Molecular biology of neurological and psychiatric disorders. I. Effect of parkinsonism, age, sex and L-dopa on platelet monoamine oxidase.

Since there is substantial evidence for a nigrostriatal dopamine defect in Parkinson's disease and since monoamine oxidase (MAO) appears to be essential for the degradation of dopamine, we investigated whether this enzyme is involved in the pathogenesis of this disease or in the therapeutic action of L-dopa. To gain a solid basis for our analysis we studied some properties of platelet MAO, at present the only practical in vivo source for human MAO. Substrate and inhibitor pattern clearly pointed to a predominant B-type character of this enzyme. By using 3 substrates, m-iodobenzylamine, p-methoxybenzylamine, and tyramine, we found a marked age and sex difference in MAO activity. In untreated parkinsonian patients, platelet MAO was slightly reduced as compared with age- and sex-matched controls. Treatment with L-dopa induced a further reduction of platelet MAO activity in both sexes, but more in men than in women. We conjecture that the action of L-dopa on parkisonian patients is twofold: L-dopa is known to enhance the release of gonadotropins and thus to increase the production of sex hormones which in turn are capable of reducing MAO activity. Dopamine, formed from L-dopa, may thus have a better chance for survival in reaching the dopaminergic receptor. A new form of therapy, based on this concept, is proposed.