Studies on the monoamine oxidase of monkey brain

Abstract

A study of the enzyme monoamine oxidase (MAO) was carried out in the monkey brain. From the monkey brain mitochondrial fraction a lysolecithin-soluble form of the enzyme (MAO_s) and an insoluble form (MAO_p) were isolated. The latter required freezing, thawing and sonication for solubilization. Both these forms of MAO had identical electrophoretic mobilities, a pH optimum of 7 and comparable thermal stabilities. The enzyme which could not be solubilized and which remained membrane-bound also gave the same pH optimum of 7 and a similar thermal stability profile. Both MAO_s and MAO_p had comparable K_m values of 2.2 × 10^-5 m and 5.0 × 10^-5 m respectively when using tyramine as substrate and 7.4 ×10^-5 M and 7.7 × 10^-5 m respectively with benzylamine as substrate. The K_m values of the membrane-bound enzyme were 1.0 × 10^-5 m with tyramine as substrate 2.5 × 10^-5 m with benzylarnine as substrate. The MAO inhibitors, tranylcypromine, isocarboxazid and iproniazid inhibited both MAO_s and MAO_p to approximately the same extent. The extent of inhibition of the membrane-bound enzyme however, was relatively different with all three inhibitors. Immunodiffusion techniques using anti-MAO_p indicated the immunological identity among MAO_p, MAO_s and the mitochondrial fraction. Substrate specificity and substrate competition experiments as well as the use of the selective inhibitor pargyline indicated the presence of both the 'A' and 'B' type of activity in the MAO isolated from monkey brain.