Abstract
A soluble form of NADH-cytochrome b 5 reductase (NADH: ferricytochrome B 5 oxidoreductase, EC 1.6.2.2) was found in the cytosolic fraction of rabbit liver. The partially purified enzyme was strictly specific for NADH. It catalyzed the reduction of several substrates such as the methemoglobin β ferrocyanide complex (Hegesh, E. and Avron, M. (1967) Biochim. Biophys. Acta 146, 91–101) (apparent K m: 8 μM), potassium ferricyanide (apparent K m: 10 μM) and ferricytochrome 5 5 (apparent K m: 15 μM). Upon acrylamide gel isoelectro-focusing followed by specific staining, the enzyme was resolved into four bands (isoelectric pH: 7.05, 6.70, 6.50 and 6.30). The optimum pH of activity with ferricytochrome b 5 as a substrate was 6.5. The estimated molecular weight was 25 000–30 000. The enzyme was unsensitive to cyanide. It was strongly inhibited by p-hydroxymercuribenzoate. The cytosolic liver cytochrome b 5 reductase was immunologically related to the soluble cytochrome b 5 reductase from human and rabbit red-cells, and to the microsomal cytochrome b 5 reductase from rabbit liver.
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