Cytoplasmic NADP-linked dehydrogenase activities in avian tissues

Cytoplasmic activities of NADP-linked malic enzyme (E.C. 1.1.1.40), glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49) and NADP-linked isocitrate dehydrogenase (E.C. 1.1.1.42) were determined in tissues of selected avian species, and compared with those in mammals. Malic enzyme was generally more active in avian liver and kidney than in the corresponding mammalian tissues. Hepatic activities as high as 200 units/g wet wt and 100 units/g wet wt were recorded in the Nectariniidae and the Ploceidae respectively. Glucose-6-phosphate dehydrogenase was generally less active in avian tissues than malic enzyme. In passerine birds activities were very low indeed, and in most cases spectrophotometrically undetectable. Malic enzyme and glucose-6-phosphate dehydrogenase were highly active in the adipose tissue of mammals but were inactive in the adipose tissue of birds. Marked increases in hepatic malic enzyme and glucose-6-phosphate dehydrogenase activities were associated in birds with premigratory fattening. Activities of isocitrate dehydrogenase were comparable in the corresponding avian and mammalian tissues, including adipose tissue.     

Dietary induction of hepatic glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme in lean and obese female Zucker rats

Responses of the hepatic lipogenic enzymes, glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME) to starvation refeeding and diet shifting were determined in lean and obese female Zucker rats. Rats were either fed nonpurified diet, starved 48 hr, and then refed nonpurified diet or one of the refined carbohydrate diets containing either glucose, fructose, cornstarch, or sucrose for 72 hr, or shifted from nonpurified diet directly to one of the refined carbohydrate diets for 72 hr. Initial activities were greater in obese than lean rats for all three enzymes studied. Similar to other strains of female rats, lean Zucker rats failed to demonstrate a starve-refeed response when refed nonpurified diet. Obese female littermates showed a statistically significant increase in enzymes when refed a nonpurified diet. Both lean and obese female Zucker rats demonstrated increases in enzyme activities above controls when starved and refed any of the refined carbohydrate diets. The greatest responses were observed when female rats were starved and refed sucrose; activities increased 2.6- to 3.5-fold in lean and 3.0- to 4.3-fold in obese Zuckers. In lean females 50-70% of the starve-refeed response observed with G6PDH and ME can be accounted for by simply shifting from a nonpurified diet to the respective refined carbohydrate diet, whereas in obese females only 33-55% of the increase could be attributed to diet shifting. Plasma testosterone/estrogen ratios were consistently 1.5 times higher in obese than in lean female rats. This phenotypic difference may potentiate the heightened starve-refeed overshoot response observed in obese rats.     

Dietary protein and the control of fatty acid synthesis in rat adipose tissue

Fatty acid synthesis in adipose tissue normally proceeds at a high rate when fasted animals are refed a diet containing carbohydrate, protein, and low levels of fat. This study investigated the effect of omitting protein from the refeeding diet. Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured. Refeeding the animals a diet devoid of protein resulted in a low rate of fatty acid synthesis from each of these substrates as well as a reduction in carbon flow over the citrate cleavage pathway. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP-malate dehydrogenase, and ATP-citrate lyase were also reduced in epididymal fat pads from these rats. On the other hand, adipose tissue phosphoenolpyruvate carboxykinase activity was five times as great as that in tissue from animals refed a balanced diet. This difference could be eliminated if actinomycin D was injected coincident with refeeding. Refeeding rats diets high in carbohydrate is not, therefore, capable of inducing high rates of fatty acid synthesis in adipose tissue in the absence of dietary proteins. Thus, liver and adipose tissue respond differently to dietary protein.     

