Influence of Mitochondrial Radical Formation on Energy‐Linked Respiration

Abstract

The earlier observation that aging promotes peroxidation of lipids within inner mitochondrial membranes by a radical‐generating mechanism has provided a model for studying the relationship of membrane alterations to energy‐linked respiration. Electron transport was impaired as a function of age only in the presence of substrates oxidized within the inner mitochondrial membrane. Respiratory control ratios and P:O ratios decreased regardless of substrate identity. Arrhenius kinetic data revealed age‐dependent shifts of temperature breaks only for membrane‐bound enzymes including oligomycin‐sensitive ATPase. Specific activities decreased for all enzymes studied except cytochrome oxidase and the glutamate oxidase system. Molecular motion data of spin‐labelled fatty acid analogues associated with phospholipid bilayer from inner mitochondrial membranes exhibited a decrease in lipid fluidity in membranes from aged rats. The exposure of isolated mitochondria to a O 2 − ‐radical generating system brought about identical changes in energy‐linked respiration and oxidative phosphorylation as did aging in the animal. The effect was completely inhibited only in the presence of both superoxide dismutase and catalase, indicating the participation of O 2 − ‐radicals and H 2 O 2 in causing the observed structural and functional changes. The results support the concept of an active mechanism in mitochondria of living tissues which can contribute to pathophysiological processes by a sequential reaction involving radical‐induced changes in the hydrophobic bonding of lipid‐dependent enzyme systems.