Multiple forms of mitochondrial monoamine oxidase in the Novikoff hepatoma and rat liver

The interaction of lipid with the two forms of monoamine oxidase was determined to be different. Arrhenius plots of monoamine oxidase activity revealed that only type B activity was sensitive to a bulk phase transition of the outer mitochondrial membrane. Delipidation with organic solvent, phospholipase and detergents resulted in selective inactivation of type A monoamine oxidase. Reconstitution of partially purified enzyme and addition of lipid to delipidated mitochondria demonstrated that type A activity could be reactivated partially by selected phospholipids. However, the maximum reactivation occurred with different lipids with the two reactivation methods. The results suggested that a net negative charge of the lipid is required for lipid to initially interact with delipidated type A monoamine oxidase, but that the charge of the lipid is not important in activating the enzyme. Type B activity seems to be dependent of the fluidity of the lipid surrounding the enzyme but retains some activity after delipidation. The requirement of a specific lipid for either type of activity seems unlikely but could not be ruled out.

Treatment of the outer mitochondrial membranes from rat liver with digitonin in the absence of sonication resulted in the formation of two types of outer membrane fragments which were resolved by DEAE cellulose chromatography. The less anionic fraction was enriched in type B monoamine oxidase activity while the more anionic fraction was enriched in type A monoamine oxidase activity. Equilibrium sedimentation showed that both fractions were of similar density and contained most of the outer membrane phospholipid. Gel filtration showed that the molecular weight of the fragments exceeded 4 x 10⁶.

Investigation of monoamine oxidase activity in coupled mitochondria showed that the activity was inhibited by rapid respiration. This study is not consistent with the hypothesis that electrons flow from monoamine oxidase to the electron transport chain.

A new purification method for pig liver monoamine oxidase is described. Immunological technique showed that monoamine oxidase from pig liver and beef kidney share identical antigenic sites.