title: A ubiquinone derivative that inhibits mitochondrial cytochrome b-c1 complex but not chloroplast cytochrome b6-f complex activity.
abstract: A ubiquinone derivative, 3-chloro-5-hydroxyl-2-methyl-6-decyl- 1,4-benzoquinone (3-CHMDB), which shows different effects on the mitochondrial cytochrome b-c1 complex and chloroplast cytochrome b6-f complex, has been synthesized and characterized. When the cytochrome b-c1 complex is treated with varying concentrations of 3-CHMDB and assayed at constant substrate (Q2H2) concentration, a 50% inhibition is observed when 2 mol of 3-CHMDB per mol of enzyme are used. The degree of inhibition is dependent on the substrate concentration. When ubiquinol-cytochrome c reductase is treated with 2 mol of 3-CHMDB per mol of enzyme, less inhibition is observed with a lower substrate concentration, suggesting the possible existence of two forms of reductases: one with a high affinity for ubiquinone and another with a low affinity. 2-Chloro-5-hydroxyl-3-methyl-6-decyl-1,4-benzoquinone (2-CHMDB), an isomer of 3-CHMDB, shows much less inhibition of the mitochondrial cytochrome b-c1 complex, suggesting that the quinone binding site in this complex is highly specific. In contrast to the inhibition observed with the cytochrome b-c1 complex, 3-CHMDB causes no inhibition of the plastoquinol-plastocyanin reductase activity of chloroplast cytochrome b6-f complex, regardless of whether plastoquinol-2 or ubiquinol-2 is used as substrate. 3-CHMDB restores the dibromothymoquinone-altered EPR spectra of iron-sulfur protein in both complexes. In the case of the cytochrome b6-f complex, 3-CHMDB also partially restores the dibromothymoquinone-inhibited activity. Reduced form 3- or 2-CHMDB is oxidizable by the cytochrome b6-f complex, but not by the cytochrome b-c1 complex. These results suggest that the quinol oxidizing sites in the cytochrome b6-f complex may differ from those in the mitochondrial cytochrome b-c1 complex.
authors.affiliation: Department of Biochemistry, Oklahoma State University, Stillwater 74078.
authors.forename: L Q
authors.initials: LQ
authors.lastname: Gu
authors.forename: L
authors.initials: L
authors.lastname: Yu
authors.forename: C A
authors.initials: CA
authors.lastname: Yu
mesh_terms: D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D002736:Chloroplasts / Q000201:enzymology*; D003573:Cytochrome b Group / Q000037:antagonists & inhibitors*; D045346:Cytochrome b6f Complex; D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III / Q000037:antagonists & inhibitors*; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000378:metabolism; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008563:Membrane Lipids / Q000502:physiology; D008928:Mitochondria / Q000201:enzymology*; D010084:Oxidation-Reduction; D010944:Plants / Q000201:enzymology; D012242:Rhodobacter sphaeroides / Q000201:enzymology; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000494:pharmacology
chemical_list: D003573:Cytochrome b Group; D007506:Iron-Sulfur Proteins; D008563:Membrane Lipids; C058820:3-chloro-5-hydroxy-2-methyl-6-decyl-1,4-benzoquinone; D014451:Ubiquinone; D045346:Cytochrome b6f Complex; D014450:Electron Transport Complex III