Walter Norman Shaw

Interrelationship of glucose and lipid metabolism.

Since 1948 evidence has accumulated which indicates that the synthesis of fatty acid requires the concomitant utilization of glucose. Within the past two years additional information about the relationship between glucose metabolism and fatty acid synthesis has given a better understanding of the defect in lipid metabolism in the diabetic animal. It has long been recognized that the oxidation of fatty acids is accelerated in the uncontrolled diabetic. This resulted in the investigative emphasis on ketone formation as the major defect in lipid metabolism of the diabetic patient. It was not until the work of Drury and of Stetten and Boxer that a marked depression in fatty acid synthesis was demonstrated. Thus, when normal glucose utilization is restricted, as in diabetes, the synthesis of fatty acids is depressed while oxidation continues unchecked. The classic work of Lynen has elucidated the steps by which long-chain fatty acids are oxidized in a stepwise fashion to form acetyl Coenzyme-A (acetyl Co-A). This so-called fatty acid cycle includes two oxido-reductive reactions which require flavin adenine dinucleotide (FAD) and diphosphopyridine nucleotide (DPN) as cofactors. It was believed that the synthesis of fatty acids resulted from the reversal of the steps of oxidation and that these same two cofactors were involved. Shaw, Dituri and Gurin demonstrated that the failure of cell-free systems prepared from the livers of alloxan diabetic rats to synthesize fatty acids was due to an inability to convert acetyl Co-A to butyryl Co-A. Their work localized a possible block in fatty acid synthesis in the diabetic to the reaction in which crotonyl Coenzyme-A is converted to butyryl CoenzymeA. Langdon demonstrated that in the synthesis of fatty acids from acetate, the reduction of crotonyl CoenzymeA to form butyryl Coenzyme-A requires reduced triphosphopyridine nucleotide (TPNH). Thus, when this reaction proceeds in the direction of the synthesis of fatty acids it requires TPNH but when this same reaction proceeds in the oxidative direction it requires FAD. This has led to the conclusion that the pathways of fatty acid synthesis and oxidation may be different. The demonstration that fatty acid synthesis requires TPNH has aroused interest in TPNH production and its relationship to lipogenesis. Two pathways for glucose utilization exist in liver and adipose tissue, major sites of fatty acid synthesis. One pathway is the Embden-Meyerhof pathway which produces DPNH but no TPNH. The other is the phosphogluconate oxidative pathway (PGO) in which the first two reactions produce TPNH. The following evidence has led to the conclusion that the utilization of glucose via the PGO pathway is important in fatty acid synthesis. Felts et al. first noted a decreased glucose utilization via the PGO pathway in liver slices prepared from alloxan diabetic rats. Siperstein demonstrated in liver homogenates that the stimulation of glucose metabolism via the Embden-Meyerhof pathway by the addition of DPN to the medium results in little or no increase in fatty acid synthesis. However, when glucose utilization via the PGO pathway is stimulated by the addition of TPN, there is a marked increase in fatty acid synthesis. He also showed that the defect in fatty acid synthesis in homogenates prepared from the livers of alloxan diabetic rats can be corrected by the stimulation of glucose utilization via the PGO pathway. Milstein has also observed a decrease in the utilization of glucose via the PGO pathway as estimated from carbon dioxide production by the epididymal fat pad obtained from the alloxan diabetic rat. Winegrad and Renold studied the relationship between glucose metabolism and fatty acid synthesis in the epididymal fat pad of the normal rat. The addition of insulin in vitro results in an increased synthesis of fatty acids from glucose at the same time that there is an increased utilization of glucose via both the Embden-Meyerhof and PGO pathways. They also showed that the stimulation of fatty acid synthesis from other precursors of acetyl Co-A (acetate and pyruvate) which results from the addition of insulin in vitro, is dependent upon the concomitant utilization of glucose. Insulin added in vitro to the fat pads of alloxan diabetic rats will correct the defect in fatty acid synthesis. Winegrad, Shaw and Renold employed the in vitro effect of growth hormone in the epididymal fat pad to elucidate the relationship between a specific pathway of glucose utilization and fatty acid synthesis. When growth hormone is added in vitro to this tissue, there is an increased oxidation of glucose to carbon dioxide but no increase in fatty acid synthesis. Studies with differentially labeled glucose (glucose-1-C and glucose-6-C) indicate that growth hormone produces a marked decrease in the utilization of glucose via the PGO pathway. The increase in the carbon dioxide formation from glu-

