Lipid-protein interactions in the outer membranes of yeast and rat liver mitochondria

The investigation of the structure and functional interrelationship of proteins and lipids of mitochondrial membranes has been centered on the energy-transducing inner membrane. We have focused our attention on two enzymes associated with the outer mitochondrial membrane, kynurenine hydroxylase and monoamine oxidase and have studied their activities in mitochondria isolated from yeast and rat liver cells which had been subjected to various experimental manipulations. Kynurenine hydroxylase activity, in contrast to monoamine oxidase, demonstrates biphasic Arrhenius kinetics which are characteristic of the lipid membrane milieu in both yeast and rat liver mitochondria. Nonionic surfactants, e.g., Tritons, drastically reduce kynurenine hydroxylase activity while increasing that of monoamine oxidase. Phospholipid depletion of these mitochondrial membranes by acetone extraction reduces kynurenine hydroxylase activity while increasing the activity of monoamine oxidase. The former activity is almost totally restored either by the addition of the extracted lipid or with asolectin. Loss of membrane neutral lipids has no effect on kynurenine hydroxylase activity, but treatment of mitochondria with phospholipase C effects a major decrease which is partially restored by asolectin. Yeast cells grown in increasing concentrations of sterol, manifest a skewed bell-shaped activity curve for kynurenine hydroxylase and a cyclic shift in the breaks in this enzyme's Arrhenius curves (11-9-11 °C). These results show that kynurenine hydroxylase activity depends on the percentage of unsaturation, the length of the fatty acid chain, as well as the sterol content for its expression and that monoamine oxidase is not dependent on lipids for its activity.