title: Thermodynamic properties of oxidation-reduction reactions of bacterial, microsomal, and mitochondrial cytochromes P-450: an entropy-enthalpy compensation effect.
abstract: An optically transparent thin-layer electrode cell with a very small volume was used for determination of the formal reduction potentials of bacterial, microsomal, and mitochondrial cytochromes P-450. At an extrapolated zero concentration of dye, the bacterial cytochrome from Pseudomonas putida catalyzing the hydroxylation of camphor and the adrenal mitochondrial cytochrome catalyzing the cholesterol side-chain cleavage reaction had formal reduction potentials of -168 and -285 mV (pH 7.5 and 25 degrees C), respectively. The oxidation-reduction potentials for the rabbit liver microsomal cytochrome P-450 induced by 3-methylcholanthrene and the mitochondrial cytochrome for steroid 11 beta-hydroxylation were found as -360 and -286 mV, respectively. Potential measurements at different temperatures allowed documentation of the standard thermodynamic parameters for cytochrome P-450 reduction for the first time. All cytochromes tested were found to have a relatively large negative entropy change upon reduction. The extent of these changes is comparable to that observed for the ferric-ferrous couple of cytochrome c. An entropy-enthalpy compensation effect was observed among the four cytochromes P-450 examined although the correlation is weaker than that observed with cytochrome c isolated from various sources.
mesh_terms: D000302:Adrenal Cortex / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification / Q000378:metabolism*; D004355:Drug Stability; D004563:Electrochemistry; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism*; D010084:Oxidation-Reduction; D011549:Pseudomonas / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D013053:Spectrophotometry / Q000379:methods; D013816:Thermodynamics
chemical_list: D003577:Cytochrome P-450 Enzyme System