The monoamine oxidases play a vital role in the metabolism of biogenic amines in the central nervous system and in peripheral tissues. Using oligonucleotide probes derived from three sequenced peptide fragments, we have isolated cDNA clones that encode the A and B forms of monoamine oxidase and have determined the nucleotide sequences of these cDNAs. Comparison of the deduced amino acid sequences shows that the A and B forms have subunit molecular weights of 59,700 and 58,800, respectively, and have 70% sequence identity. Both sequences contain the pentapeptide Ser-Gly-Gly-Cys-Tyr, in which the obligatory cofactor FAD is covalently bound to cysteine. Based on differences in primary amino acid sequences and RNA gel blot analysis of mRNAs, the A and B forms of monoamine oxidase appear to be derived from separate genes.Monoamine oxidases A and B [MAO A and MAO B, respectively; amine:oxygen oxidoreductase (deaminating) (flavin-containing), EC 1.4.3.4] in the central nervous system and in peripheral tissues catalyze the oxidative deamination of neuroactive and vasoactive amines (1) and the oxidation of xenobiotics, including the parkinsonism-producing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (2, 3). These enzymes, which are integral proteins of the outer mitochondrial membrane (4), are distinguished by differences in substrate preference (5), inhibitor specificity (6), tissue and cell distribution (7), and immunological properties (8,9). MAO A preferentially oxidizes the biogenic amine serotonin and is inactivated irreversibly by the acetylenic inhibitor clorgyline. MAO B preferentially oxidizes phenylethylamine and benzylamine and is inactivated by the irreversible inhibitors pargyline and deprenyl. The level of MAO activity in almost all human tissues consists of a mixture of both forms of the enzyme, but placental tissue contains predominantly MAO A (10), whereas platelets and lymphocytes express only MAO B (11,12). MAO A and B from several tissue sources and species appear to consist of two subunits with approximate molecular masses of 60 kDa (13,14 (15,19). Peptide maps obtained from proteinase digestion of [3H]pargyline-labeled crude or partially purified MAO A and B suggest that these enzymes differ in their amino acid sequences (17,20). Furthermore, differences in degrees of photo-dependent inactivation of these two enzymes suggest the existence of conformational or structural differences in their active sites (21-23).To clarify the molecular basis of structural and functional differences between these important enzymes, we have isolated and characterized cloned cDNAsI encoding these proteins. The nucleotide and deduced amino acid sequences for human liver MAO A and B show that these two proteins are derived from separate genes.
MATERIALS AND METHODSConstruction and Screening of the Human Liver cDNA Library. A Agt1O library was constructed from poly(A)+ mRNA isolated from human liver (24). The phage library contained 2 x 106 individual clones of which 5 x 105 clones were subjected to hy...
