Comparative metabolism of dibenzo[a,l]pyrene by liver microsomes from rainbow trout and rats

Abstract

In order to assess the differences in the ability of fish and rat liver to metabolize carcinogenic polycyclic aromatic hydrocarbons (PAHs), we have investigated the metabolism of dibenzo[a,l]pyrene (DB[a,l]P), a highly potent carcinogenic PAH, by liver microsomes from 3-methylcholanthrene-treated Shasta rainbow trout ( Oncorhynchus mykiss) and rats. Rat liver microsomes metabolized DB[a,l]P at a slightly higher rate (1.3-fold) than trout liver microsomes. Compared to benzo[a]pyrene (B[a]P), DB[a,l]P was metabolized at a significantly lower rate by both rat and trout liver microsomes. Although the microsomes from the two species metabolized DB[a,l]P to qualitatively similar metabolites, they showed significant differences in the profile of the metabolites formed. The proportion of DB[a,l]P-11,12-diol, the proximate carcinogen of DB[a,l]P, formed by trout microsomes was over two-fold greater (32.6%) than the corresponding value for rat microsomes (15.6%). Unlike rat microsomes, trout microsomes metabolized DB[a,l]P to its K-region diol (8,9-diol) to a small extent (26.1 vs 3.6%). As previously noted with B[a]P, trout liver, compared to rat liver, appears to be more efficient in forming the proximate carcinogenic metabolite of DB[a,l]P but less efficient in producing its K-region diol, a non-carcinogenic metabolite.