Reduced platelet MAO activity and vulnerability to psychiatric disorders.

Platelet monoamine oxidase (MAO) is a genetically controlled mitochondrial enzyme that metabolizes amines in the brain by deamination. We investigated the relationship between MAO activity and the presence or absence of psychopathology by determining MAO activity and the psychiatric diagnosis in 61 first-degree relatives of 31 patients with bipolar (manic-depressive) illness. Relatives who themselves were diagnosed as having bipolar I illness or alcoholism had a significantly lower mean MAO activity level than did other relatives or 255 normal individuals. These findings indicate that a relationship exists within pedigrees between MAO activity and psychiatric disorders. Reduced MAO activity may play a role in vulnerability to some types of psychopathology.