Enzymatic activities related to intermediary metabolism of glucose in circulating mononuclear cells from obese humans: relationship to enzyme activity in adipose tissue

In order to assess whether enzyme activities of glucose metabolism measured in mononuclear blood cells reflect those in a typical insulin target tissue, we studied hexokinase, 6-phosphofructokinase, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase activities in lymphomonocytes and in hypogastric adipose tissue from 15 nondiabetic obese women. Statistically significant relationships were found in the activities of hexokinase (r = 0.53, p less than 0.05), 6-phosphofructokinase (r = 0.85, p less than 0.01), and 6-phosphogluconate dehydrogenase (r = 0.72, p less than 0.01) between the two tissues. These results suggest that mononuclear blood cells may be suitable as a model for studying cytosolic key enzymes involved in the glucose metabolism of humans.     

Regulation of glucose-6-phosphate dehydrogenase activity during caloric restriction in human adipose tissue

Glucose-6-phosphate dehydrogenase activity was significantly lower in adipose tissue of human subjects after 7 days of severe caloric restriction on low-carbohydrate diets and had returned to normal values 4 days after the subjects resumed normal diets. Three other enzyme activities (malate-NADP dehydrogenase, oxaloacetate decarboxylating; ATP-citrate lyase and 6-phosphofructokinase) were not significantly affected by these dietary changes. These results are consistent with separate control of glucose-6-phosphate dehydrogenase activity versus other 'lipogenic' enzyme activities in human adipose tissue.     

Regulation of carbohydrate metabolism by indole-3-carbinol and its metabolite 3,3′-diindolylmethane in high-fat diet-induced C57BL/6J mice

Indole glucosinolates, present in cruciferous vegetables have been investigated for their putative pharmacological properties. The current study was designed to analyse whether the treatment of the indole glucosinolates—indole-3-carbinol (I3C) and its metabolite 3,3′-diindolylmethane (DIM) could alter the carbohydrate metabolism in high-fat diet (HFD)-induced C57BL/6J mice. The plasma glucose, insulin, haemoglobin (Hb), glycosylated haemoglobin (HbA1c), glycogen and the activities of glycolytic enzyme (hexokinase), hepatic shunt enzyme (glucose-6-phosphate dehydrogenase), gluconeogenic enzymes (glucose-6-phosphatase and fructose-1,6-bisphosphatase) were analysed in liver and kidney of the treated and HFD mice. Histopathological examination of liver and pancreases were also carried out. The HFD mice show increased glucose, insulin and HbA1c and decreased Hb and glycogen levels. The elevated activity of glucose-6-phosphatase and fructose-1,6-bisphosphatase and subsequent decline in the activity of glucokinase and glucose-6-phosphate dehydrogenase were seen in HFD mice. Among treatment groups, the mice administered with I3C and DIM, DIM shows decreased glucose, insulin and HbA1c and increased Hb and glycogen content in liver when compared to I3C, which was comparable with the standard drug metformin. The similar result was also obtained in case of carbohydrate metabolism enzymes; treatment with DIM positively regulates carbohydrate metabolic enzymes by inducing the activity of glucokinase and glucose-6-phosphate dehydrogenase and suppressing the activity of glucose-6-phosphatase and fructose-1,6-bisphosphatase when compared to I3C, which were also supported by our histopathological observations.     

Lipogenesis in the liver and adipose tissue of the cardiomyopathic hamster.

The activities of fatty acid synthetase, citrate cleavage enzyme, glucose-6-phosphate dehydrogenase and malic enzyme were measured in the liver and adipose tissue of cardiomyopathic and normal hamsters at age 33, 68 and 108 days. There was no difference in the activity of hepatic fatty acid synthetase between the diseased animals and the controls at any stage in their development. The activity of glucose-6-phosphate dehydrogenase was not different until age 108 days where it was significantly elevated in the BIO 82.62 strain. Citrate cleavage enzyme in the liver was depressed at all stages in the diseased animals as was malic enzyme. In adipose tissue, all enzyme activities were significantly depressed in the cardiomyopathic animals at the three stages. These data suggest that lipogenesis was depressed in the cardiomyopathic hamster.     

