Autoxidizability of beef heart cytochrome c₁ lacking the hinge protein c₁-c

The autoxidizability of beef heart cytochrome c₁ was investigated in terms of the integrity of the binding of the hinge protein to the heme subunit. Cytochrome c₁ was isolated as a subcomplex consisting of the heme subunit and the hinge protein. Treatment of the cytochrome c₁ subcomplex with p-chloromercuribenzoate (pCMB) under mild conditions lessened the binding strength between the two subunits. They were dissociated on polyacrylamide gel electrophoresis (PAGE) under non-denaturing conditions, but were not separated by gel filtration chromatography. The pCMB-treated subcomplex had a slight autoxidizabiity. This was repressed to the level of the native subeomplex, when the mercurial compound bound to the subcomplex was removed by the addition of 2-mercaptoethanol. Concomitantly, the less stable binding between the subunits was apparently reversed to the native state. After pCMB treatment of the subcomplex, the heme subunit recovered from PAGE showed marked autoxidizabiity, even if it was treated with 2-mercapto-ethanol. Addition of cholate repressed the autoxidizability of the heme subunit after the removal of the mercurial compound. These results confirmed that the stable binding of the hinge protein to the heme subunit was essential for the non-autoxidizability of cytochrome c₁ subcomplex. In addition, it was suggested that cysteinyl residues in the subcomplex must be involved to a great extent in the stable binding between the two subunits.