Photoreduction of cytochrome c1.

1. Ferricytochrome c1 solution was reduced completely between pH 7 and 10 by illumination under anaerobic conditions. Photoreduction was not affected by the ionic strength of the medium. However, it did not take place at pH lower than 6 or higher than 10, or in the presence of p-hydroxymercuric benzoate. The ferricyanide-reoxidized photoreduced c1 was not further reduced upon illumination. The reductant was most probably a specific sulfhydryl group in the subunit containing the heme of the cytochrome since this subunit contained one less p-HMB-titratable group in the photoreduced sample than in the untreated preparation. 2. The photoreduced cytochrome c1 showed the same spectra as the native cytochrome, and was not reactive with carbon monoxide. The equilibrium constant of the reaction c12+ + c3+ equilibrium c13+ + c2+ for the photoreduced c1 was found to be slightly lower (Keq = 2.6) than that for the native c1 (Keq = 3.5). The antimycin A-sensitive electron acceptor activity of ferricyanide-reoxidized photoreduced c13+ catalyzed by succinate-cytochrome c reductase was about 80% of that of the native c1. 3. A somewhat simplified method for isolation of cytochrome c1 was developed. Anaerobic ammonium sulfate fractionation and calcium phosphate gel chromatography were still used in order to achieve the purity level of about 25 nmol of heme/mg of protein. The cytochrome c1 prepared by this procedure showed the same properties tested as that by the beta-mercaptoethanol method (Yu, C.A., Yu, L., and King, T.E. (1972) J. Biol. Chem. 247, 1012-1019).