Cytochrome c probably belongs in the class of adaptive enzymes, in the sense that the cellular concentration of this chromoprotein appears to respond to the demand for its function in the utilization of oxygen (3). The substrate-induced synthesis of mammalian cytochrome c may be deduced from the findings of Drabkin (2, 4, 5) in experimental hypothyroid and hyperthyroid (thyrotoxic) rats, in which decreases and increases of oxygen consumption were attended by marked parallel alterations in the quantity of the tissue pigment. A similar inference for the cytochrome c of brewers' yeast adapting to life in an atmosphere of oxygen was drawn by Chin (6) in Keilin's laboratory and by Ephrussi and Slonimski (7) for the chromoprotein in bakers' yeast grown anaerobically, then exposed to oxygen. These investigators correlated increases in the cyanide-sensitive respiratory exchange with qualitative spectroscopic changes which occurred when these yeasts in the non-proliferative state were shifted from their anaerobic environment to oxygen. Before exposure to oxygen two absorption bands were present, one at 556 to 566 microns attributed to cytochrome b1 and a second weak band at 535 to 599 microns ascribed to cytochrome a1. The latter may be owing to the more recently described yeast hemoglobin (8). After exposure to oxygen the above spectrum was replaced by another, characteristic of cytochromes a, b, and c. Similar spectroscopic changes in anaerobic cells adapting to aerobic existence had been discovered much earlier by Tamiya (9) in Aspergillus oryzue and by Fink (10) in brewers' yeast, but they had been attributed by these early workers to the selective growth and multiplication of certain cells already present in the anaerobic state. Recently, it has been found that two other hemin chromoproteins in yeast, cytochrome peroxidase (11) and catalase (12), are adaptively produced on exposure to oxygen. Aside from ancillary considerations of interest, the present study was undertaken with the view of securing quantitative information on the biosynthesis of oytochrome c in yeast adapting to oxygen by means of labeling with glycine-2-C14. The incorporation of this isotope in mammalian cytochrome c had previously been demonstrated (13, 14). It was also desirable to learn whether adaptation to life in oxygen involved a de nova total synthesis of cytochrome c, whether the process was largely confined to the fabrication of specific cytochrome c protein, with hemin transfer from pre-existing chromoproteins, suggested as a possibility by earlier workers (6, 7), or whether inert precursors present before adaptation were transformed into cytochrome c. The evidence which will be presented demonstrates that yeast cells adapting to oxygen have a highly active biosynthetic capa- bility for cytochrome c and that the hemin moiety of this chromoprotein can be fabricated de novo in such cells.

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Journal Article

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YCAS M, DRABKIN DL

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