Formate‐Nitrite Reduction in <i>Escherichia coli</i> K12

Abstract

Components of the electron transfer pathway from formate to nitrite were identified and compared with those of the formate‐nitrate system. Information was derived from studies on various mutants, different electron donors, and effects of inhibitors. An explanation is proposed for the loss activity in spheroplasts or cell free extract. Formate and d ‐lactate with E′ o (pH 7) values of –420 mV and –180 mV respectively reduce both nitrite and nitrate. On the contrary ascorbate/phenazine methosulfate (+ 80 mV) does not reduce nitrite. Formate dehydrogenase and hemin‐deficient mutants lack both formate nitrite and nitrate reductase activities. Quinone‐lacking mutants lost 40% of the formate‐nitrite reduction and all of the formate‐nitrate reductase activity. Strains affected on nitrate‐reductase cytochrome b or nitrate reductase enzyme have retained a normal capacity to reduce nitrite with formate. Mutant nir 197 has simultaneously lost the formate‐nitrite reductase activity and the biosynthesis of the cytochrome c 552 . With HpHOQnO one inhibitory site was found. Ultraviolet irradiation revealed the presence of quinones at more than one site. Carbon monoxide inhibited formate‐nitrite reductase but not d ‐lactate‐nitrite reductase activity. This gas had no effect on the formate dehydrogenase. Action of inhibitors on the redox steady state showed the presence in the electron pathway of cytochrome c 552 as well as a b ‐type cytochrome. The formate‐nitrite electron pathway proposed shares with that of nitrate a membrane‐bound part constituted by formate‐dehydrogenase, cytochrome b , and quinones. It also involves a soluble specific part containing at least the cytochrome c 552 . It is suggested that the loss of a soluble periplasmic element or its dilution lead to the loss of the activity, as is the case in spheroplasts and osmotic‐shock‐treated cells or in cell‐free extracts and sucrose‐treated cells.