SUMMARY: Cells harvested from exponentially growing cultures of the budding yeast were fractionated according to their age in the cell cycle by isopycnic-zonal centrifugation in Urografin gradients. Activities of five enzymes were assayed in cell-free extracts, prepared using a French pressure cell. The activities of catalase and three enzymes of the mitochondrial inner membrane (NADH dehydrogenase, succinate dehydrogenase and cytochrome c oxidase) doubled during the cell cycle but showed complex oscillatory patterns. Acid phosphatase activity increased continuously during the cell cycle. Fourth-order finite difference analysis of low-temperature difference spectra showed that the levels of three -type cytochromes increased continuously during the cycle. In contrast, amounts of cytochromes and oscillated over one cycle, with cytochromes c and aa3 rising to two maxima per cycle in phase with cytochrome oxidase activity. Liganding of redox-active metals, and discontinuous synthesis and proteolysis of apocytochromes are discussed as possible mechanisms for cell cycle-dependent fluctuations in the composition and function of the respiratory chain.


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