REDUCTIONS BY FERROCYTOCHROME-C PEROXIDASE .4. KINETICS OF YEAST CYTOCHROME-C REDUCTION AT HIGH BUFFER PHOSPHATE CONCENTRATION

The kinetics of reduction of yeast ferricytochrome c from Saccharomyces cerevisiae by ferrocytochrome c peroxidase (CCPII) were investigated at high ionic strength. In 200-500 mM phosphate (pH 7.0) first-order kinetics were observed with an average rate of 0.025 +/- 0.003 s-1 at 24 +/- 1-degrees-C. Previously (Inorg. Chem., 27 (1988) 1078; Rev. Port. Quim., 27 (1985) 237) it was reported that the reduction of horse cytochrome c by CCPII in greater-than-or-equal-to 100 mM phosphate also exhibited first-order kinetics with a rate of 0.02 s-1. These observations are consistent with a reduction mechanism in which the rate-limiting step involves a conformation change in free CCP. The fluorescence polarization of yeast porphyrin cytochrome c in the presence of CCPIII was also examined, and the results indicate that the two proteins do not form a complex in 200 mM phosphate.