Activity of enzymes of carbon metabolism during the induction of Crassulacean acid metabolism in Mesembryanthemum crystallinum L.

The maximum extractable activities of twenty-one photosynthetic and glycolytic enzymes were measured in mature leaves of Mesembryanthemum crystallinum plants, grown under a 12 h light 12 h dark photoperiod, exhibiting photosynthetic characteristics of either a C₃ or a Crassulacean acid metabolism (CAM) plant. Following the change from C₃ photosynthesis to CAM in response to an increase in the salinity of in the rooting medium from 100 mM to 400 mM NaCl, the activity of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) increased about 45-fold and the activities of NADP malic enzyme (EC 1.1.1.40) and NAD malic enzyme (EC 1.1.1.38) increased about 4- to 10-fold. Pyruvate, Pi dikinase (EC 2.7.9.1) was not detected in the non-CAM tissue but was present in the CAM tissue; PEP carboxykinase (EC 4.1.1.32) was detected in neither tissue. The induction of CAM was also accompanied by large increases in the activities of the glycolytic enzymes enolase (EC 4.2.1.11), phosphoglyceromutase (EC 2.7.5.3), phosphoglycerate kinase (EC 2.7.2.3), NAD glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12), and glucosephosphate isomerase (EC 2.6.1.2). There were 1.5- to 2-fold increases in the activities of NAD malate dehydrogenase (EC 1.1.1.37), alanine and aspartate aminotransferases (EC 2.6.1.2 and 2.6.1.1 respectively) and NADP glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). The activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39), fructose-1,6-bisphosphatase (EC 3.1.3.11), phosphofructokinase (EC 2.7.1.11), hexokinase (EC 2.7.1.2) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) remained relatively constant. NADP malate dehydrogenase (EC 1.1.1.82) activity exhibited two pH optima in the non-CAM tissue, one at pH 6.0 and a second at pH 8.0. The activity at pH 8.0 increased as CAM was induced. With the exceptions of hexokinase and glucose-6-phosphate dehydrogenase, the activities of all enzymes examined in extracts from M. crystallinum exhibiting CAM were equal to, or greater than, those required to sustain the maximum rates of carbon flow during acidification and deacidification observed in vivo. There was no day-night variation in the maximum extractable activities of phosphoenolpyruvate carboxylase, NADP malic enzyme, NAD malic enzyme, fructose-1,6-bisphosphatase and NADP malate dehydrogenase in leaves of M. crystallinum undergoing CAM.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate     

Regulation of malic enzyme and mitochondrial alpha-glycerophosphate dehydrogenase by thyroid hormones, insulin, and glucocorticoids in cultured hepatocytes

Hepatocytes isolated from normal adult rats were cultured under serum-free conditions. Induction of mitochondrial alpha-glycerophosphate dehydrogenase (glycerol 3-phosphate dehydrogenase) (EC 1.1.99.5; sn-glycerol-3-phosphate: (acceptor) oxidoreductase) and cytosolic malic enzyme (EC 1.1.1.40; L-malate-NADP+ oxidoreductase (decarboxylating)) by 3,3'-5-triiodo-L-thyronine (triiodothyronine) in the culture medium follows the same time course as the in vivo response to thyroid hormones. The addition of 1 microM cycloheximide blocks the triiodothyronine response. Thyroxine is also capable of stimulating the activities of both enzymes. Although increases in alpha-glycerophosphate dehydrogenase and malic enzyme activities are observed when triiodothyronine is added to the culture medium for 3 days (62% and 36%, respectively), in the presence of insulin and cortisol the response is significantly greater. Dexamethasone is more potent than cortisol in increasing triiodothyronine action. In the presence of bovine serum albumin, to prevent metabolism of triiodothyronine, hepatocytes show increased enzyme activity at concentrations as low as 10(-10) M triiodothyronine.     

