Selective effects on catalysis by the multiple forms of monoamine oxidase produced by interactions of acidic phospholipids with mitochondrial membranes

The effect of acidic phospholipids on the A and B multiple forms of membrane-bound mitochondrial monoamine oxidase has been investigated by incubating liposomes with isolated rat liver mitochondrial outer membrane preparations at lipid:protein ratios of 0.01 to 1. A strong inhibition of monoamine oxidase B was observed with phosphatidylserine and a moderate activation of monoamine oxidase A with phosphatidylinositol, while cardiolipin had no significant effect on either form. The specificity of phosphatidylserine inhibition for monoamine oxidase B was also confirmed in mitochondrial outer membrane isolated from tissues containing exclusively the A or B form of the enzyme (human placenta and bovine liver). Levels of incorporation were comparable for all the phospholipids and tissues studied and could not account for the different effects observed. Inhibition of monoamine oxidase B was found to be similar in an intact mitochondria preparation to that observed in the isolated outer membrane. A recent report of activation of both monoamine oxidase forms in delipidated whole mitochondria by the acidic phospholipids was re-examined and found to involve release of monoamine oxidase from the mitochondria. The details of the effects of phosphatidylserine and phosphatidylinositol on membrane-bound monoamine oxidase are consistent with the concept of the multiple forms as two distinct peptides, and suggest a second possible mode of in vivo regulation of substrate specificity.