any one of several enzymes of the oxidoreductase class that splits hydrogen ions in animal and plant cells from the reduced forms of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). In doing so, the electron is transferred to the iron-containing proteins of the cytochrome, for example, to the cytochrome c from the microsomes during the functioning of NADPH cytochrome c reducíase.

All cytochrome reductases are flavoproteins and constituents of multienzyme complexes forming a chain of respiratory enzymes and hydroxylation systems in plant and animal cells. Cytochrome reductases from mitochondria are not individual enzymes but a complex of dehydrogenases and electron carriers like ubiquinones, which are localized in the membrane in a very definite way.