Depression and monoamine oxidase

1. The first part of the paper is devoted to a critical review on the possible relationships between depression and monoamine oxidase. 2. This study describes the results of an investigation of MAO-activity in depression, using new approaches and methodology. This methodology was developed because the literature data indicated that a) previous results are difficult to compare because of varying methods and diverse target populations used; b) previous methodological deficiencies do not allow to draw definite conclusions about the relationship between MAO-activity and depression. 3. The present investigation selected 35 psychiatric patients according to clearly defined diagnostic criteria (20 endogenous depressive, 10 neurotic depressive, and 5 manic patients) matched to 25 healthy control-subjects. 4. The Michaelis-constant (Km), the maximum reaction speed (Vmax), and the 50% enzyme inhibition by tranylcypromine (IC50) of platelet-MAO were determined during and after recovery from the depressive or manic episode using 3 substrates (tyramine, tryptamine, and phenylethylamine). 5. The present investigation, in contrast to conventional methodology, utilized three different substrates at different concentrations. Significant correlations were demonstrated for the Vmax-values of each of the three substrates, whereas the Km and the IC50 (tranylcypromine)-values varied for each substrate. 6. The results show that there were no differences between the characteristics of the platelet-MAO in depressive or manic patients and those of normal subjects. Furthermore, treatment with tricyclic antidepressants had no effect on MAO-activity. A previous investigation indicated that the MAO-properties in human brain tissue were similar to that in human platelets.