAT induction by ethanol administration is not dependent upon an increase in adrenal corticosteroid production, nor does it require ethanol metabolism.

Protection of tyrosine aminotransferase against proteolytic digestion by nucleotide derivatives

The abilities of several nucleotides to protect tyrosine aminotransferase (L-tyrosine: 2-oxoglutarate aminotransferase, EC 2.6.1.5) against proteolytic inactivation in vitro have been examined as part of an ongoing investigation of the role of cyclic GMP in the intracellular degradation of the hepatic enzyme. Although neither cyclic GMP nor cyclic AMP was found to exert such a protective effect, certain nucleotide analogs were observed to inhibit the inactivation of tyrosine aminotransferase by trypsin and chymotrypsin. The nucleotides which conferred the strongest protection were the dibutyryl derivatives of cyclic GMP and cyclic AMP. This phenomenon appears to require a purine nucleotide with hydrophobic substituent(s), while the cyclic phosphate is not essential. The nucleotides probably act by direct interaction with tyrosine aminotransferase as indicated by changes in kinetic properties and heat stability of the enzyme and by their failure to inhibit trypsin when other protein substrates, including another aminotransferase, were used. Dibutyryl cyclic AMP was shown to block the appearance of a characteristic 43 kDa tryptic cleavage product of tyrosine aminotransferase but not the conversion of the native 54 kDa form to a size of 50 kDa. Arguments are presented against the involvement of the protective effect in the actions of dibutyryl cyclic nucleotides on tyrosine aminotransferase in cells.

Gene-specific acquisition of hormonal responsiveness in rat liver during development

Cloned cDNAs were used in hybridization analyses to assess hormonal responsiveness of two similarly regulated genes in livers of late-term fetal rats. Transcription of the tyrosine aminotransferase gene and of gene 33 (Lee et al.: J Biol Chem 260:16433-16438, 1985) is enhanced by glucocorticoids and by each of the usually antagonistic hormonal agents, insulin and cAMP, in adult liver, and that of both genes is developmentally activated at or just prior to birth. The mRNA of gene 33 was found to be significantly increased by each of the hormonal regulators in livers of fetuses treated in utero. Expression of the nearly silent aminotransferase gene in fetal liver was appreciably increased by cAMP but was refractory to control by either glucocorticoids or insulin; capacity of this gene to respond to insulin was not realized until several days postpartum. The data indicate specificity in the developmental acquisition of the capacity of individual genes to respond to hormonal regulators.

Mechanisms of hormonal regulation of hepatic glucose metabolism

Acute hormonal regulation of liver carbohydrate metabolism mainly involves changes in the cytosolic levels of cAMP and Ca2+. Epinephrine, acting through beta 2-adrenergic receptors, and glucagon activate adenylate cyclase in the liver plasma membrane through a mechanism involving a guanine nucleotide-binding protein that is stimulatory to the enzyme. The resulting accumulation of cAMP leads to activation of cAMP-dependent protein kinase, which, in turn, phosphorylates many intracellular enzymes involved in the regulation of glycogen metabolism, gluconeogenesis, and glycolysis. These are (1) phosphorylase b kinase, which is activated and, in turn, phosphorylates and activates phosphorylase, the rate-limiting enzyme for glycogen breakdown; (2) glycogen synthase, which is inactivated and is rate-controlling for glycogen synthesis; (3) pyruvate kinase, which is inactivated and is an important regulatory enzyme for glycolysis; and (4) the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme, phosphorylation of which leads to decreased formation of fructose 2,6-P2, which is an activator of 6-phosphofructo-1-kinase and an inhibitor of fructose 1,6-bisphosphatase, both of which are important regulatory enzymes for glycolysis and gluconeogenesis. In addition to rapid effects of glucagon and beta-adrenergic agonists to increase hepatic glucose output by stimulating glycogenolysis and gluconeogenesis and inhibiting glycogen synthesis and glycolysis, these agents produce longer-term stimulatory effects on gluconeogenesis through altered synthesis of certain enzymes of gluconeogenesis/glycolysis and amino acid metabolism. For example, P-enolpyruvate carboxykinase is induced through an effect at the level of transcription mediated by cAMP-dependent protein kinase. Tyrosine amino-transferase, serine dehydratase, tryptophan oxygenase, and glucokinase are also regulated by cAMP, in part at the level of specific messenger RNA synthesis. The sympathetic nervous system and its neurohumoral agonists epinephrine and norepinephrine also rapidly alter hepatic glycogen metabolism and gluconeogenesis acting through alpha 1-adrenergic receptors. The primary response to these agonists is the phosphodiesterase-mediated breakdown of the plasma membrane polyphosphoinositide phosphatidylinositol 4,5-P2 to inositol 1,4,5-P3 and 1,2-diacylglycerol. This involves a guanine nucleotide-binding protein that is different from those involved in the regulation of adenylate cyclase. Inositol 1,4,5-P3 acts as an intracellular messenger for Ca2+ mobilization by releasing Ca2+ from the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin enhances transcription of the tyrosine aminotransferase gene in rat liver

