Endogenous modulation of monoamine oxidase in schizophrenic and normal humans.

Plasma from normal subjects and chronic schizophrenic patients produced nearly equal inhibitory effects on platelet and bovine brain monoamine oxidase (MAO) activity. Plasma from controls and patients with high platelet MAO activity caused less decrease in platelet MAO kinetic variables (Km, Vmax) than plasma from subjects with midrange or low MAO activity. When bovine striatal MAO was used, with serotonin as substrate, plasma from schizophrenic patients caused less decrease in Vmax than plasma from controls. These inhibitory effects are associated with the presence in plasma of one or more nonalbumin proteins with low molecular weight.