Abstract
P:O ratios have been determined in mitochondria from the liver and skeletal muscle of chicks ranging in age from eight days of incubation through hatching. The values obtained remain constant throughout development and are the same as those of mitochondria from corresponding tissues of the adult. Developmental changes in respiratory control (RC) show variations depending on the tissue and the substrate oxidized. RC is constant in liver mitochondria with all substrates; in muscle mitochondria, RC increases with glutamate and ฮฑketoglutarate, while with malate, RC shows a marked decrease prior to hatching. Similarly, changes in specific activity of oxidation and phosphorylation in mitochondria vary with tissue and developmental stage. In liver mitochondria, specific activities of oxidation and phosphorylation with glutamate and ฮฑketoglutarate decrease during development; in muscle mitochondria, specific activities with these substrates increase. With malate, specific activities increase in liver, but decrease in muscle mitochondria; in the latter tissue this occurs at about the time of hatching. Added cytochrome c increases oxidation rate of muscle mitochondria, the increase being progressively greater the older the animals from which the mitochondria are derived.