On the mechanism of action of cytochrome P450: evaluation of hydrogen abstraction in oxygen-dependent alcohol oxidation.
The mechanisms of oxidation of primary and secondary benzylic alcohols to the corresponding carbonyl compounds by purified rabbit liver cytochrome P450 forms 2B4 and 2E1 in a reconstituted enzyme system has been examined by linear free energy relationships, intramolecular and steady-state deuterium isotope effects, and the incorporation of an O2-derived oxygen atom or solvent-derived deuterium. The kcat and Km values were found to be relatively insensitive to the presence of electronic perturbations at the para position. The Hammett reaction constants for the oxidation of benzyl alcohols by P450s 2B4 and 2E1 are -0.46 and -0.37, respectively, and with 1-phenylethyl alcohols the corresponding reaction constants are -1.41 and -1.19, respectively. With [1-2H1]benzyl alcohol, P450s 2B4 and 2E1 show similar intramolecular deuterium isotope effects of 2.6 and 2.8, respectively, whereas with [1-2H2]benzyl alcohol under steady-state conditions, the deuterium isotope effects on the catalytic constants are 2.8 and 1.3, respectively. No significant isotope effect on the catalytic constant was noted for either form of P450 with 1-phenylethyl alcohol. In D2O, acetophenone formed by either form of P450 from 1-phenylethyl alcohol does not contain a deuterium atom at the methyl group, whereas under an atmosphere of 18O2 approximately 30% of the labeled oxygen is incorporated into the carbonyl group with either form of the cytochrome. The results are consistent with a mechanism that involves stepwise oxidation of the alcohol to a carbon radical alpha to the alcohol function, followed by oxygen rebound to yield the gem-diol, dehydration of which gives the carbonyl product.(ABSTRACT TRUNCATED AT 250 WORDS)
Vaz
A D
AD
Department of Biological Chemistry, Medical School, University of Michigan, Ann Arbor 48109-0606.
Coon
M J
MJ
D000098:Acetophenones / Q000737:chemistry; D000818:Animals; D001592:Benzyl Alcohols / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry*; D003903:Deuterium; D008401:Gas Chromatography-Mass Spectrometry; D006859:Hydrogen / Q000737:chemistry*; D007527:Isoenzymes / Q000737:chemistry*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010626:Phenylethyl Alcohol / Q000737:chemistry*; D011817:Rabbits; D012997:Solvents
D000098:Acetophenones; D001592:Benzyl Alcohols; D007527:Isoenzymes; D012997:Solvents; D006859:Hydrogen; D003577:Cytochrome P-450 Enzyme System; D003903:Deuterium; D010626:Phenylethyl Alcohol; C038699:acetophenone; D010100:Oxygen