Intestinal metabolism of tyramine by both forms of monoamine oxidase in the rat

The two forms of monoamine oxidase (MAO) in rat intestine and brain homogenates were found to have different K_m and V_max values towards tyramine. The K_m values for the A-form of the enzyme towards this substrate were around 120 μM in both cases, whereas the values for the B-form were about 240 μM. 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 K_i values of 3.4 μM 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.