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J Biol Chem, Vol. 273, Issue 26, 16038-16042, June 26, 1998

Distinct Roles of Two Heme Centers for Transmembrane Electron Transfer in Cytochrome b₅₆₁ from Bovine Adrenal Chromaffin Vesicles as Revealed by Pulse Radiolysis

Kazuo Kobayashi, Motonari Tsubaki^¶, and Seiichi Tagawa

From the  Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan and the ^¶ Department of Life Science, Faculty of Science, Himeji Institute of Technology, Kamigoori-cho, Akou-gun, Hyogo 678-1297, Japan

The reaction of monodehydroascorbate (MDA) radical with purified cytochrome b₅₆₁ from bovine adrenal chromaffin vesicles was investigated by the technique of pulse radiolysis. Radiolytically generated MDA radical oxidized rapidly the reduced form of cytochrome b₅₆₁ to yield the oxidized form. Subsequently the oxidized form of cytochrome b₅₆₁ was re-reduced by ascorbate in the medium. The second-order rate constants of the reaction of MDA radical were increased with decreasing pH, whereas a maximum of the second-order rate constant for the reaction with ascorbate was obtained around pH 6.8. At excess MDA radical to cytochrome b₅₆₁ concentration, only half of the heme in cytochrome b₅₆₁ was oxidized, indicating that only one of the two heme centers can react with MDA radical. On the other hand, when the reactions were examined using cytochrome b₅₆₁ pretreated in a mild alkaline condition in the oxidized state, the cytochrome b₅₆₁ could not be oxidized with MDA radical, suggesting that the heme center specific for the electron donation to MDA radical is selectively modified upon the alkaline treatment. These results suggest that the two heme b centers have distinct roles for the electron donation to MDA radical and the electron acceptance from ascorbate, respectively.

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