Biochemical and immunological studies of the monoamine-oxidizing activities of cultured human cells.

The enzyme monoamine oxidase [monoamine+ oxidoreductase (deaminating), EC 1.4.3.41 is believed to exist in several isoenzymic forms (see Sandler & Youdhim, 1972). On the basis of differential inhibitor sensitivity, Johnston (1968) suggested the existence of two forms of monoamine oxidase, designated A and B. Form A was highly sensitive to the inhibitor clorgyline (M & B 9302), whereas the form B was much less sensitive. These two forms also differed in substrate affinities, the A form showing a greater affinity for 5-hydroxytryptamine. Multiple enzymically active forms can be resolved by electrophoresis in polyacrylamide gels (Sandler & Youdhim, 1972). These forms exhibited different substrate affinities and inhibitor sensitivities. However, they were found to be conterted into a single form by treatment with chaotropic agents, suggesting that they represent a single enzyme species whose behaviour is modulated by attached membrane fragments (Houslay & Tipton, 1973). Other studies have attempted to define different forms of monoamine oxidase by immunological procedures. One of these (McCaulay & Racker, 1973) indicated that two immunologically distinct forms of the enzyme exist in bovine brain, one of which cross-reacts with bovine Iiver monoamine oxidase and exhibits low activity with 5-hydroxytryptamine and noradrenaline.