Inhibition of NAD biosynthesis by paraquat in Escherichia coli

Niacin significantly reduced the bacteristatic effect of 1 mM paraquat for Escherichia coli. Without niacin (an intermediate in the salvage pathway for pyridine nucleotide coenzyme biosynthesis), the NAD concentration was decreased rapidly and significantly in E. coli during paraquat poisoning. Niacin prevented the decline in NAD in paraquat-poisoned cells; quinolinate (an intermediate in de novo NAD biosynthesis prior to the entry point of niacin) did not. These data suggest that paraquat poisons the de novo pathway of pyridine nucleotide coenzyme biosynthesis. Similar consequences have been reported to result from hyperbaric oxygen poisoning of E. coli; thus, there is growing evidence for a common mechanism of toxicity for hyperoxia and paraquat.