Is CYP1B1 involved in the metabolism of dioxins in the pig?

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most difficult to biodegradate and the most toxic dioxin congener. Previously, we demonstrated in silico the ability of pig CYP1A1 to hydroxylate 2,7-dichlorodibenzo-p-dioxin (DiCDD), but not TCDD. To increase our knowledge concerning the low effectiveness of TCDD biodegradability, we analyzed in silico the binding selectivity and affinity between pig CYP1B1 and the two dioxins by means of molecular modeling. We also compared the effects of TCDD and DiCDD on CYP1B1 gene expression (qRT-PCR) and catalytic (EROD) activity in porcine granulosa cells. It was found that DiCDD and TCDD were stabilized within the pig CYP1B1 active site by hydrophobic interactions. The analysis of substrate channel availability revealed that both dioxins opened the exit channel S, allowing metabolites to leave the enzyme active site. Moreover, DiCDD and TCDD increased the CYP1B1 gene expression and catalytic activity in porcine granulosa cells. On the other hand, TCDD demonstrated higher than DiCDD calculated affinity to pig CYP1B1, hindering TCDD exit from the active site. The great distance between CYP1B1's heme and TCDD also might contribute to the lower hydroxylation effectiveness of TCDD compared to that of DiCDD. Moreover, the narrow active site of pig CYP1B1 may immobilize TCDD molecule, inhibiting its hydroxylation. The results of the access channel analysis and the distance from pig CYP1B1's heme to TCDD suggest that the metabolizing potential of pig CYP1B1 is higher than that of pig CYP1A1. However, this potential is probably not sufficiently high to considerably improve the slow TCDD biodegradation.