1. Subcellular fractionation of sphaeroplasts produced at different stages during the first 4h of respiratory adaptation of anaerobically grown glucose-de-repressed Saccharomyces carlsbergensis gave mitochondrial fractions that contained all the detectable c- and a-type cytochromes. 2. The rates of cytochrome formation were studied; individual cytochromes were produced at different rates so as to give respiratory chains having widely differing cytochrome ratios. A CO-reacting haemoprotein other than cytochrome a3 also increased throughout 8h of respiratory adaptation. 3. Even after short periods of aeration, organisms contained mitochondria in which cytochrome–cytochrome interactions and the reaction of cytochrome a3 with O2 proceeded at rates almost as fast as in organelles from aerobically grown cells. 4. The technique of flow–flash photolysis enabled kinetic resolution of the reoxidation of cytochromes a3 and a to be achieved and their individual contributions to extinction changes in the Soret region were assessed. The ratio cytochrome a3/cytochrome a increased over the early stages of adaptation.

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