Nitrite metabolism by skeletal muscle mitochondria in relation to haem pigments

Abstract

1. Nitrite is reduced anaerobically by skeletal muscle mitochondria, a product of the action being nitrosylferricytochrome c (ref. 1). The anaerobic incubation of nitrite with skeletal muscle minces at pH 6.0 leads to the formation of nitric oxide 2, part of which is located ultimately in combination with endogenous myoglobin. 2. Pig oxymyoglobin is readily oxidised to the met- form by low concentrations of nitrite, whereas nitrosylmyoglobin is relatively stable. 3. Under argon, metmyoglobin is insensitive to reduction by pig muscle mitochondrial enzyme systems and NADH, whilst nitrosylmetmyoglobin is readily reduced to nitrosylmyoglobin. 4. The incubation of nitrosylferricytochrome c and metmyoglobin with muscle mitochondria and NADH under N 2 has resulted in the formation of ferrocytochrome c, which does not form a nitrosyl- complex, and a stoichiometric equivalent of nitrosylmyoglobin, with nitrosylmetmyoglobin as an intermediate. 5. Thus, nitrosylferricytochrome c, formed by anaerobic cytochrome oxidase (EC 1.9.3.1) action in the presence of nitrite 1, is reduced by the reduced NAD dehydrogenase (EC 1.6.99.3) system of muscle, the nitrosyl-group being transferred to metmyoglobin arising from the chemical oxidation of endogenous myoglobin by nitrite. Mitochondrial reduction of nitrosylmetmyoglobin to the stable complex nitrosylmyoglobin follows even in the presence of sufficient nitrite to oxidise oxymyoglobin.