RT Journal Article SR Electronic T1 EXAMINATION OF EXTINCTION COEFFICIENTS OF SOLUBLE AND MEMBRANE-BOUND CYTOCHROMES P-450 AND P-420 JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 516 OP 522 VO 1 IS 2 A1 JOHN B. SCHENKMAN A1 YOUNG-NAM CHA A1 PETER MOLDEUS A1 DOMINICK L. CINTI YR 1973 UL http://dmd.aspetjournals.org/content/1/2/516.abstract AB The measurement of the extinction coefficient of cytochrome P-450 is fraught with difficulties, among which are contaminating hemoproteins and incomplete reduction of the hemoprotein; both of these problems will cause lower than true extinction coefficients. Complete reduction of microsomal cytochrome P-450 by the chemical reductant dithionite is not instantaneous, but requires about 2-5 min. After tryptic solubilization of cytochrome P-450 and stabilization by 25% glycerol, complete reduction can take up to 30 min. The extinction coefficient of 91 mM-1 cm-1 was obtained for cytochrome P-450 of liver microsomes of phenobarbital-treated, 3-methylcholanthrene-treated, and untreated male rats. A value of 122 mM-1 cm-1 was obtained for the 3-methylcholanthrene-treated rabbit and values of 114-122 mM-1 cm-1 were obtained for phenobarbital-treated rabbit liver cytochrome P-450. In all cases, similar values were found in microsomes and P-450 particles. The method of measurement of the extinction coefficient of soluble cytochrome P-420 causes another problem to become manifest because of rapid degradation of the reduced hemoprotein in aerobic medium. When measured in open cuvets, the apparent extinction coefficient increases from 115 mM-1 cm-1 to a maximum of 160 mM-1 cm-1. This change is due to destruction of the heme of cytochrome P-420 in the aerobic reference cuvet when reduced by dithionite; carbon monoxide stabilizes the reduced cytochrome P-420 in the sample cuvet. Copyright © 1973 by The American Society for Pharmacology and Experimental Therapeutics