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Monoamine oxidase activity (MAO) was determined in platelets in samples from a North-Swedish pedigree with a high frequency of schizophrenia. The MAO assay was performed with phenethylamine as substrate and with two concentrations of oxygen (0.06 and 0.12 mM). A tendency to lowered Vmax and increased apparent Km values was observed among the schizophrenic subjects, although there were no statistically significant differences in apparent Km and Vmax values between schizophrenics and their non-schizophrenic relatives.

Monoamine oxidase activity was assayed in platelets from 32 apparently healthy subjects using phenethylamine as substrate and two concentrations of oxygen (0.06 and 0.12 mM). Apparent Km (microM) and Vmax (nmol/mg protein/5 min) values were estimated from double reciprocal plots. The means of the Km and Vmax values were both increased by a factor of about 1.6 when the oxygen concentration was elevated from 0.06 mM (Km = 2.96 and Vmax = 4.47) to 0.12 mM (Km = 4.82 and Vmax = 7.18). The frequency distribution of the Vmax values was not clearly unimodal, especially at the higher oxygen concentration. The Vmax values of the women were significantly higher than those of the men but no sex difference was obtained for the Km values. The overall velocity (v) at 10 microM phenethylamine was highly correlated to the Vmax values both at low and high oxygen concentrations (r = 0.99 and 0.98, respectively).

Monoamine oxidase activity was assayed in platelets from 22 (8 monozygotic and 14 dizygotic) twin pairs. At least one twin of each pair had a serious psychiatric disorder (proband). Phenethylamine was used as substrate and the assay was performed at two different oxygen concentrations (0.06 and 0.12 nM). Apparent Km and Vmax values were estimated from double reciprocal plots. The correlation between proband and control twins was high for both apparent Km (r = 0.83) and Vmax (r = 0.69) when the enzyme was assayed at the higher concentration of oxygen. The monozygotic twins pairs showed very high correlations (Km r = 0.93; Vmax r = 0.86) as compared to dizygotic twins (Km r = 0.81, Vmax r = 0.50) and apparently healthy subjects (Km r = 0.46, Vmax r = 0.33). No difference in kinetic properties was found between schizophrenic and non-schizophrenic twins.