• Concomitant measures of blood indole metabolism were conducted in 33 chronic schizophrenics who showed significantly elevated mean platelet serotonin (5-HT) values and lower platelet monoamine oxidase (MAO) and plasma amine oxidase activities than normals. When subdivided according to Research Diagnostic Criteria diagnosis, the 20 chronic undifferentiated (CU) schizophrenics showed these same deviations from normal; the 13 chronic paranoid (CP) schizophrenics also had significantly higher mean 5-HT values but significantly lower plasma concentrations of total and bound tryptophan. In CP schizophrenics, platelet MAO activity, but not plasma amine oxidase activity, was significantly lower than in CU schizophrenics and controls. Hyperserotonemia occurred in 11 of the chronic patients (33%); nine were CU schizophrenics. In the latter, total tryptophan concentration was significantly lowered. Hyperserotonemia was not associated with reduced liver tryptophan pyrrolase activity or platelet MAO or plasma amine oxidase activities; rather, it may be a consequence of enhanced tissue tryptophan uptake and utilization.
Freedman DX, Belendiuk K, Belendiuk GW, Crayton JW. Blood Tryptophan Metabolism in Chronic Schizophrenics. Arch Gen Psychiatry. 1981;38(6):655-659. doi:10.1001/archpsyc.1981.01780310055005