The enzymatic conjugation of glutathione with bay-region diol-epoxides of benzo[a]pyrene, benz[a]anthracene and chrysene

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Abstract
The kinetics of the enzymatic conjugation of glutathione (GSH) with the anti-diastereoisomers of trans-7, 8-dihydroxy-9, 10-epoxy-7, 8, 9, 10-tetrahydrobenzo[a]pyrene (BPDE), trans-3, 4-dihydroxy-1, 2-epoxy-1, 2, 3, 4-tetrahyd-robenz[a] anthracene (BADE) and trans-1, 2-dihydroxy-3, 4-epoxy-1, 2, 3, 4-tetrahydrochrysene (CDE) catalyzed by transferase 4-4 from rat liver have been compared. When the concentration of these diol-epoxides was varied (using 2mM GSH) the apparent Vmax values were 560, 2100 and 1500 nmol/ing/min for (±)-anti-BPDE, (±)-anti-BADE and (±)-anti-CDE, respectively, with corresponding apparent Km values of 11, 125 and 105 μM. The catalytic efficiency of transferase 4–4 in the GSH conjugation of (±)-anti-BADE and (±)-anti-CDE is thus approximately one-third of (±)-anti-BPDE (0.014 and 0.012 s-1 μM-1 respectively versus 0.042 s-1μM-1). Similar non-linear Lineweaver-Burk plots were obtained with each diol-epoxide when the concentration of GSH was varied, and two apparent. Km values of 0.02–0.04 and 0.4–0.9 mM GSH were estimated. The GSH-conjugates formed with the individual enantiomers of the recemic substrates used were resolved by h.p.l.c. The data indicate that with each diol-epoxide transferase 4–4 is highly selective(⋛95%) towards the biologically most active (+)-enantiomer.