Enhanced glycerol 3-phosphate dehydrogenase activity in adipose tissue of obese humans

The primary purpose of this investigation was to determine whether adipose tissue glycerol 3-phosphate dehydrogenase activity is associated with human obesity. The data presented in this paper indicate that the glycerol 3-phosphate dehydrogenase activity in adipose tissue from morbidly obese subjects is approximately 2-fold higher than from lean individuals. Moreover, positive correlation between adipose tissue glycerol 3-phosphate dehydrogenase activity and body mass index (BMI) (r = 0.5; p < 0.01) was found. In contrast, the adipose tissue fatty acid synthase (FAS) and ATP-citrate lyase (ACL) activities in morbidly obese patients are significantly lower than in lean subjects. Furthermore, negative correlation between adipose tissue FAS activity and BMI (r = –0.3; p < 0.05) as well as between ACL activity and BMI (r = –0.3; p < 0.05) was found.These data indicate that elevated glycerol 3-phosphate dehydrogenase might contribute to the increase of triacylglycerol (TAG) synthesis in obese subjects, however, fatty acids necessary for glycerol 3-phosphate esterification must be derived (because of lower FAS and ACL activities) mainly from TAG in circulating lipoproteins formed in liver (VLDL), and/or from the intake with food (chylomicrons).The conclusion is, that the enhanced activity of glycerol 3-phosphate dehydrogenase, and hence the generation of more glycerol 3-phosphate in adipose tissue offers a novel explanation for increased TAG production in adipose tissue of obese subjects.     

Sex differences in the control of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Interaction of estrogen, testosterone and insulin in the regulation of enzyme levels in vivo and in cultured hepatocytes

Control of the activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malate dehydrogenase was investigated in intact rats and in hepatocyte cultures. 1) Adult females had 2-fold greater activities of hepatic glucose-6-phosphate- and 6-phosphogluconate dehydrogenases than adult males, but similar activities of malate dehydrogenase. Castrated males showed decreased activities of all three enzymes in comparison to age- and weight-matched intact controls. In starved animals the activities of all three enzymes decreased significantly. After refeeding with nonpurified diet the activities returned to the prestarved levels in females, but increased to clearly higher values in intact and castrated males. 2) Estrogen levels were in the same range in immature and adult male and female rats. Testosterone levels were highest in adult males, clearly lower in adult females (1/8) and immature males (1/8), still lower in immature females (1/15) and lowest in castrated males (1/40). A simple correlation of the sex differences in these hormone levels to sex differences in glucose-6-phosphate- and 6-phosphogluconate dehydrogenase activities was not apparent. 3) In serum-free, dexamethasone-supplemented 48-h cultures of hepatocytes from both male and female rats the basal activities of glucose-6-phosphate dehydrogenase were the same; they were increased 2-3 fold by insulin alone, 1.5 fold by estrogen alone and 4-5 fold by insulin plus estrogen. Apparently sex differences did not persist in 48-h cell cultures. 4) In 48-h cultures of male hepatocytes, then used as the experimental model, insulin alone increased the activity not only of glucose-6-phosphate dehydrogenase but also of 6-phosphogluconate and malate dehydrogenases.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glycogen metabloism in the developing chick glycogen body: functional significance of the direct oxidative pathway.

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and glucose-6-phosphatase were quantitatively determined for the first time in glycogen body tissue from late embryonic and neonatal chicks. For comparative purposes, the activities of these enzymes were examined also in liver and skeletal muscle from pre- and post-hatched chicks. The present data show that both the embryonic and neonatal glycogen body lack glucose-6-phosphatase, but contain relatively high levels of glucose-6-phosphate dehydrogenase. The activity of each dehydrogenase in either embryonic or neonatal glycogen body tissue is two- to five-fold greater than that found in muscle or liver from pre- or post-hatched chicks. The relatively high activities observed for both dehydrogenases in the glycogen body, together with the absence of glucose-6-phosphatase activity in that tissue, suggest that the direct oxidative pathway (pentose phosphate cycle) of glucose metabolism is a functionally significant route for glycogen utilization in the glycogen body. It is hypothesized that the glycogen body is metabolically linked to lipid synthesis and myelin formation in the central nervous system of the avian embryo.     

