Kinetic aspects of monoamine oxidase activity in twins with psychoses.

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.