Decreased Platelet Monoamine Oxidase Activity in Chronic Schizophrenia, Shown With Novel Substrates

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Abstract
• Platelet monoamine oxidase (MAO) was kinetically evaluated in chronic schizophrenics and matched controls, using substrates of major physiologic importance and substrates of particular interest in the study of schizophrenia, such as serotonin (5-HT),N,N-dimethyltryptamine(DMT), 5-methoxytryptamine (5-MT), and dopamine (DA). Substrates were measured at six concentrations; values for maximal velocity (Vmax) and Michaelis constant (Km) were obtained by using Lineweaver-Burk plots. TheVmaxwas decreased for all substrates in chronic schizophrenia and the Kmwas decreased for DA, 5-HT, and DMT, but remained unchanged for 5-MT. The value of Km/Vmaxwas similar for schizophrenics and normal persons when DA, 5-HT, and DMT were used as substrates, which may indicate that "uncompetitive" inhibition is responsible for the observed decrease in activity among chronic schizophrenics. The finding of a decreased Vmaxbut unchanged Kmwith 5-MT would be consistent with noncompetitive inhibition.