Abstract
The deamination of methylamine (MA) by amine oxidase enzymes has been studied and compared with that of benzylamine (BZ) in homogenates of rat aorta and human umbilical artery by means of a radiochemical assay to estimate the radiolabelled deaminated metabolites produced, and also a spectrophotometric assay to measure H 2O 2 formation during the metabolism of these substrates. The effects of various inhibitors used in these assays suggest that a semicarbazide-sensitive amine oxidase (SSAO) is predominantly if not wholly responsible for the deamination of both MA and BZ in these tissues. MA was found to have a relatively higher apparent K m (102 μM in aorta; 779 μM in umbilical artery) than BZ (6.8 μM in aorta; 207 μM in umbilical artery) for metabolism by SSAO in these tissues. However, these large differences between species in the apparent K m values for each amine indicate that the biochemical properties of SSAO in human and rat vasculature are not identical. SSAO in human umbilical artery was particularly active towards MA, with a V max which was approximately 70% greater than that for BZ as substrate, whereas in rat aorta the V max for MA was around 60% of that for BZ. MA is known to occur endogenously in man and other species, and the possibility that it may be a physiological substrate in vivo for SSAO is discussed.
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