Studies of Nicotinic Acid Metabolism in Astasia longa: I. INCORPORATION OF NICOTINIC ACID INTO PYRIDINE DERIVATIVES IN EXPONENTIALLY GROWING AND IN SYNCHRONIZED CULTURES

Abstract

Abstract Astasia longa grown in synthetic media incorporate labeled nicotinic acid into deamido-NMN, deamido-NAD, NAD, and NADP. The specific activities of these pyridine nucleotides after a 2-min pulse are compatible with the operation of the Preiss-Handler pathway of NAD biosynthesis. Label also appeared in nicotinamide, NMN, and the N-methyl derivatives of nicotinic acid and nicotinamide, indicating a high turnover of NAD in this protozoan cell. The relative amounts of deamido-NMN, deamido-NAD, NAD, and NADP were about 1:5:20:2 in both exponentially growing cultures and in cultures synchronized by means of a repetitive temperature cycle. Appreciable synthesis of pyridine nucleotides did not begin until the middle of the warm period, when almost half of the population had divided, and synthesis continued throughout the cold period. The NAD and NADP contents of the cell were relatively constant throughout the synchrony cycle, but the ratio of deamido-NMN to deamido-NAD changed. Growth in the presence of nicotinic acid leads to an increased intracellular concentration of pyridine derivatives, suggesting that the rate of endogenous supply of deamido-NMN precursors is one rate-limiting step in the biosynthesis of NAD in Astasia. The maintenance of a high and relatively constant level of deamido-NAD indicates that the supply of endogenous deamido-NMN precursors is not the only regulatory process controlling the level of pyridine nucleotides. Growth of Astasia in the presence of high nicotinic acid concentrations leads to the synthesis of a pink pigment which does not appear to be present under normal conditions of growth, i.e. in the absence of nicotinic acid. High nicotinic acid during growth also leads to a large increase in the level of an unidentified guanine-containing nucleotide.