Metabolic pathways for nitrate reduction in Escherichia coli

Abstract

1. An assay for the production of formate from pyruvate has been described. The presence of the enzyme involved in this reaction has been established for Escherichia coli K 12 grown anaerobically in the presence of nitrate. 2. Formate, lactate, pyruvate and NADH were effective donors for nitrate reduction. 3. Cell membrane preparations catalysed the reduction of nitrate to nitrite when NADH or formate was the electron donor. Reduction by lactate required both “soluble” and “membrane-bound” enzymes and cofactors. The NADH-nitrite reductase also present in these extracts was soluble. 4. Increasing concentrations of nitrate supplied during anaerobic growth in a chemostat were paralleled by increased activities of NADH-nitrate reductase (EC 1.6.6.2) and NADH oxidase activity. In contrast, cytochrome c 552 and a 2 synthesis and NADH-nitrite reductase activity (EC 1.6.6.4) were highest at low concentrations of nitrate. 5. The oxidation and reduction of cytochromes in whole cells with various substrates and inhibitors has been followed spectrophotometrically. Cytochrome c 552 was oxidised by both nitrite (azide insensitive) and nitrate. Other cytochromes were oxidised only by nitrate (azide sensitive). 6. The possible involvement of cytochrome c 552 in nitrite or nitrate reduction has been discussed.