EVIDENCE OF A COUPLED MECHANISM BETWEEN MONOAMINE-OXIDASE AND PEROXIDASE IN THE METABOLISM OF TYRAMINE BY RAT INTESTINAL MITOCHONDRIA

The relationship between monoamine oxidase (EC 1.4.3.4; MAO) and perox idase (EC 1.11.1.7; POD) in the metabolism of tyramine was investigate d using the crude mitochondrial fraction of rat intestine. When tyrami ne was incubated with mitochondria, the formation of the peroxidase-ca talysed oxidation product, 2,2'-dihydroxy-5,5'-bis(ethylamino)diphenyl (dityramine), identified by mass spectrometric analysis, was monitore d spectrophotometrically. After an initial lag time, the formation rat e of dityramine was linear up to 2 hr, amounting to 17 nmol X hr(-1) X mg protein(-1). A similar value was found for the oxidative deaminati on of tyramine catalysed by intestinal MAO. Either 10(-3) M clorgyline or 10(-3) M NaCN suppressed this reaction by completely inhibiting MA O or POD, respectively. In the former case, however, addition of H2O2 to the incubation mixture promptly started the reaction. Selective inh ibition of MAO-A and MAO-B was achieved with 3 X 10(-7) M clorgyline a nd 3 X 10(-7) M deprenyl, respectively, and the formation rate of dity ramine decreased in a corresponding manner. Preincubation with histami ne or spermidine reduced the lag time without affecting the steady-sta te reaction rate. Higher levels of dityramine were also detected in vi vo in rat intestine after oral administration of tyramine. These resul ts indicate that the peroxidase-dependent metabolism of tyramine in th e gut may be driven by H2O2 produced by MAO activities and that MAO-A is mainly responsible for this process, as well as for the oxidative d eamination of tyramine. (C) 1998 Elsevier Science Inc.