The mechanism of insulin-mediated induction of tyrosine aminotransferase in rat liver was investigated using a cloned cDNA probe. The level of aminotransferase mRNA increases about fourfold following administration of the hormone. This induced mRNA accumulation does not require de novo protein synthesis. Nuclear runoff transcription assays in isolated liver nuclei demonstrate that insulin has a rapid and time-dependent stimulatory effect on aminotransferase gene transcription. The magnitude of enhanced transcription can fully account for the increase in the mRNA. We conclude that the induction of tyrosine aminotransferase in rat liver by insulin is primarily a consequence of a selective increase in the rate of transcription of the aminotransferase gene.

Changes in hepatic levels of tyrosine aminotransferase messenger RNA during chemical hepatocarcinogenesis

The activity of tyrosine aminotransferase (TAT) decreased biphasically in livers of rats fed 3'-methyl-4-dimethylaminoazobenzene (3'-MDAB). TAT activity decreased to an extremely low level at later stages of hepatocarcinogenesis. The activity of TAT is negatively correlated with alpha-fetoprotein (AFP) levels. The level of TAT enzyme activity in precancerous liver and hepatoma is a reflection of the amount of TAT mRNA. Dexamethasone increased the TAT enzyme activity and TAT mRNA concentration in rat livers during chemical carcinogenesis.

Enzymes of serine metabolism in normal, developing and neoplastic rat tissues

The cellular pattern of serine metabolism was conceptualized into four main areas of metabolic sequences: the biosynthesis of serine from intermediates of the glycolytic pathway (the so-called "phosphorylated pathway"); and alternative pathways of serine utilization initiated by serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. The known regulatory and adaptive properties of the enzymes involved in these pathways were reviewed in detail and key enzymes associated with each pathway (phosphoserine aminotransferase, serine dehydratase, serine aminotransferase, and serine hydroxymethyltransferase, respectively) were selected for further investigation. Tissue distribution studies in the rat revealed that whereas serine dehydratase and serine aminotransferase activities were largely confined to the liver, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were more broadly distributed. In particular in tissues with a high rate of cell turnover, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were coordinately increased. An increase in serine hydroxymethyltransferase activity coincided temporally with the incorporation of [3-14C]serine and thymidine into DNA in normal human lymphocytes during proliferation after mitogenic stimulation by phytohemagglutinin. The evidence suggested a primarily gluconeogenic role for serine dehydratase and serine aminotransferase. Serine hydroxymethyltransferase has a role in providing glycine and one-carbon folate co-factors as precursors for nucleotide biosynthesis and in some situations serves to metabolically couple the pathway of serine biosynthesis to utilization for de novo purine and pyrimidine synthesis. Multiple enzymic forms were distinguished for serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. For serine dehydratase the two cytosolic multiple forms had no apparent functional significance; the multiple forms were catalytically unmodified by conditions promoting phosphorylation-dephosphorylation in vitro. The mitochondrial form of serine aminotransferase showed adaptive responses in gluconeogenic situations, and the hypothesis was proposed that the mitochondrial isoenzyme of serine hydroxymethyltransferase is associated together with serine aminotransferase in a pathway for gluconeogenesis from protein-derived amino acids such as glycine and hydroxyproline. The adaptive behaviour of the enzymes during the neonatal development of rat liver revealed that serine aminotransferase reached a peak in the mid-suckling period at a time when gluconeogenesis is known to be increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin inhibition of tyrosine aminotransferase induction by dexamethasone is species-related

