Abstract
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.
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