title: Intestinal metabolism of tyramine by both forms of monoamine oxidase in the rat.
abstract: The two forms of monoamine oxidase (MAO) in rat intestine and brain homogenates were found to have different Km and Vmax values towards tyramine. The Km values for the A-form of the enzyme towards this substrate were around 120 microM in both cases, whereas the values for the B-form were about 240 microM. As a consequence, the ratio of activities (MAO-A: MAO-B) towards tyramine are dependent upon the substrate concentration. The MAO-A-selective inhibitors, toloxatone and cimoxatone, were found to be competitive inhibitors of the oxidation of tyramine by the A-form of this enzyme in the rat intestine, with Ki values of 3.4 microM and 3.7 nM respectively. The significance of these results in relation to the "cheese effect", a pressor response to tyramine after monoamine oxidase inhibition, are discussed.
mesh_terms: D000818:Animals; D001921:Brain / Q000201:enzymology; D003010:Clorgyline / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007422:Intestines / Q000201:enzymology*; D007700:Kinetics; D008297:Male; D008995:Monoamine Oxidase / Q000378:metabolism*; D008996:Monoamine Oxidase Inhibitors / Q000494:pharmacology; D010080:Oxazoles / Q000494:pharmacology; D023303:Oxazolidinones; D051381:Rats; D011919:Rats, Inbred Strains; D014439:Tyramine / Q000378:metabolism*
chemical_list: D008996:Monoamine Oxidase Inhibitors; D010080:Oxazoles; D023303:Oxazolidinones; C010798:toloxatone; D008995:Monoamine Oxidase; D003010:Clorgyline; D014439:Tyramine