Dexamethasone induces TAT in the embryonic chick liver, but this effect is not blocked by prior administration of insulin at a dose sufficient to increase the hepatic protein/DNA ratio. Similarly, insulin does not block dexamethasone induction of TAT in chick embryo hepatocytes in vitro. In contrast, insulin reduces TAT induction by dexamethasone in fetal rat hepatocytes by 35-40%. No significant differences in insulin binding were noted between chick and rat hepatocytes.

Possible involvement of cGMP in the control of tyrosine aminotransferase degradation in rat hepatocytes

Addition of theophylline to primary cultures of rat hepatocytes in which tyrosine aminotransferase had been preinduced with dexamethasone caused a further increase in specific activity of the enzyme. This increase was due in part to a reduction in the rate of tyrosine aminotransferase degradation that began about 2 hr after theophylline was added. The level of cGMP also increased with a similar time lag following the addition of theophylline. The concentration of theophylline which produced the above effects (1 mM) did not alter the rate of general protein degradation in hepatocytes. Addition of 8-bromo-cGMP (0.5 mM) resulted in an immediate reduction in the rate of tyrosine aminotransferase degradation and in an increase in the activity of the enzyme. Treating hepatocytes with MnCl2 (0.9 mM) caused an elevation of cGMP and a concomitant slowing of tyrosine aminotransferase degradation without changing the level of cAMP significantly. These results suggest an inverse relationship between the level of cGMP and the rate of tyrosine aminotransferase degradation in hepatocytes.

Effect of glucagon on alanine 2-oxoglutarate aminotransferase

Alanine-2-oxoglutarate aminotransferase activity in mouse liver is stimulated by the intravenous injection of glucagon. The stimulation is abolished by pretreatment with actinomycin D indicating that the increased activity is probably due to new enzyme formation. Administration of dibutyryl cyclic AMP, isoproterenol, an activator of adenyl cyclase and theophylline, an inhibitor of phosphodiesterase also increases the enzyme activity suggesting the involvement of cyclic AMP in glucagon-mediated increase of enzyme activity.

Glycerol-3-phosphate dehydrogenase is induced by glucocorticoids in hepatocytes and hepatoma cells in vitro

Previous studies have shown that cytosolic glycerol-3-phosphate dehydrogenase (GPDH; EC 1.1.1.8) can be induced by glucocorticoids in mammalian brain, mammary gland, and thymus, but it was thought that no induction occurred in liver. We report here that GPDH is induced by glucocorticoids in several lines of hepatoma cells and in rat hepatocytes cultured in vitro. When rat hepatoma cells of clone FU5AH were exposed to 3 microM hydrocortisone (HC) for 3 days, GPDH specific activity increased greater than sixfold over control. The rate and extent of induction were similar in exponentially growing and stationary-phase cultures of cells. Four other hepatoma cell lines were inducible to a lesser extent, and three lines were not inducible. GPDH was also induced by glucocorticoids in cultures of hepatocytes isolated from livers of 6-day-old rats. The enzyme was induced three-to fourfold by the synthetic glucocorticoid, dexamethasone, in the presence of 1 nM insulin, but the induction was not observed in the absence of insulin.

Factors affecting induction of tyrosine aminotransferase in isolated rat liver cells

1. The response of tyrosine aminotransferase activity in isolated liver cells has been studied under several conditions. 2. Activity is increased over a 5 h period by both glucagon and glucocorticoids in cells from adrenalectomized rats. The results do not support the view that glucagon action is dependent on preexposure of cells to steroid. 3. In cells from fed animals, significant stimulation is seen only when both glucagon and steroid are present together. 4. In cells from 48 h fasted rats steroid is effective, but glucagon is not significant so. 5. These anomalies are attributed to the differences in hormonal and nutritional status between the animals from which the cells are isolated.

