It is becoming increasingly apparent that many well-known enzymes have alternative non-enzymic functions. Similarly, several proteins that were identified as having non-catalytic functions were subsequently found to have enzyme activities. Some examples are considered to illustrate the diversity of alternative functions. The semicarbazide-sensitive amine oxidase (EC 1.4.3.6) is considered in more depth as an example. It was originally believed to be a detoxifying enzyme, but the reaction products may have important signalling functions. Furthermore, this enzyme, from some sources, also behaves as a vascular-adhesion protein. Finally, the challenges posed by such multiplicity of functions for the interpretation of genetic deletion, in vivo inhibition and the development of functional protein databases are briefly considered.

Abbreviations used: SSAO, semicarbazide-sensitive amine oxidase; MAO, monoamine oxidase; TOPA, 2,4,5-trihydroxyphenylalanine; 5-HT, 5-hydroxytryptamine; VAP-1, vascular-adhesion protein 1.

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Author notes

The Behaviour of Enzymes in Cells, a Biochemical Society-supported meeting held at Trinity College Dublin, 14 September 2002