Development of hepatic and adipose tissue lipogenic enzymes and insulinemia during suckling and weaning on to a high-fat diet in Zucker rats

This study was designed to monitor the developmental changes in insulinemia and lipogenic enzyme activities in both inguinal adipose tissue and liver during suckling (7, 9, 14, and 17 days of age) and weaning (22 and 30 days of age) on to either a low-fat or a high-fat diet in lean (Fa/fa) and obese (fa/fa) rats. Tissues were removed through surgery and genotypes were retrospectively determined. During suckling, there was no difference in liver enzyme activities between the two groups. In contrast, adipose tissue fatty acid synthetase was increased by 50% and citrate cleavage enzyme and malic enzyme by 30% by 9 days of age. By 17 days of age, there was a threefold elevation in these enzyme activities and 6-phosphogluconic dehydrogenase and a twofold increase in glucose-6-phosphate dehydrogenase per inguinal fat pad in fa/fa versus Fa/fa. Consistent with these results, fat pad weight was increased by 20%, 50%, and 100% at 9, 14, and 17 days of age, respectively, in obese as compared to lean pups. However only by 17 days of age could a slight but significant increase in insulin level be detected in obese pups. Enlargement of inguinal fat pad accelerated after weaning on to a low-fat diet and still more after weaning on to a high-fat diet. Weaning on to a low-fat diet elicited an induction of hepatic lipogenic enzymes two or three times greater in fa/fa than in lean pups, while weaning on to a high-fat diet blunted the differences between genotypes. The lipogenic enzyme activities displayed per total inguinal fat were three to ten times greater in obese than in lean pups, regardless of the diet. However, adipose tissue lipogenic enzyme activities were much lower after weaning on to a high-fat than on to a low-fat diet in obese pups. The high-fat diet was as effective as the low-fat diet in triggering hyperinsulinemia in obese pups. The increased adipose tissue capacity for lipogenesis, starting during the suckling period, could play an important etiologic role in the development and maintenance of obesity in the Zucker rat.-Bazin, R., and M. Lavau. Development of hepatic and adipose tissue lipogenic enzymes and insulinemia during suckling and weaning on to a high-fat diet in Zucker rats.     

Refeeding-Induced Brown Adipose Tissue Glycogen Hyper-Accumulation in Mice Is Mediated by Insulin and Catecholamines

Brown adipose tissue (BAT) generates heat during adaptive thermogenesis through a combination of oxidative metabolism and uncoupling protein 1-mediated electron transport chain uncoupling, using both free-fatty acids and glucose as substrate. Previous rat-based work in 1942 showed that prolonged partial fasting followed by refeeding led to a dramatic, transient increase in glycogen stores in multiple fat depots. In the present study, the protocol was replicated in male CD1 mice, resulting in a 2000-fold increase in interscapular BAT (IBAT) glycogen levels within 4–12 hours (hr) of refeeding, with IBAT glycogen stores reaching levels comparable to fed liver glycogen. Lesser effects occurred in white adipose tissues (WAT). Over the next 36 hr, glycogen levels dissipated and histological analysis revealed an over-accumulation of lipid droplets, suggesting a potential metabolic connection between glycogenolysis and lipid synthesis. 24 hr of total starvation followed by refeeding induced a robust and consistent glycogen over-accumulation similar in magnitude and time course to the prolonged partial fast. Experimentation demonstrated that hyperglycemia was not sufficient to drive glycogen accumulation in IBAT, but that elevated circulating insulin was sufficient. Additionally, pharmacological inhibition of catecholamine production reduced refeeding-induced IBAT glycogen storage, providing evidence of a contribution from the central nervous system. These findings highlight IBAT as a tissue that integrates both canonically-anabolic and catabolic stimulation for the promotion of glycogen storage during recovery from caloric deficit. The preservation of this robust response through many generations of animals not subjected to food deprivation suggests that the over-accumulation phenomenon plays a critical role in IBAT physiology.     