Premature appearance of hepatic phosphoenolpyruvate carboxykinase in fetal rats, not mediated by adenosine 3':5'-monophosphate

Injection of streptozotocin in utero to fetuses elicited a premature appearance of cytosolic hepatic activity of phosphoenol pyruvate carboxykinase. This was due to a precocious initiation of the synthesis of the enzyme. The streptozotocin-induced appearance of enzyme activity was not mediated by adenosine 3':5'-monophosphate since the concentration of the cyclic nucleotide in the liver was unaffected by the antibiotic, the administration of dibutyryladenosine 3':5'-monophosphate to streptozotocin-treated fetuses elicited an additive increase in enzyme activity, and insulin administration in utero repressed the streptozotocin effect while the effect due to dibutyryladenosine 3':5'-monophosphate was not inhibited by simultaneous insulin injection. Streptozotocin treatment also caused a small but consistent retardation of fetal growth and a marked reduction of liver wet weight. Histological analysis of the liver demonstrated a premature loss of some hematopoietic elements, while hepatocytes appeared normal. Hepatic protein synthesis was unaffected by the streptozotocin treatment. Streptozotocin treatment had no effect on fetal renal phosphoenol pyruvate carboxykinase activity or kidney wet weight.

Multihormonal control of tyrosine aminotransferase in isolated liver cells

The regulation of tyrosine aminotransferase activity by glucocorticoids and cyclic AMP was investigated in isolated liver parenchymal cell suspensions. The induction and maintenance of elevated levels of tyrosine aminotransferase activity in liver cells were completely dependent upon the presence of both the synthetic glucocorticoid, dexamethasone, and glucagon of dibutyryl cyclic AMP. No induction was observed when any of these compounds were tested alone. Immunotitration experiments revealed that the 6- to 7-fold increase in tyrosine aminotransferase activity following the addition of dexamethasone and glucagon was accompanied by a parallel increase in the amount of immunologically reactive enzyme protein. Pulse-labeling experiments established that this increase in enzyme protein could be fully accounted for by a corresponding increase in rate of synthesis of tyrosine aminotransferase. Neither hormone had any effect on the rate of degradation of the enzyme. The increase in tyrosine aminotransferase synthesis evoked by the presence of both hormones was blocked by the addition of actinomycin D or cycloheximide to the medium, demonstrating that RNA and protein synthesis were required for the induction of enzyme activity. By varying the time and order of addition of the inducers and inhibitions, evidence was obtained that the hormones act sequentially. The steroid hormone acts first, presumably to increase the level of functional tyrosine aminotransferase mRNA or its precursor. The processing of this precursor to a translatable form or the specific translation of tyrosine aminotransferase mRNA is apparently dependent upon a specific cyclic AMP-controlled process. In vivo experiments demonstrated that both glucocorticoids and cyclic AMP increase the level of functional tyrosine aminotransferase mRNA in the liver. The actions of the steroid hormone and cyclic nucleotide were blocked by alpha amanitin, establishing the requirement for ongoing gene transcription. The protein synthesis inhibitors, cycloheximide, emetine, and puromycin, were as effective as cyclic AMP in increasing tyrosine aminotransferase mRNA activity. The action of these inhibitors is probably related to their ability to elevate hepatic intracellular cyclic AMP levels, thus mimicking cyclic AMP administration. Extension of these in vivo studies to isolated liver cells will provide a valuable system for investigating the regulation of gene expression by glucocorticoids and cyclic AMP.

