TOLUENE METABOLISM BY CDNA-EXPRESSED HUMAN HEPATIC CYTOCHROME-P450

The metabolism of toluene in human liver microsomes and by cDNA-expres sed human cytochrome P450s (CYPs) was investigated. Toluene was metabo lized mainly to benzyl alcohol and slightly to o- and p-cresol by huma n liver microsomes. Formation of o-cresol was elevated in microsomes f rom human livers derived from cigarette smokers, but the induced CYP i soforms were not clear. Of the eleven human CYP forms studied, CYP2E1 was the most active in forming benzyl alcohol, followed by CYP2B6, CYP 2C8, CYP1A2, and CYP1A1, in that order. The activities of CYP2A6, CYP2 C9. CYP2D6, CYP3A3, CYP3A4, and CYP3A5 were negligible. In addition, C YP2B6 and CYP2E1 catalyzed the formation of p-cresol (11-12% of total metabolites), and CYP1A2 catalyzed the formation of both o-(22%) and e -cresol (35%). The relationship between the amino acid sequence of rat CYP2B1 cDNA and the activity for toluene metabolism was investigated using variants, because of great differences in the forming of toluene ring products between CYP2B1 and CYP2B6. These results suggest that t he structure of CYP2B1 at the site of Leu 58 rather than Ile-114 and G lu-282 plays an important role in the formation of toluene ring produc ts, whereas in CYP2B1 Ile-114 plays an important role in the formation of benzyl alcohol. These results may explain, In part, the lower acti vity of CYP2B6, which has Phe at position 58 of the protein, for tolue ne ring oxidations than that of CYP2B1. Copyright (C) 1997 Elsevier Sc ience Inc.