title: Inhibition of electron transfer from adrenodoxin to cytochrome P-450scc by chemical modification with pyridoxal 5'-phosphate: identification of adrenodoxin-binding site of cytochrome P-450scc.
abstract: Covalent modification of cytochrome P-450scc (purified from bovine adrenocortical mitochondria) with pyridoxal 5'-phosphate (PLP) was found to cause inhibition of the electron-accepting ability of this enzyme from its physiological electron donor, adrenodoxin, without conversion to the "P-420" form. Reaction conditions leading to the modification level of 0.82 and 2.85 PLP-Lys residues per cytochrome P-450scc molecule resulted in 60% and 98% inhibition, respectively, of electron-transfer rate from adrenodoxin to cytochrome P-450scc (with beta-NADPH as an electron donor via NADPH-adrenodoxin reductase and with phenyl isocyanide as the exogenous heme ligand of the cytochrome). It was found that covalent PLP modification caused a drastic decrease of cholesterol side-chain cleavage activity when the cholesterol side-chain cleavage enzyme system was reconstituted with native (or PLP-modified) cytochrome P-450scc, adrenodoxin, and NADPH-adrenodoxin reductase. Approximately 60% of the original enzymatic activity of cytochrome P-450scc was protected against inactivation by covalent PLP modification when 20% mole excess adrenodoxin was included during incubation with PLP. Binding affinity of substrate (cholesterol) to cytochrome P-450scc was found to be increased slightly upon covalent modification with PLP by analyzing a substrate-induced spectral change. The interaction of adrenodoxin with cytochrome P-450scc in the absence of substrate (cholesterol) was analyzed by difference absorption spectroscopy with a four-cuvette assembly, and the apparent dissociation constant (Ks) for adrenodoxin binding was found to be increased from 0.38 microM (native) to 33 microM (covalently PLP modified).(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, Kagawa Medical School, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Tsubaki
authors.forename: Y
authors.initials: Y
authors.lastname: Iwamoto
authors.forename: A
authors.initials: A
authors.lastname: Hiwatashi
authors.forename: Y
authors.initials: Y
authors.lastname: Ichikawa
mesh_terms: D000302:Adrenal Cortex / Q000201:enzymology*; D000325:Adrenodoxin / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002786:Cholesterol Side-Chain Cleavage Enzyme / Q000037:antagonists & inhibitors*; D004579:Electron Transport / Q000187:drug effects; D007700:Kinetics; D008928:Mitochondria / Q000201:enzymology*; D008969:Molecular Sequence Data; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000302:isolation & purification; D010449:Peptide Mapping; D011732:Pyridoxal Phosphate / Q000494:pharmacology*; D014357:Trypsin
chemical_list: D010446:Peptide Fragments; D000325:Adrenodoxin; D009249:NADP; D011732:Pyridoxal Phosphate; D002786:Cholesterol Side-Chain Cleavage Enzyme; D014357:Trypsin