any member of a class of enzymes, commonly known as dehydrogenases or oxidases, that catalyze the removal of hydrogen atoms and electrons from the compounds on which they act. Substances called coenzymes, associated with the oxidoreductase enzymes and necessary for their activity, accept the hydrogen and electrons, which—in metabolic systems of animals—eventually are transferred to oxygen. Other enzymes of this group catalyze such reactions as the oxidation of aldehydes and ketones to carboxylic acids and the dehydrogenation of amino acids.