Development of intra- and intermuscular adipose tissue in growing large white and Meishan pigs

Lipogenic enzyme activities of porcine intra- and intermuscular adipose tissues were determined in growing lean (Large White) and fat (Meishan) pigs. The activities of acetyl-CoA-carboxylase (ACX), malic enzyme (ME) and glucose-6-phosphate dehydrogenase (G6PDH) were compared in both breeds and at both adipose sites. All three enzyme activities were much lower in the intramuscular adipose tissue than in the intermuscular site. Although the lipogenic activity of the intramuscular adipose site was low, it appeared, however, to possess adequate levels of enzymes for in situ lipid synthesis. The highest differences in lipogenic enzyme activities between Meishan and Large White pigs were found in intramuscular adipose tissue, and essentially concerned the activity of malic enzyme which was much higher in Meishan pigs. A close relationship between ME activity and lipid content of intramuscular adipose tissue was observed in both breeds. It was concluded that ME appeared to be a major factor affecting the incidence of higher intramuscular fat in the pig.     

Adaptation to a high protein, carbohydrate-free diet induces a marked reduction of fatty acid synthesis and lipogenic enzymes in rat adipose tissue that is rapidly reverted by a balanced diet

We have previously shown that in vivo lipogenesis is markedly reduced in liver, carcass, and in 4 different depots of adipose tissue of rats adapted to a high protein, carbohydrate-free (HP) diet. In the present work, we investigate the activity of enzymes involved in lipogenesis in the epididymal adipose tissue (EPI) of rats adapted to an HP diet before and 12 h after a balanced diet was introduced. Rats fed an HP diet for 15 days showed a 60% reduction of EPI fatty acid synthesis in vivo that was accompanied by 45%-55% decreases in the activities of pyruvate dehydrogenase complex, ATP-citrate lyase, acetyl-CoA carboxylase, glucose-6-phosphate dehydrogenase, and malic enzyme. Reversion to a balanced diet for 12 h resulted in a normalization of in vivo EPI lipogenesis, and in a restoration of acetyl-CoA carboxylase activity to levels that did not differ significantly from control values. The activities of ATP-citrate lyase and pyruvate dehydrogenase complex increased to about 75%-86% of control values, but the activities of glucose-6-phosphate dehydrogenase and malic enzyme remained unchanged 12 h after diet reversion. The data indicate that in rats, the adjustment of adipose tissue lipogenic activity is an important component of the metabolic adaptation to different nutritional conditions.     

Enzyme alteration in skeletal muscle of mice with muscular dystrophy.

1. Developmental enzyme alterations were investigated in skeletal muscle of the hereditary progressive muscular dystrophy (PMD) mice of C57BL/6J strain. 2. Enzymes examined were classified into three groups according to changes of activities in dystrophy muscle during ageing. Activities of creatine kinase (EC 2.7.3.2), pyruvate kinase (EC 2.7.1.40), glycogen phosphorylase (EC 2.4.1.1), and fructose-biphosphate aldolase (EC 4.1.2.13), each of which had the respective muscle specific isoenzyme of extremely high activity in normal adult skeletal muscle, decreased rapidly in dystrophy muscle from the early stage of the disease with ageing. Activities of glycogen synthase (EC 2.4.1.11) and hexokinase (EC 2.7.1.1) were higher in dystrophy muscle in the early stage but decreased gradually to lower levels than those in the control with ageing. Activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) were always much higher in dystrophy muscle than in the control, with no relation to ageing. 3. Isoenzymes of creatine kinase, pyruvate kinase and phosphorylase in dystrophy muscle were mainly the muscle types, indicating that muscle differentiation was not blocked profoundly even in dystrophy muscle. In limited cases, especially in the early stage of the disease, very weak activities of the non-muscle fetal type isoenzymes of creatine kinase and phosphorylase were detected, apparently associated with partial muscle regeneration in dystrophy muscle.     

