Cytochrome P450 hydroxylation of hydrocarbons: variation in the rate of oxygen rebound using cyclopropyl radical clocks including two new ultrafast probes.
The oxidation of eight methyl-substituted and three alkyl-substituted cyclopropanes by rat liver microsomal cytochrome P450 and pure reconstituted rabbit P450 2B4 was studied. Alkane hydroxylation catalyzed by P450 is generally believed to proceed by hydrogen abstraction followed by reaction of the carbon-centered radical with an iron-bound hydroxyl radical, a process called oxygen rebound. Hydrogen abstraction from methylcyclopropanes generates cyclopropylcarbinyl radicals whose solution rate constants for ring opening are known [Bowry, V.W., et al. (1991) J. Am. Chem. Soc. 113, 5687-5698]. Rearranged products were only observed with the five substrates which, upon hydrogen abstraction, would generate a cyclopropylcarbinyl radical that undergoes ring opening with a rate constant > or = 2.0 x 10(9) s-1 in solution. Values of the rate constants for oxygen rebound (kOH) were calculated by determining the ratio of unrearranged products (cyclopropylmethanols) to rearranged products (alkenols). For each substrate this ratio was generally about the same for the oxidations catalyzed by microsomal P450 and by P450 2B4. It is concluded that all of the substrates are oxidized via an intermediate cyclopropylcarbinyl radical. Two ultrafast probes, trans-1-methyl-2-phenylcyclopropane and 1,1-diphenyl-2-methylcyclopropane, gave alcohol product ratios which yielded unreasonably high values for kOH, viz., ca. 1.5 x 10(12) and ca. 7 x 10(12) s-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Atkinson
J K
JK
Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario.
Ingold
K U
KU
D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D003521:Cyclopropanes / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005609:Free Radicals; D006900:Hydroxylation; D008862:Microsomes, Liver / Q000201:enzymology*; D008956:Models, Chemical; D015335:Molecular Probes; D010084:Oxidation-Reduction; D011817:Rabbits; D051381:Rats; D013250:Steroid Hydroxylases / Q000378:metabolism; D013997:Time Factors
D003521:Cyclopropanes; D005609:Free Radicals; D015335:Molecular Probes; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C050356:steroid 15-alpha-hydroxylase