Biol. Pharm. Bull. 29(12) 2362—2366 (2006)

living organisms. This class of enzymes is divided into two subclasses: FAD-containing amine oxidases (FAD-AOs) and Cu-containing amine oxidases (Cu-AOs). Two types of mitochondrial monoamine oxidases, monoamine oxidases (MAO)A and MAO-B, and cytosolic polyamine oxidase are included in FAD-AOs. The Cu-AOs consist of retina amine oxidase, diamine oxidase (DAO), and semicarbazide-sensitive amine oxidase (SSAO), which is known as the tissue-bound or plasma form of amine oxidase. The classification of MAO into an A and B form was primarily based on Johnston’s presentation that both enzymes could be differentiated with the aid of the specific inhibition of MAO-A by clorgyline and of MAO-B by deprenyl. Classification by means of specific inhibitors has been more convenient for the identification of different types of amine oxidase. However, this concept has been considered somewhat of an oversimplification and the premises on which it was based have come under increasing scrutiny. One major problem was that serotonin and b-phenylethylamine, which are generally used to distinguish MAO-A and MAO-B, are not entirely specific for the respective MAO form. Another problem was that the substrate selectivity of MAO seems to be affected by minor cross-inhibition caused by specific inhibitors. SSAO essentially metabolizes amine compounds as well as MAO-A and MAO-B. However, the enzyme activity is not inhibited by clorgyline and deprenyl, whereas it is almost completely inhibited by semicarbazide. In contrast, semicarbazide has a weak inhibitory effect on MAO at a concentration of 0.1—1 mM. Clorgyline and deprenyl have been regarded as virtually inactive against the tissue-bound SSAO, nevertheless they showed moderate reversible competitive inhibition against bovine and human plasma SSAO at a concentration of 0.1—1 mM. DAO, belonging to the Cu-AOs, is responsible for histamine metabolism. Comparative substrate selectivity studies of DAO have not always been performed in detail. Aminoguanidine has been used for many years as an inhibitor of Cu-AO, but it is a relatively weak and nonselective inhibitor of SSAO. Thus, the results of the substrate selectivity of these amine oxidases using specific inhibitors are very confused and complex, and might not reflect natural biochemical properties. In this study, we utilized a single amine oxidase expressed in COS-1 cells to investigate the substrate selectivity of amine oxidases in the absence of chemical inhibitors.