Pyridine nucleotide synthesis by rat adipose tissue in vitro

The synthesis of NAD and NADP by rat adipose tissue was measured in vitro. Nicotinamide-7-(14)C and NaH(2)(32)PO(4) were incorporated together into NAD with a (32)P/(14)C ratio of 1.82 and nicotinic-7-(14)C acid and NaH(2)(32)PO(4) with a ratio of 1.94. Nicotinic acid stimulated, by 90%, lipogenesis from glucose-U-(14)C by rat adipose tissue in vitro. Glucose plus insulin and refeeding for 48 hr after a 48 hr fast markedly increased the incorporation of nicotinic-7-(14)C into NAD in rat epididymal fat pads in vitro, but neither fructose, L-glutamine, nor insulin alone increased the synthesis of NAD in this tissue. Glucose-1-(14)C, ribose-1-(14)C, and to a greater extent glucose-6-(14)C are incorporated into the NAD of rat adipose tissue. Fasting followed by refeeding sharply increased the radioactivity of NAD-(14)C formed from glucose-1-(14)C and glucose-6-(14)C but not from ribose-1-(14)C. Increasing the ribose concentration from 2 mM to 10 mM increased its incorporation into adipose tissue NAD twofold. The nicotinic-7-(14)C acid incorporation into NAD increased over the 1st hr of incubation and remained constant for the next 3 hr. The concentration of NAD in the fat pads showed a similar response to the time of incubation. NADP concentrations increased over the entire 4 hr incubation period as did the incorporation of nicotinic-7-(14)C acid into NADP. The results of this study suggest that NAD is synthesized de novo by rat adipose tissue in vitro and that this synthesis is increased by factors which stimulate lipogenesis.     

Comparative changes in the lipogenic enzyme activities and in the in vivo fatty acid synthesis in liver and adipose tissues during the post-weaning growth of male rats

Changes in the specific activities of acetyl-CoA-carboxylase (ACX), malic enzyme (ME) and glucose-6-phosphate dehydrogenase (G-6-PD) were compared to changes in de novo lipogenesis measured by in vivo incorporation of [3H] of tritiated water into fatty acids of liver and of perirenal and dorsal subcutaneous adipose tissues. In the adipose tissues, the specific activities of the three enzymes rather closely followed fluctuations in the rate of fatty acid synthesis. In the liver, ACX and especially ME activities were satisfactory indicators of de novo lipogenesis; G-6-PD activity did not depend on de novo lipogenesis.     

Insulin-like effect of vanadate on malic enzyme and glucose-6-phosphate dehydrogenase activities in streptozotocin-induced diabetic rat liver

The effect of oral administration of sodium orthovanadate on hepatic malic enzyme (EC 1.1.1.40) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) activities was investigated in nondiabetic and diabetic rats. Streptozotocin-induced diabetic rats were characterized by 4.7-fold increase in plasma glucose and 82% decrease in plasma insulin levels. The activities of hepatic malic enzyme and glucose-6-phosphate dehydrogenase were also diminished (P less than 0.001). Vanadate treatment in diabetic rats led to a significant decrease (P less than 0.001) in plasma glucose levels and to the normalization of enzyme activities, but it did not alter plasma insulin levels. In nondiabetic rats vanadate decreased the plasma insulin level by 64% without altering the enzyme activities. Significant correlation was observed between plasma insulin and hepatic lipogenic enzyme activities in untreated and vanadate-treated rats. Vanadate administration caused a shift to left in this correlation suggesting improvement in insulin sensitivity.