Effect of Porcine Proinsulin in Vitro on Adipose Tissue and Diaphragm of the Normal Rat

The effect in vitro of porcine proinsulin and single component porcine insulin on the epididymal adipose tissue and the isolated diaphragm of the normal rat has been investigated. Both proteins enhance the oxidation of labeled glucose to carbon dioxide and its conversion to fatty acids by adipose tissue as well as glycogen formation from glucose by the hemidiaphragm. Kunitz pancreatic trypsin inhibitor (KPTI) but not the purified Kazal or soybean trypsin inhibitor block these effects of proinsulin on adipose tissue and diaphragm. KPTI has no effect on the response of these tissues to the single component insulin. Anti-insulin serum blocks the response of both adipose tissue and hemidiaphragm to proinsulin and single component insulin. The enhancement of the oxidation of carbon one of glucose to carbon dioxide by adipose tissue in the presence of proinsulin is also blocked by KPTI. It is concluded that porcine proinsulin itself cannot affect the glucose metabolism of adipose tissue and hemidiaphragm of the normal rat. Proinsulin exerts the biologic effect seen in this study because it is converted by an enzyme or group of enzymes present in adipose tissue and diaphragm muscle, to a molecule which is similar to or identical with insulin.

Effect of Streptozocin-Induced Diabetes on Insulin-Receptor Tyrosine Kinase Activity in Obese Zucker Rats

We examined insulin binding, insulin-stimulated autophosphorylation, and phosphorylation of poly(GIu · Na,Tyr)4:1 by liver and skeletal muscle insulin receptor from lean, obese, and obese streptozocin-induced diabetic Zucker rats. Induction of diabetes with streptozocin (30 mg/kg) lowered the fasting insulin level from 11.4 to 3.8 ng/ml, which was not significantly greater than the lean control level. Autophosphorylation and tyrosine kinase activity of liver insulin receptors were increased 70–100% in the obese control group (relative to lean rats), but diabetes reversed this hyperresponsiveness to insulin. In muscle, obesity was associated with a 40–50% decrease in autophosphorylation and tyrosine kinase activity, which was also reversed in the diabetic state. Autophosphorylation and tyrosine kinase activity were significantly correlated in liver and muscle and were also correlated with fasting insulin levels. These data suggest that insulin-receptor tyrosine kinase activity is regulated differently in liver and muscle and that the abnormalities in kinase activity associated with the obese Zucker rat are at least partly secondary to hyperinsulinemia.

LY79771: A novel compound for weight control

Abstract Compound LY79771 given subcutaneously reduced weight or decreased weight gain of genetically obese rats and mice, gold thioglucose obese mice, and obese beagles. The compound had no effect on body weight of lean rats. Food consumption was not decreased. Obese rats showed a transient rise in body temperature after each administration of the drug. The change in body weight was due mainly to a decrease in body fat mass.

Comparative study of the effects of porcine proinsulin and insulin on lipolysis and glucose oxidation in rat adipocytes.

Proinsulin blocked lipolysis activation by epinephrine, glucagon or theophylline. The antilipolytic effect of pro-insulin was not blocked by Kunitz pancreatic trypsin inhibitor (KPTI). The-concentrations of proinsulin and insulin required to cause 50 per cent inhibition of epinephrine-stimulated lipolysis were estimated to be approximately 1 × 10−9 M and 3 × 10−11 M respectively. Proinsulin which had been reduced and allowed to reoxidize was found to have approximately the same antilipolytic activity as native proinsulin, indicating that the antilipolytic effect was due to proinsulin and not insulin contamination. The concentrations of proinsulin and insulin required to give 50 per cent of maximal stimulation of glucose oxidation were estimated to be approximately 5 × 10−9 M and 1 × 10−11 M respectively. The effect of proinsulin on glucose oxidation did not appear to be caused by any insulin contamination since reduced-reoxidized proinsulin had approximately the same activity as native proinsulin. KPTI had no effect on proinsulin-stimulated glucose oxidation in isolated fat cells, in contrast to the marked inhibitory effect observed using intact epididymal fat pad. This suggests that the KPTI-sensitive proteolytic activity of the epididymal fat pad is not located in the fat cell but in some other type cell or extracellular space.