PROTEIN-A OF QUINOLINATE SYNTHETASE IS THE SITE OF OXYGEN POISONING OF PYRIDINE-NUCLEOTIDE COENZYME SYNTHESIS IN ESCHERICHIA-COLI

De novo biosynthesis of pyridine nucleotide coenzymes in Escherichia coli is initiated by an enzyme complex (quinolinate synthetase) containing protein B which converts L-aspartate into iminoaspartate and protein A, which then generates quinolinate on the pathway to the coenzymes. This complex has been shown to be poisoned by hyperbaric oxygen.7,8 We performed assays made dependent on both proteins B and A versus only protein A, using cell-free extracts of hyperbaric-oxygen poisoned and aerobically grown cells. The specific activities were reduced by similar amounts of 68% and 60%, respectively, when measured in assays made dependent on enzymes B and A versus only protein A that was derived from oxygen-poisoned extract. Thus, protein A is the oxygen-sensitive component.