Plasma MAO activity unrelated to genetic vulnerability to primary affective illness.

Plasma monoamine oxidase (MAO) activity was studied in 37 patients with primary affective illness, 105 relatives and spouses, and 37 normal controls, using carbon 14-labeled benzylamine as substrate. No differences between patients and controls were found, and there was no association of altered enzyme activity with the ill or well persons within families. Plasma MAO activity is heritable, and there are no effects of sex or of age in the observed age range (18 to 82 years).