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Chloroplast cytochrome b6: Molecular composition as a lipoprotein

1975; Elsevier BV; Volume: 376; Issue: 3 Linguagem: Inglês

10.1016/0005-2728(75)90175-9

1879-2650

Allan L. Stuart, Aaron R. Wasserman,

Mitochondrial Function and Pathology

Disc electrophoretically homogeneous spinach-chloroplast cytochrome b6 was found to be a lipoprotein whose redox potential was essentially unchanged during isolation. These results further support the hypothesis of Triton X-1004M urea, pH 8, as a useful extracting medium for membrane lipoproteins. Cytochrome b6 was found to have a heme equivalent dry weight of 1 mol of heme per 60 000 g. Of this, 20 000 g was lipid-extractable. The molecular weight was 60 000 with a partial specific volume of 0.84 ml/g. The protein portion of the molecule (40 000) consisted of 1 polypeptide chain of 20 000 daltons, 1 of 9600 daltons and 2 of 6600 daltons. A simple lipid composition (relative to the original membrane) was found consisting of 7 mol of chlorophyll a and 6 mol of cardiolipin per mol of cytochrome; these two lipids thus account for about 75–80% of the lipid content. An unidentified minor neutral lipid and minor polar lipid were also detected. At pH 7.0 in the presence of 0.5% Triton X–100, E′0 was −0.080 V, and in the absence of Triton X–100, E′0 was −0.120 V. At pH 8 in 0.5% Triton X–100, E′0 was −0.084 V, thus indicating that the redox potential is independent of pH in the region 7–8. The redox reaction proceeded via a one-electron-transfer.