title: Spin state transitions of liver microsomal cytochrome P-450.
abstract: Spin state transitions of membrane-bound cytochrome P-450 were investigated by difference spectrophotometry using the 'D'-charge transfer absorbance band at 645 nm as a measure of the amount of hemin iron present in the 5-coordinated state. The magnitude of the 'D'-absorbance band in the absence of exogenous substrates, e.g., the concentration of native high spin cytochrome P-450, was evaluated from the difference in absorbance at 645 nm between ferric cytochrome P-450 and the carbon monoxide derivative of the pigment in its ferrous state. The contribution of the native high spin species to the total cytochrome P-450 content of microsomes was calculated to be between 40% and 65% after induction with phenobarbital and polycyclic hydrocarbons, respectively. Up to 80% of the cytochrome P-450 was found to be present in the high spin state after the addition of exogenous substrates. Further, the steady state concentrations of high spin cytochrome P-450, observed in the presence of reduced pyridine nucleotides, suggest that the rate limiting step for microsomal mixed function oxidation reactions is variable and dependent on the substrate under investigation.
mesh_terms: D000818:Animals; D002164:Camphor; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology; D008797:Metyrapone; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D010634:Phenobarbital / Q000494:pharmacology; D011284:Pregnenolone / Q000494:pharmacology; D011485:Protein Binding; D051381:Rats; D013053:Spectrophotometry
chemical_list: D008748:Methylcholanthrene; D011284:Pregnenolone; D002164:Camphor; D003577:Cytochrome P-450 Enzyme System; D010634:Phenobarbital; D008797:Metyrapone