Oxygen dependence of mitochondrial nitric oxide synthase activity

Abstract

The effect of O 2 concentration on mitochondrial nitric oxide synthase (mtNOS) activity and on O 2 − production was determined in rat liver, brain, and kidney submitochondrial membranes. The K m O 2 for mtNOS were 40, 73, and 37 μM O 2 and the V max were 0.51, 0.49, and 0.42 nmol NO/min mg protein for liver, brain, and kidney mitochondria, respectively. The rates of O 2 − production, 0.5–12.8 nmol O 2 −/min mg protein, depended on O 2 concentration up to 1.1 mM O 2. Intramitochondrial NO, O 2 −, and ONOO − steady-state concentrations were calculated for the physiological level of 20 μM O 2; they were 20–39 nM NO, 0.17–0.33 pM O 2 −, and 0.6–2.2 nM ONOO − for the three organs. These levels establish O 2/NO ratios of 513–1000 that correspond to physiological inhibitions of cytochrome oxidase by intramitochondrial NO of 16–25%. The production of NO by mtNOS appears as a regulatory process that modulates mitochondrial oxygen uptake and cellular energy production.