Journal zyxwvutsrqponm of Neurochemistry zyxwvutsrqponm 37(2):363-372, August. Raven Press, New York zyxwvutsrq 0 1981 International Society for Neurochemistry 0022-3042’81/0201-0363/$02.75/0 zy Differences in the Structure of A and B Forms of Human Monoamine Oxidase Richard M. Cawthon, John E. Pintar, *Florence P. Haseltine, and Xandra 0. Breakefield Departments of Human Genetics and *Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut U.S.A. Abstract: [3H]Pargyline-labeled polypeptides associated with the A and B types of monoamine oxidase (MAO) activity in human tissues were analyzed by sodium dodecyl sulfate -polyacrylamide gel electrophoresis (SDS-PAGE). [3H]Pargyline was bound to M A 0 zyxwvut A in a crude mitochondrial fraction from the placental trophoblast of a male newborn and to MA0 B in blood platelets from the umbilical vein of the same newborn. [3H]Pargyline was also bound to MA0 A and B in a crude mitochondrial fraction from cultured skin fibroblasts of a male adult and to MA0 B in blood platelets from the same individual. Specific labeling of proteins associated with type A or type B activity in fibroblast cells was achieved by preincubation with selective B or A inhibitors, respectively. For all tissues, SDS-PAGE of [3H]pargyline-bound samples revealed a labeled protein band of apparent molecular weight 63,000 for MA0 A and 60,000 for MA0 B. When SDS-solubilized, [3H]pargyline-labeled M A 0 A and B proteins from the same male newborn were subjected to limited proteolysis and one- dimensional peptide mapping in SDS gels, different patterns of [3H]pargyline- labeled peptides were obtained. These findings indicate that distinct enzyme molecules are associated with the A and B types of human M A 0 activity. Key Words: Monoamine zyxwvut oxidase-Isoenzymes-Pargyline-Sodium dodecyl sul- fate -polyacrylamide gel electrophoresis-Peptide mapping. Cawthon R. M. et al. Differences in the structure of A and B forms of human monoamine oxidase. J. Neurochem. 37, 363-372 (1981). Monoamine oxidase (MAO) [monoamine: 0, oxi- doreductase, EC 1.4.3.41 is a mitochondrial outer membrane enzyme that degrades amine transmit- ters in the nervous system, and biogenic amines throughout the body. At least two types of MA0 activity have been distinguished, based on differ- ences in substrate specificity and inhibitor sensitiv- ity (Johnston, 1968; Knoll and Magyar, 1972; Neff and Yang, 1974). Type A MA0 preferentially deaminates 5-hydroxytryptamine and norepineph- rine and is more sensitive to inhibition by clor- gyline; type B preferentially degrades phenylethyl- amine and benzylamine and is more sensitive to in- hibition by deprenyl. In individuals of several species, some tissues express just one type of MA0 activity, while other tissues express both types. A flavin cofactor, covalently linked to the apoenzyme by a cysteinyl thioether, is required for both MA0 A and B activities (Erwin and Hellerman, 1967; Salach and Detmer, 1979). The molecular basis for the differences between the two types of MA0 ac- tivity is not known. Early studies of active enzyme preparations (Youdim et al., 1969; Collins et al., 1970; Houslay and Tipton, 1973) gave rise to two opposing views regarding the molecular nature of MA0 A and B. One theory is that different enzyme molecules are associated with the A and B types of activity; the Received December 2, 1980; accepted February zyxwvutsrq 15, 1980. Address correspondence and reprint requests to Dr. X. 0. Breakefield, Department of Human Genetics, Yale University School of Medicine, New Haven, Connecticut 06510. Abbrer-iutions zyxwvu used: MAO, Monoamine oxidase; MW, Molec- ular weight; SDS, Sodium dodecyl sulfate: SDS-PAGE, Sodium dodecyl sulfate-polyacrylamide gel electrophoresis. 363