Interaction of beta-lactoglobulin and cytochrome c: complex formation and iron reduction.
Abstract β-Lactoglobulin forms a soluble complex with cytochrome c in mildly alkaline solutions of low ionic strength. Sedimentation velocity experiments suggest that the complex (maximum s 20 = 3.7) consists of one cytochrome c molecule per β-lactoglobulin monomer unit. At pH 8 or higher, the presence of β-lactoglobulin causes reduction of ferri- to ferrocytochrome c . The initial rate of reduction at a single temperature depends primarily on the concentration of β-lactoglobulin, although the final percentage ferrocytochrome c obtained is constant at molar ratios of three or more β-lactoglobulin monomers to one cytochrome c molecule. The temperature dependence of the initial rate of iron reduction resembles that for alkaline denaturation of β-lactoglobulin. The displacement of N -dansylaziridine, a sulfhydryl specific dye, from bovine β-lactoglobulin during iron reduction, and the formation of nonreducing complexes between the analogous swine protein (no sulfhydryls) and cytochrome c suggest that the sulfhydryl group of β-lactoglobulin is the electron donor.