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Avian and Mammalian Skeletal Muscle Mitochondrial Enzyme and Electron Transport Chain Activities

Birds are able to fuel flight, a moderate‐high intensity exercise, with fat, while mammals depend on carbohydrates for fuel at the same relative intensity. This difference in fuel selection persists in mitochondria (MITO) isolated from the skeletal muscle of these species, suggesting fuel selection can be controlled at the level of the MITO matrix. To compare the capacity rat and sparrow MITO to harvest electrons to the ability of the electron transport chain (ETC) to oxidize them, we assessed the activities of CAC and related enzymes, the Vmax of NADH oxidation from Complex I → IV, and cytochrome b content via myxothiazol titration. The Vmax of NADH oxidation (1074 ± 118 vs 516 ± 42 nmol O2·mg−1·min−1), as well as the cytochrome b content (1.50 vs 1.06 nmol·mg mitochondrial protein−1), were greater in sparrow compared to rat. These data suggests a greater capacity of sparrow ETC to oxidize electrons compared to the capacity of the dehydrogenases to produce them. This balance may result in a lower oxidation‐reduction potential in sparrow MITO, decreasing the inhibition of NADH on β‐oxidation and allowing fatty acid oxidation to proceed.