The role of glucocorticoid hormones as biological amplifiers

Recent research in hormone action has been aimed at studying single effects in well-defined systems. As exemplified in several chapters of this book, it has been possible to deduce a general mechanism of action of the glucocorticoids using this approach. Most hormones, and the glucocorticoids in particular, do not act as independent agents in the intact animal. Although the best known example of how glucocorticoids interact with other hormones is the amplification of the effect of those whose action is mediated by cAMP, these steroids also augment the effects of a variety of other hormones and effectors. Such interactions are of interest in clinical medicine as well, since glucocorticoid hormones are used in combination with other drugs in a number of conditions, including the treatment of asthma, allergies, and certain kinds of shock and cancer. Neither the biochemical nor the pharmacologic basis for the effects of the glucocorticoids is known. In some cases the actions of other hormones are not observed unless the tissue has first been exposed to glucocorticoids. In these instances the glucocorticoids are said to exert a "permissive effect," since they allow a process to proceed at a maximal rate even though the steroid itself has no effect on this process. There is no doubt that such examples exist, as documented above: thus the concept of a "permissive effect" does have utility. The term fails to describe the more general role the glucocorticoids play, since in many instances the steroid also has a direct effect on the process itself, or optimizes a process in which the primary effector is not as yet known. Because of these cases, and because the historically more general usage first proposed by INGLE [1] seems to have been forgotten, use of the term "permissive effect" has been avoided in this chapter. An ultimate goal in glucocorticoid hormone research is to identify the mechanisms involved in the amplification effect these hormones exert. Now that the actions of these hormones and of the hormones they interact with are being defined, such work is within the realm of feasibility.

Adaptive responses of enzymes of carbohydrate and lipid metabolism to dietary alteration in genetically obese Zucker rats (fa/fa)

1. Measurements have been made of the activities of enzymes of the glycolytic route, the pentose phosphate pathway, the tricarboxylic acid cycle and lipogenesis in liver and adipose tissue from genetically obese (fa/fa) rats and their lean litter mates (fa/ --). The effect of food restriction for a period of three weeks on the enzyme profile of liver and adipose tissue of the obese rat was also studied. 2. The most striking increases in enzyme activity in livers from obese rats were: (a) among enzymes of lipogenesis; ATP-citrate lyase, acetyl-CoA carboxylase, fatty acid synthetase, malate dehydrogenase (decarboxylating) and cytoplasmic glycerolphosphate dehydrogenase; (b) within the pentose phosphate pathway; glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase; (c) within the glycolytic pathway; glucokinase, pyruvate kinase and lactate dehydrogenase. All of these enzymes showed a significant increase in activity on the basis of U/g liver and U/mg DNA. In adipose tissue all the enzymes of lipogenesis, of the glycolytic route, of the oxidative segment of the pentose phosphate pathway and of the tricarboxylic acid cycle were increased when expressed as U/2 fat pads or as U/mg DNA. 3. The restriction of the food intake of obese rats to that consumed by their lean litter mates for periods of three weeks did not produce the expected adaptive decrease in enzymes of lipogenesis; in adipose tissue, only ATP-citrate lyase and malate dehydrogenase (decarboxylating) showed a marked decrease; no significant change was found in adipose tissue or liver of the activities of acetyl-CoA carboxylase and fatty acid synthetase, when expressed on a cell basis (U/mg DNA). The non-oxidative enzymes of the pentose phosphate pathway and enzymes involved in glycerogenesis (pyruvate carboxylase, malate dehydrogenase and phosphoenolpyruvate carboxykinase) all increased in adipose tissue from limit-fed obese rats. 4. The rate of conversion of specifically labelled glucose to (14C)O2 and 14C-labelled lipid by pieces of adipose tissue and by liver slices was also measured. Insulin caused an increase in the conversion of (1-14C)glucose to (14C)O2 and 14C-labelled lipid in obese rats fed ad libitum, limit-fed rats and in their lean litter mates. 5. The results are discussed in relation to the raised insulin and hypothyroid state of the obese rat. The effect of this altered hormonal status on the activity of cyclic nucleotide phosphodiesterases and cellular levels of adenosine 3' :5'-monophosphate and guanosine 3' :5'-monophosphate and guanosine 3' :5'-monophosphate in relation to the obese syndrome is considered.