Oxidation Rate of Monomeric Radish Cytochrome f with Plastocyanin

The reaction rate of reduced monomeric cytochrome f with oxidizedplastocyanin, both purified from Japanese radish, was determinedby a stopped-flow method. The oxidation rate constant was 6.0x 10⁷ M¹sec¹ at pH 7.0 and 25°C, which is slightly higherthan the value reported by Wood [(1974) Biochim. Biophys. Acta357 : 370] for oligomeric parsley cytochrome f Thermodynamicparameters also were determined to be 56 KJ M^–1 for activationenthalpy and 90 J M^–1 K^–1 for activation entropy.Neither a pH from 6 to 9 nor the addition of NaCl, polylysine,histone or polyaspartate affected the rate constant. ¹Present address: The National Institute for Environmental Studies,Yatabe, Ibaraki 305, Japan. (Received October 9, 1980; Accepted November 17, 1980)