Nitrate reductase and related enzymes in Escherichia coli.

Several reports about nitrate reduction in Escherichia coli have appeared during the last few years. Taniguchi, Sato & Egami (1956) have found in cell-free extracts of E. coli, grown in a peptone broth-agar medium containing nitrate, a particulate system which reduces nitrate to nitrite, only under anaerobic conditions and with reduced diphosphopyridine nucleotide, formate or reduced methylene blue as electron donors and flavin nucleotides as electron carriers. Wainwright (1955) found that nitrate-reductase activity in E. coli, strain 1431, was enhanced in anaerobiosis by vitamin K3 and that the maximal activity was obtained when both vitamin K3 and flavin nucleotides were added to the incubation mixtures. Cheniae & Evans (1958, 1959) have also observed an activation by vitamin K3 of nitrate reductase by preparations of Rhizobium japonicum when reduced diphosphopyridine nucleotide was the donor. Nicholas & Nason (1955) found in cell-free extracts of E. coli grown in a synthetic medium a soluble, flavindependent diphosphopyridine nucleotide-nitrate reductase which reduces nitrate to nitrite aerobically. Medina & Heredia (1958) found that cellfree extracts of E. coli, grown either in a synthetic medium or in a complex broth medium, contain a reduced diphosphopyridine nucleotide-dependent system which reduces nitrate to nitrite aerobically with vitamin K3 as electron carrier without any apparent participation of flavin in the process. The work presented in this paper shows that nitrate reduction takes place under aerobic as well as under anaerobic conditions and in both cases is catalysed by a menadione-reductase-nitrate-reductase system which uses vitamin K3 as electron carrier. In addition to this pathway, nitrate can also be reduced to nitrite, under completely anaerobic conditions, through an apparently flavin-dependent system, presumably related to the reduced diphosphopyridine nucleotide-oxidase system.