Escherichia coli Mutant with Elevated Nicotinamide Adenine Dinucleotide Phosphate (Reduced) Oxidase Activity

A slow-growing mutant of Escherichia coli with greatly elevated nicotinamide adenine dinucleotide phosphate (reduced; NADPH) oxidase activity has been isolated. The oxidase activity of the wild-type organism, normally low at pH 7.5, was increased when the assay was performed at pH 6.0. Sucrose density gradients of sonic extracts of the mutant and wild-type strains revealed several peaks of NADPH oxidase activity at pH 6.0. The parent organism had a peak of activity of high molecular weight which was absent from the mutant. The mutant strain had an activator capable of increasing the activity of all wild-type density gradient peaks, especially the one of high molecular weight. The activator was either missing or masked in the wild type. Agar gel electrophoresis of the extracts uncovered a rapidly moving band from the wild type, missing from the mutant; the material in this band had weak NADPH-diaphorase activity and strongly inhibited the activity of the mutant NADPH oxidase. It was concluded that, in wild-type E. coli, NADPH oxidase activity is regulated by a proper balance of an activator and an inhibitor. The absence of the inhibitor, as in the mutant, or the inactivation of the inhibitor at acid pH levels, results in a high level of NADPH oxidase activity. The relation of high NADPH oxidase levels and subsequent decrease of the NADPH pool to the decrease in growth rate is considered.