Nicotinic Acid Biosynthesis in Prototrophs and Tryptophan Auxotrophs of Saccharomyces cerevisiae

Yeasts utilize tryptophan for the synthesis of nicotinic acid under aerobic, but not under anaerobic, conditions. Under aerobic conditions of growth, radioactivity from tryptophan-14C uniformly labeled in the benzene ring and 14C-carboxyl-labeled 3-hydroxyanthranilic acid is incorporated into nicotinic acid. Under anaerobic conditions, little radioactivity from these compounds is incorporated into nicotinic acid. The presence of a tryptophan-nicotinic acid pathway in yeast is further supported by the establishment of the presence of formylkynurenine formamidase and 3-hydroxyanthranilic acid oxidase, two of the enzymes known to mediate the conversion of tryptophan to nicotinic acid in mammalian systems. Under anaerobic conditions, yeasts incorporate radioactivity from uniformly labeled aspartate-14C and glutamate-14C, suggesting that an alternative pathway similar to that shown in several bacterial systems is operative. Evidence is presented which establishes quinolinic acid as a precursor to nicotinic acid adenine dinucleotide under either aerobic or anaerobic conditions. Quinolinic acid appears, therefore, to be a common intermediate in the alternative pathways for nicotinic acid biosynthesis in yeast.