The relationship between human monoamine oxidase A and B

Monoamine oxidase (EC 1.4.3.4) exists in at least two forms (termed type A and type B) with different substrate and inhibitor specificities (Johnston, 1968; Knoll & Magyar, 1972). Amine neurotransmitters are among a wide range of substrates (Blaschko, 1974) and abnormalities of monoamine oxidase activity may therefore contribute to neurological disease. Most studies of such diseases have looked at monoamine oxidase activity in platelets (see Sandler et al., 198 l), where only B-type activity is found (Donnelly & Murphy, 1977). In contrast, neurological tissue in many mammals has been reported to contain both Aand B-type activities (see Achee et al., 1977). Type A monoamine oxidase is sensitive to clorgyline at ~O-’M (Johnston, 1968) while type B is inhibited by 10-6M-deprenyl (Knoll & Magyar, 1972). Both of these inhibitors bind irreversibly into the active site of the enzyme molecule and compete with the substrate and with a third inhibitor, pargyline. Aand B-type activity can therefore be blocked selectively by preincubation with the appropriate inhibitor. 3H-labelled pargyline also binds irreversibly to the active site and can therefore be used to estimate the number of active sites or to follow the presence of the subunit containing the active site after gel electrophoresis. We have employed these approaches to investigate the nature of the monoamine oxidase activity found in human brain and compared it with the A-type activity of human placental mitochondria and with the B-type activity of human platelets. Mitochondria were prepared from human autopsy brain (temporal cortex) or fresh placenta (as described by Brown et al., 1980). Platelets were obtained from human blood (Brown et al., 1980). Assay of monoamine oxidase activity was performed using 13Hltryptamine as substrate and the number of active sites was estimated by incubating with 1.33 x 10-6~-[3Hlpargyline (Summers et al., 1982). Preincubation with M-deprenyl totally inhibits platelet monoamine oxidase activity and abolishes the specific binding of I ’Hlpargyline to active sites. This concentration of deprenyl has no effect on the activity or the pargyline binding of placental (A-type) enzyme. Studies on brain mitochondria with selective inhibitors indicated that about 60% of the activity is resistant to clorgyline (lo-’ M) but sensitive to deprenyl ( M) and presumably therefore represents type A enzyme. However, binding assays with [ 3H]pargyline employing similar inhibitors and concentrations revealed that most (>90%) of the monoamine oxidase molecules in the brain preparation were of type B. Thus the molecular turnover numbers for the two enzyme forms are quite different in this tissue. The turnover number calculated for the type B enzyme in brain (15 molecules/min) is very similar to the average value of 12 obtained with human platelets and is subject to the same limitations in estimation (Summers et al., 1982). The type A enzyme from brain had a molecular turnover number of about 300, which is comparable with a value Origin-