Regulation of NAD biosynthesis via the pyridine nucleotide cycle.

Evidence collected in a number of laboratories is consistent with the synthesis, breakdown and resynthesis of NAD+ via the series of reactions shown in Figure 1. The de novo biosynthesis of NAD proceeds from tryptophan in some organisms and from the condensation of 3 carbon and 4 carbon moieties in others, but QA is a common intermediate in both pathways. In mammals, Neurospora, aerobically grown yeast and Xanthomonas pruni tryptophan is the precursor of QA. In higher plants and many microorganisms, QA is formed from aspartate and glycerol (or closely related compounds) (1), by a series of reactions which have not yet been elucidated. The PRPP dependent conversion of QA to NaMN was first demonstrated in liver homogenates by Nishizuka and Hayaishi (2). The responsible enzyme, quino linate phosphoribosyl transf erase, was purified at Oklahoma State University and