Kinetic analyses of the pyruvate dehydrogenase complex from Candida 107 (NCYC 911).

Candida 107 (NCYC 911) accumulates up to 45% of the biomass as triglycerides under conditions of nitrogenous substrate limitation in the medium. In oilseeds and adipocytes, lipid accumulation is preceded and accompanied by increased activity of key enzymes such as pyruvate dehydrogenase. However, in Candida 107, the activity of this complex was greatly reduced during lipogenesis. The initial velocity patterns were in accordance with a Hexa Uni Ping Pong mechanism. The Km values for the various substrates were similar to those found for the yeast Saccharomyces cerevisiae, but much higher than those reported for the mammalian enzyme. Product inhibition studies indicated that the Ki for acetyl coenzyme A and NADH were higher than those reported for other yeasts. The values for Ki were similar to those found for the liver enzyme, whereas the enzyme complex from heart had much lower Ki values for products. It has been suggested that in the heart and kidney, pyruvate dehydrogenase is regulated by product inhibition whereas in the liver this does not appear to be the mechanism. Therefore, it is probable, that like the liver enzyme, pyruvate dehydrogenase from Candida 107 may not be regulated by product inhibition.

Mode of action of lipoic acid in diabetes.

Metabolic aberrations in diabetes such as hyperglycemia, ketonemia, ketonuria, reduced glycogen in tissues and reduced rates of fatty acid synthesis in the liver are corrected by the administration of lipoic acid. Dithiol octanoic acid is formed from lipoic acid by reduction and substitutes for Coenzyme A in several enzymatic reactions such as pyruvate dehydrogenase, citrate synthase, acetyl Coenzyme A carboxylase, fatty acid synthetase, and triglyceride and phospholipid biosynthesis; but not in the oxidation of fatty acids because of the slow rates of thiolysis of ß-keto acyl dithioloctanoic acid. The overall effect of these changes in the key enzymic activities is seen in the increased rates of oxidation of glucose and a reduction in fatty acid oxidation in diabetes following lipoic acid administration.

Lipoic acid and diabetes—Part III: Metabolic role of acetyl dihydrolipoic acid.

Rat liver lipoyl transacetylase catalyzes the formation of acetyl dihydrolipoic acid from acetyl coenzyme A and dihydrolipoic acid. In an earlier paper the formation of acetyl dihydrolipoic from pyruvate and dihydrolipoic acid catalyzed by pyruvate dehydrogenase has been reported. Acetyl dihydrolipoic acid is a substrate for citrate synthase, acetyl coenzyme A carboxylase and fatty acid synthetase. The Vmax. for citrate synthase with acetyl dihydrolipoic acid was identical to acetyl coenzyme A (approximately 1 μmol citrate formed/min/mg protein) while the apparent Km was approximately 4 times higher with acetyl dihydrolipoic acid as the substrate. This may be due to the fact that synthetic acetyl dihydrolipoic acid is a mixture of 4 possible isomers and only one of them may be the substrate for the enzymatic reaction. While dihydrolipoic acid can replace coenzyme A in the activation of succinate catalyzed by succinyl coenzyme A synthetase, the transfer of coenzyme A between succinate and acetoacetyl dihydrolipoic acid catalyzed by succinyl coenzyme A: 3 oxo-acid coenzyme A transferase does not occur.

Lipoic acid and diabetes II: Mode of action of lipoic acid.

Intraperitoneal administration of lipoic acid (10 mg/100 g) does not effect changes in serum insulin levels in normal and alloxan diabetic rats, while normalising increased serum pyruvate, and impaired liver pyruvic dehydrogenase characteristic of the diabetic state. Dihydrolipoic acid has been shown to participate in activation of fatty acids with equal facility as coenzyme A. Fatty acyl dihydrolipoic acid however is sparsely thiolyzed to yield acetyl dihydrolipoic acid. Also acetyl dihydrolipoic acid does not activate pyruvate carboxylase unlike acetyl coenzyme A. The reduced thiolysis of ß-keto fatty acyl dihydrolipoic acid esters and the lack of activation of pyruvic carboxylase by acetyl dihydrolipoic acid could account for the antiketotic and antigluconeogenic effects of lipoic acid.

Nutrients in the shadow-nutrients of substance.

While the dietary importance of proteins, essential fatty acids, vitamins and trace elements has been well recognised, the role of shadow nutrients, a class of metabolites, which are biosynthesized in the body and serve vital functions, such as lipoic acid, choline, inositol, taurine and carnitine, has not been adequately appreciated. There are reasons to believe that during infancy and in ageing, biosynthesis of these metabolites may be limited. The objective of this review is to highlight the essentiality of these nutrients and the need for their supplementation in the diets of infants and in elderly people. Provision of shadow nutrients where the necessary biosynthetic machinery might not have developed to full stature or might have slowed down, is a new concept in nutrition which needs attention.

Lipoic acid and diabetes: Effect of dihydrolipoic acid administration in diabetic rats and rabbits.

Relative α-lipoic acid content of diabetic livers was considerably less than that of normal livers as determined by gas chromatography. It was not possible to detect any dihydrolipoic acid in the livers. Biochemical abnormalities such as hyperglycaemia, ketonemia, reduction in liver glycogen and impaired incorporation of [2-14C] -acetate into fatty acids in alloxan diabetic rats were brought to near normal levels by the oral or intraperitoneal administration of dihydrolipoic acid. The effect of α-lipoic acid was comparable to that of dihydrolipoic acid in reducing the blood sugar levels of diabetic rabbits during a glucose tolerance test. The results suggest that the mode of action of lipoic acid was through stimulation of pyruvate dehydrogenase.