Biochemical differences between subsarcolemmal and interfibrillar mitochondria from rat cardiac muscle: effects of...

Differences in oxidative metabolism between subsarcolemmal and interfibrillar heart mitochondria were investigated. Interfibrillar mitochondria oxidized substrates donating reducing equivalents at Complex I (NADH-CoQ reductase), Complex II (succinate-CoQ reductase), and Complex III (CoQH2-cytochrome c reductase) more rapidly than did subsarcolemmal mitochondria. There was no difference in oxidation of substrates entering the electron transport chain at Complex IV (cytochrome c oxidase). Differences expressed in normal-ionic-strength medium at Complexes II and III but not I were eliminated in low-ionic-strength medium. The concentrations of cytochromes and activities of NADH and cytochrome c oxidase were virtually the same in the two populations. In permeabilized mitochondria, activities of succinate-duroquinone and TMPD plus ascorbate oxidase were significantly lower in the subsarcolemmal mitochondria. Differences in membrane permeability between the populations were suggested by the greater permeability of subsarcolemmal mitochondria to exogenous NADH. The influence of isolation buffers and preparative procedures on the two classes of mitochondria were also examined. Characteristic biochemical and morphological properties of the two populations were unchanged by exposing each to the preparative procedure used to isolate the alternate population; the oxidative performance of the two populations cannot be equalized by experimental manipulation.