Properties and functions of the semicarbazide-sensitive amine oxidases.

The semicarbazide-sensitive amine oxidases (SSAOs) are grouped together under the Enzyme Commission classification, EC 1.4.3.6 [amine: oxygen oxidoreductase (deaminating) (copper containing)] and have a history dating back to the discovery of histaminase by Best in 1929 [l]. This enzyme was subsequently renamed diamine oxidase (DAO), to help establish its separate identity from monoamine oxidase (EC 1.4.3.4; MAO). Doubts about this terminology were expressed when it was shown that some long chain diamines were substrates for MA0 and not DAO, while other enzyme activities capable of deaminating mono-, diand polyamines were beginning to appear (see [2] for review). This led Blaschko [3] to introduce terminology based on the sensitivity of a particular amine oxidase to carbonyl reagents such as semicarbazide and hydroxylamine. One group contained ‘enzymes resistant to inhibition by carbonyl groups’ and included the classical MA0 enzymes as well as a number of other FAD-dependent enzymes. The other group contained ‘enzymes inhibited by carbony1 reagents’. Its members included DAO and enzymes in plants and bacteria as well as enzymes in blood plasma that were capable of deaminating monoamines such as benzylamine and ruminant