Catecholamine metabolism in the brain by membrane-bound and soluble catechol-o-methyltransferase (COMT) estimated by enzyme kinetic values

A kinetic model was constructed to reevaluate the catecholamine metabolism in hypothetical brain homogenates. Earlier published kinetic values of reco mbinant membrane-bound (MB-) COMT and soluble (S-) COMT were combined with data suggesting that MB-COMT represents 70% and 30% of total COMT protein i n human and rat brain, respectively. In the rat brain model L-DOPA and 3,4- dihydroxybenzoic acid were O-methylated mainly via S-COMT, while dopamine a nd noradrenaline, at low concentrations, were O-methylated slightly more by MB-COMT. In the human brain model dopamine and noradrenaline were metaboli zed primarily by MB-COMT. The ratio of meta (3-methoxy) over para (4-methox y) product formation from 3,4-dihydroxybenzoic acid was higher for MB-COMT than S-COMT. It is suggested that MB-COMT clearly predominates the O-methyl ation of dopamine and noradrenaline also in vivo. Additionally, meta/para r atios could support the enrichment of either isoform of COMT in a homogenat e sample. (C) 2001 Harcourt Publishers Ltd.