Inherited variations in monoamine oxidase (MAO) activity are thought to affect human behavior and expression of disease. The present study has established the chromosomal location of one of the structural genes coding for this enzyme. Mapping was carried out by somatic cell hybridization between normal human skin fibroblasts and mouse neuroblastoma cells. Selective media for growth of cells with or without hypoxanthine phosphoribosyltransferase (HPRT) activity were used to obtain hybrid lines which had retained or lost the human X chromosome, respectively. Cytogenetic techniques, isozyme analysis, and limited proteolysis and peptide mapping of [3H]pargyline-labeled MAO were used to characterize hybrid lines. With one exception, only lines containing the human X chromosome and human forms of two X-linked enzymes (phosphoglycerate kinase and glucose-6-phosphate dehydrogenase) expressed the human form of the flavin polypeptide of type A MAO. The exceptional hybrid line contained a putative translocation of part of the human X chromosome, since it expressed human forms of both MAO and phosphoglycerate kinase but neither the human form of glucose-6-phosphate dehydrogenase nor HPRT activity. This evidence indicates that the structural gene for the flavin polypeptide of MAO-A is on the human X chromosome. This represents the first chromosomal assignment of a human gene coding for an enzyme of neurotransmitter metabolism. This information will help to elucidate the structure of MAO and modes of its inheritance in the human population.

Abstract— Monoamine oxidase (MAO) activity against tryptamine was compared in a number of continuous rodent lines, including neuroblastoma, hepatoma, melanoma, nephroma, sarcoma and L cells. Activities against tryptamine varied over 300‐fold in homogenates of different lines, being highest in hepatoma line MH1C1 and lowest in a neuroblastoma line lacking hypoxanthine phosphoribosyltransferase (HPRT) activity. The amount, but not the type, of MAO activity varied with the stage of growth in homogenates of neuroblastoma and hepatoma cells. Measurements of succinate‐cytochrome c reductase (SCCR), another mitochondrial enzyme, also showed 20‐fold variations between lines, being highest in neuroblastoma line N1E‐115 and lowest in hepatoma line MH1C1; SCCR and MAO activities appeared to be regulated independently. The relative proportions of the A and B types of MAO activity were determined in homogenates and living cultures. Clorgyline inhibition of tryptamine deamination in homogenates indicated that in all lines except MH1C1, greater than 95% of the MAO activity was of the A type. In MH1C1 homogenates, using clorgyline or deprenyl, 40–70% of the activity appeared to be of the A type and 30‐60% of the B type. In cultures of neuroblastoma N1E‐115 cells, deamination of tryptamine and dopamine was sensitive to inhibition by low concentrations of clorgyline, indicating that the A type of activity is present intracellularly. as in homogenates. In MH1C1 hepatoma cultures, tryptamine deamination showed a biphasic sensitivity to clorgyline. We interpret this to mean that A and B types of MAO activity occur together in living hepatoma cells.

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