Evidence of a coupled mechanism between monoamine oxidase and peroxidase in the metabolism of tyramine by rat intestinal mitochondria

Abstract

The relationship between monoamine oxidase (EC 1.4.3.4; MAO) and peroxidase (EC 1.11.1.7; POD) in the metabolism of tyramine was investigated using the crude mitochondrial fraction of rat intestine. When tyramine was incubated with mitochondria, the formation of the peroxidase-catalysed oxidation product, 2,2′-dihydroxy-5,5′-bis(ethylamino)diphenyl (dityramine), identified by mass spectrometric analysis, was monitored spectrophotometrically. After an initial lag time, the formation rate of dityramine was linear up to 2 hr, amounting to 17 nmol × hr −1 × mg protein −1. A similar value was found for the oxidative deamination of tyramine catalysed by intestinal MAO. Either 10 −3 M clorgyline or 10 −3 M NaCN suppressed this reaction by completely inhibiting MAO or POD, respectively. In the former case, however, addition of H 2O 2 to the incubation mixture promptly started the reaction. Selective inhibition of MAO-A and MAO-B was achieved with 3 × 10 −7 M clorgyline and 3 × 10 −7 M deprenyl, respectively, and the formation rate of dityramine decreased in a corresponding manner. Preincubation with histamine or spermidine reduced the lag time without affecting the steady-state reaction rate. Higher Ievels of dityramine were also detected in vivo in rat intestine after oral administration of tyramine. These results indicate that the peroxidase-dependent metabolism of tyramine in the gut may be driven by H 2O 2 produced by MAO activities and that MAO-A is mainly responsible for this process, as well as for the oxidative deamination of tyramine.