All enzymes that catalyse oxido-reductions belong in this class. The substrate oxidized is regarded as a hydrogen or electron donor. The classification is based on 'donor:acceptor oxidoreductase'. The common name is 'dehydrogenase', wherever this is possible; as an alternative, 'acceptor reductase' can be used. 'Oxidase' is used only where O2 is an acceptor. Classification is difficult in some cases, because of the lack of specificity towards the acceptor.