Studies of Nicotinic Acid Metabolism in Astasia longa

Cell-free extracts of Astasia longa possess the following enzymes at the indicated relative activities: (a) quinolinic acid phosphoribosyl transferase, 0.5; (b) nicotinic acid mononucleotide pyrophosphorylase, 1; (c) nicotinic acid dinucleotide pyrophosphorylase, 1; (d) nicotinamide adenine dinucleotide synthetase, 0.003; (e) nicotinamide dinucleotide kinase, 1; (f) nicotinamide deamidase, 1; (g) nicotinic acid methyltransferase, 0.0001. The presence of Enzymes a, c, d, and e, in conjunction with data on the intracellular levels of deamido-NMN, deamido-NAD, NAD, and NADP, establishes this sequence of compounds as the pathway of biosynthesis of pyridine nucleotides in Astasia and indicates that the conversion of deamido-NAD to NAD is a rate-limiting step. Cell-free extracts also catalyzed the degradation and pyrophosphorolysis of deamido-NAD and of NAD, and catalyzed an exchange reaction between nicotinic acid and the nicotinamide moiety of NAD. These reactions, in conjunction with the presence of Enzymes b and f, and with data showing a high rate of turnover of NAD in exponentially growing cultures, indicate the operation of a nicotinic acid salvage pathway in Astasia. The finding that nicotinic acid is an inhibitor of Enzyme a and that quinolinic acid is an inhibitor of Enzyme b suggests a possible means of control of the relative rates of the biosynthetic and the salvage pathways. The finding that deamido-NAD, present in relatively high concentrations in Astasia, is an inhibitor of Enzyme b suggests another possible control mechanism. Growth in the presence of nicotinic acid or quinolinic acid did not lead to repression of either Enzyme a or b. Nicotinamide and high concentrations of nicotinic acid inhibited the growth of Astasia and nicotinamide repressed the level of Enzymes a and b.