Purification of endogenous modulators of monoamine oxidase from plasma.

A partial purification of endogenous modulators of monoamine oxidase-A (MAO-A) and MAO-B from human plasma has been achieved through Sephadex, ion-exchange and affinity chromatography. The MAO-A modulator had a molecular weight of about 4,000, was acidic or neutral, and did not contain the carbohydrate moiety mannose. It was thermostable but sensitive to trypsin treatment. It inhibited MAO-A activity in a sonically disrupted mitochondrial preparation prepared from bovine striatum in a dose-dependent manner and altered the kinetic parameters of MAO-A by increasing the Km and decreasing the Vmax. The concentration of the MAO-A modulator was higher than that of the MAO-B modulators, which had molecular weights of about 7,000, 14,000 and greater than 30,000 respectively. The MAO-B modulators increased the Km for tryptamine without changing the Vmax. These data indicate that human plasma contains peptides that may function as endogenous regulations of platelet MAO. The significance of this finding in relation to psychiatric disorders is discussed.