Synaptosomal tryptophan-hydroxylase activity in rat brain measured over 24-hours

The activity of tryptophan-5-hydroxylase was measured in synaptosomes prepared from rat wholebrain homogenates. Activity was assessed by incubating the synaptosomal preparation with tryptophan radioactively labelled with (14)C on the carboxylic acid group; the f(14)C-CO(2)f released by decarboxylation of the 5-hydroxytryptophan formed from hydroxylation of tryptophan was measured. Measurement of tryptophan-5-hydroxylase activity in synaptosomes prepared from rats killed at different clock-hours provided no evidence of any significant circadian rhythm in enzyme activity. These results suggest that the synaptosomal enzyme is protected from factors, such as the postulated 'inhibitory factor', which generate a circadian oscillation in the activity of the soluble enzyme.