Studies on the de novo Biosynthesis of NAD in Escherichia coli

Quinolinic acid (pyridine 2,3-dicarboxylic acid) which is an immediate precursor of the pyridine nucleotides, is synthesised from L-aspartate and dihydroxyacetone phosphate in Escherichia coli. Extracts from certain nadB mutants complement the extracts prepared from all nadA mutants for the enzymic synthesis of quinolinate. Using the complementation assay, the quinolinate synthetase B protein has been purified more than 300-fold. The quinolinate synthetase B protein exists in all nadA and nadC mutants examined. The quinolinate synthetase A protein was present in all nadC mutants and most (but not all) nadB mutants. The facile separation of the wild-type quinolinate synthetase A and B proteins out of a nadC mutant suggests that quinolinate synthetase does not exist as a tightly bound complex. The partially purified quinolinate synthetase is inhibited by physiological concentrations of NAD and NADH but not by NADP or NADPH.