The class of all enzymes catalyzing oxidoreduction reactions (EC class 1). The substrate that is oxidized is regarded as a hydrogen donor. The systematic name is based on donor: acceptor oxidoreductase. The recommended name will be dehydrogenase, wherever this is possible; as an alternative, reductase can be used. Oxidase is only used in cases where O2 is the acceptor. (Enzyme Nomenclature, 1992, p9)