Deamination of 5-hydroxytryptamine by both forms of monoamine oxidase in the rat brain

Km and Vmax values of monoamine oxidase (MAO) A and B towards 5-hydroxytryptamine were determined for rat brain homogenates after the in vitro inhibition of one of the two forms by the selective inhibitors clorgyline and l-deprenyl. Km values of 178 and 1170 microM, and Vmax values of 0.73 and 0.09 nmol . mg protein-1 . min-1 towards 5-hydroxytryptamine were found for MAO-A and -B, respectively. The Ki for 5-hydroxytryptamine as a competitive inhibitor of beta-phenethylamine oxidation by MAO-B was found to be 1400 microM. The significance of these findings is discussed.