Further Studies on the Metabolism of Methylamine by Semicarbazide-sensitive Amine Oxidase Activities in Human Plasma, Umbilical Artery and Rat Aorta

Abstract

An ion exchange radiochemical assay has been developed to study the deamination of [l4C]methyIamine (MA) in homogenates of rat aorta and human umbilical artery, as well as in samples of human plasma. MA metabolism was found to be inhibited almost completely by 1 mM semicarbazide, but virtually unaffected by 0.1 mM clorgyline, suggesting that MA is a substrate for the semicarbazide-sensitive amine oxidase (SSAO) activities which also metabolize benzylamine (BZ) in these sources. Mean Km values for MA metabolism by aorta, umbilical artery and plasma were 182, 832 and 516 μM, respectively, with corresponding Vmax values in aorta and umbilical artery of 100 and 590 nmol (mg prot.)−1 h−1, and in plasma of 48 nmol (mL serum)−1 h−1. Kinetic constants determined for [14C]BZ metabolism in plasma (by an organic solvent extraction assay) and in umbilical artery (by the ion exchange assay) yielded mean Km values of 225 μM (plasma), 222 μM (umbilical artery), and Vmax values of 28 nmol (mL serum)−1 h−1 (plasma) and 377 nmol (mg prot.)−1 h−1 (umbilical artery). The deamination of [14C]MA was inhibited competitively by unlabelled BZ, with Ki values in umbilical artery and plasma of 220 and 172 μM, respectively. Also, metabolite formation from mixtures of [14C]BZ (200 μM) and [14C]MA (800 μM) was extremely close to that predicted for a single enzyme capable of metabolizing two alternative substrates in a competitive fashion. β-Aminopropionitrile was found to be a reversible, competitive inhibitor (Ki of 165 μM) of [14C]MA metabolism in umbilical artery, inhibitory properties characteristic of those found previously for the effects of β-aminopropionitrile upon BZ-metabolizing SSAO activities in other tissues. The possibility that vascular and plasma SSAO activities may be involved in the endogenous turnover of the biogenic amine MA is discussed.