Properties of a Semicarbazide-Sensitive Amine Oxidase in Human Umbilical Artery

Abstract

The metabolism of some aromatic amines by amine oxidase activities in human umbilical artery homogenates has been studied. The inhibitory effects of clorgyline showed that 5-hydroxytryptamine (5-HT) and tryptamine, 1 mM, were predominantly substrates for monoamine oxidase (MAO) type A, whereas MAO-A and B were both involved in the metabolism of β-phenylethylamine (PEA), 100 μM, and tyramine, 1 mM. About 20–30% of tyramine and PEA metabolism was resistant to 1 mM clorgyline, but sensitive to inhibition by semicarbazide, 1 mM, indicating the presence of a semicarbazide-sensitive amine oxidase (SSAO). Benzylamine, 1 mM, appeared to be metabolized exclusively by SSAO with a Km (161 μM) at pH 7.8 similar to that found for SSAO in other human tissues. Tyramine and PEA were relatively poor substrates for SSAO, with very high apparent Km values of 17.6 and 13.3 mM, respectively, when determined in the presence of clorgyline, 10−3 M, added to inhibit any metabolism of those amines by MAO activities. However, kinetic studies with benzylamine indicated that clorgyline, 10−3 M, also appears to inhibit SSAO competitively such that the true Km values for tyramine and PEA may be about 60% of those apparent values given above. No evidence for the metabolism of 5-HT or tryptamine by SSAO was obtained. The aliphatic amine methylamine was recently shown to be a specific substrate for SSAO in umbilical artery homogenates. We have used benzylamine and methylamine as SSAO substrates in histochemical studies to localize SSAO in tissue sections. Both amines promoted tissue staining which occurred predominantly over the medial layers of the vessel wall, and this staining was prevented by the presence of semicarbazide, 1 mM, but not by pargyline, 1 mM, in the reaction medium. The results support the notion that smooth muscle cells are an important site of SSAO activity in human blood vessels, and reinforce the possibility that methylamine, an endogenously-occurring amine, may be a better candidate as a physiological substrate for SSAO in man, than several aromatic biogenic amines so far examined.