Abstract
Cytochrome P-450 is only produced during growth of Saccharomyces cerevisiae at high glucose concentrations, and not at low glucose concentrations even under semi-anaerobic conditions. The removal of oxygen during exponential growth phase of S. cerevisiae leads to a repression of cytochrome P-450 synthesis. Oxygen may be acting as a substrate inducer of yeast cytochrome P-450. Genetic analysis of yeast cytochrome P-450 production has revealed that biosynthesis of this enzyme is regulated by a single nuclear gene and one or more modifying factors. This regulatory gene may exert its effect through the intracellular cyclic AMP level. Growth of yeast in the presence of benzo(a)pyrene and several other compounds results in only a small change in cytochrome P-450 levels yet a dramatic increase in the activity of benzo(a)-pyrene hydroxylase as demonstrated by an increased Vmax and a decreased Km. This is interpreted as being due to the induction of a new cytochrome P-450 enzyme, demonstrating that multiple forms of cytochrome P-450 can occur in yeast. Yeast cytochrome P-450 has been purified to homogeneity and found to have a molecular weight of 55,500. Purified enzyme showed a spectral carbon monoxide peak at 448nm, and could undergo both type I and type II spectral interactions. The mid-point redox potential of yeast cytochrome P-450 is close to that of both mammalian and bacterial cytochrome P-450. The substrate specificity of yeast cytochrome P-450 was found to be limited to benzo(a)pyrene hydroxylase. Some biphenyl hydroxylase activity was found in yeast spheroplasts but could not be recovered after cell disruption. Benzo(a)pyrene hydroxylase was inhibited by compounds which give binding spectra with cytochrome P450 and by flavonoids. Microsomal cytochrome P-450 was more stable towards thermal denaturation than solubilized or purified enzyme. Triton X-100 was shown to protect cytochrome P-450 against thermal denaturation.