Stereoselectivity of rat liver cytochrome P-450 c on formation of benzo[a]pyrene 4,5-oxide

Abstract

The facial selectivity of purified rat liver cytochrome P-450 c toward epoxidation of benzo[a]pyrene (BP) at its 4,5-position (K-region) was determined by formation, separation and quantitation of diastereomeric trans-addition products of glutathione with enzymatically produced benzo[a]pyrene 4,5-oxide (BP 4,5-oxide). Non-enzymatic trans addition of glutathione to synthetic (+)-(4S,5R)- and (−)-(4R,5S)-BP 4,5-oxides occurs equally at carbons 4 and 5 to give two diastereomeric pairs of positional isomers. The position of the glutathionyl moiety in each isomer was determined by acetylation, oxidation of the thioether to the sulfoxide, cis -elimination of the sulfoxide, and identification of the acetoxybenzo[a]pyrene formed. Correlation of the glutathione conjugates obtained from BP 4,5-oxide derived from cytochrome P-450 c catalyzed oxidation of BP with those obtained from the synthetic enantiomers indicated that ≥97% of the enzymatically formed arene oxide was the (+)-(4S,5R) enantiomer.