The reaction of the trifluoromethylphenylcarbamylated lysine-13 derivative of horse cytochrome c with cytochrome oxidase

Abstract

The kinetics of oxidation of horse cytochrome c and the trifluoromethylphenylcarbamylated lysine-13 derivative by cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) were compared using both spectrophotometric and polarographic methods under different experimental conditions. The rate constants measured spectrophotometrically in 0.025 M Tris-cacodylate buffers were similar with the two cytochromes at pH 7.8, but those with the derivative were slightly higher at pH 6. Rates measured with polarographic assays in these buffers were the same with the horse and the derivative cytochromes c at pH 6, but at pH 7.8 the rates with file derivative were less at cytochrome c concentrations between 0.05 and 0.5 μM and were greater at higher concentrations. The pH optima in the polarographic assays of the derivative and the native pigments were different in 0.025 M Tris-cacodylate buffers; in spectrophotometric assays at pH 7.8 the trifluoromethylphenylcarbamylated lysine-13cytochrome c showed a greater sensitivity to changes in ionic strength than did file native cytochrome. The variations in apparent K m and V values calculated from spectrophotometric and polarographic assays with the two cytochromes cannot be explained as due to changes in binding of cytochrome c to cytochrome oxidase. The large excess of O 2 uptake seen in polarographic assays with horse cytochrome c over that expected from spectrophotometric measurements was not apparent with the trifluoromethylphenylcarbamylated lysine-13 derivative. Thus, the derivative seems to have decreased ability to form the combination of cytochrome c with the oxidase giving high turnover rates.