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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. The Vmax was decreased for all substrates in chronic schizophrenia and the Km was decreased for DA, 5-HT, and DMT, but remained unchanged for 5-MT. The value of Km/Vmax was similar for schizophrencis 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 Vmax but unchanged Km with 5-MT would be consistent with noncompetitive inhibition.

A radioimmunoassay using N-[3-(4-hydroxy-3-[4-hydroxy-3- [125I]iodophenylpropionyl)]-5-methoxytryptamine as tracer for determination of melatonin in the serum of different species is described. Melatonin antisera were raised in rabbits by immunization with a bovine serum albumin conjugate of N-[3-(2-aminoethyl)-5-methoxy indole] hemisuccinamide. A single high affinity, specific antiserum was obtained. In contrast to previous studies, the tracer was synthesised in one step in the absence of water, giving an excellent yield of highly pure product. No chromatographic purification step was needed. Polyethylene glycol in combination with goat antirabbit immunoglobulins was used to separate bound and unbound tracer. Sera were delipidized with Lipoclean prior the extraction of melatonin with diethyl ether. This sample preparation allows the determination of melatonin in the presence of widely varying amounts of lipids in human, rat and hamster serum. Using this extraction procedure, the sensitivity of the radioimmunoassay was approximately 1 ng/l of serum. Dilutions of sera and of synthetic melatonin gave the same parallel response in the radioimmunoassay. High performance liquid chromatography analysis of a serum extract showed only one immunoreactive peak co-eluting with synthetic melatonin. Characteristic diurnal rhythms of melatonin were observed in all species. All assay components including standards and serum controls are stable for at least 1 year at 4 degrees C, thus facilitating the determination of melatonin in a routine laboratory.

The relative potencies of analogues of tryptamine and 5-hydroxytryptamine have been determined on the rat fundus preparation. This tissue had an amine oxidase activity, which, in the homogenate, was able to inactivate both tryptamine and 5-hydroxytryptamine to about the same degree. Amine oxidase inhibitors potentiated the action of tryptamine and many analogues on the isolated rat fundus preparation, but not the action of 5-hydroxytryptamine or of other hydroxytryptamines. This suggested that, in the isolated organ, the amine oxidase was unable to inactivate 5-hydroxytryptamine, but could inactivate tryptamine, 5-methoxytryptamine and many others. These results may be explained if it is supposed that tryptamine entered the cell, but because of the polar hydroxyl group 5-hydroxytryptamine did not. This hypothesis is supported by the oil/water partition coefficients. The structure/activity of the various tryptamine derivatives is discussed in the light of this assumption.