Differential Inhibition of Xenobiotic-Metabolizing Carboxylesterases by Organotins in Marine Fish

Abstract

The hydrolytic metabolism of xenobiotics in the liver of two tropical marine fish, Siganus canaliculatus and Acanthopagrus latus, was found to be catalyzed by both microsomal and cytosolic carboxylesterases; the latter forms were more active than the former. Remarkably greater efficiency of S. canaliculatus for p-nitrophenylacetate hydrolysis was attributed to manyfold higher Vmax and lower Km values of hepatic microsomal and cytosolic carboxylesterases of S. canaliculatus as compared with those of A. latus. Comparative characterization of the in vitro responses of hepatic microsomal and cytosolic carboxylesterases to the organotin group of marine pollutants—tributyltin (TBT), triphenyltin (TPT), and dibutyltin (DBT), a relatively persistent metabolite of TBT—revealed species-, isozymic form-, and organotin structure-related differences in the hydrolytic detoxication. In general, carboxylesterases of S. canaliculatus exhibited severalfold greater susceptibility to organotin inhibition and DBT was the most potent inhibitor (IC50 in micromolar range). Notably, the IC50 of SnCl2, a metal present in all the compounds studied, was higher than 2 mM. Cytosolic forms in both species were more sensitive to organotin inhibition than microsomal counterparts. In line with these differences the nature of inhibition of cytosolic and microsomal carboxylesterases by organotins was competitive and noncompetitive, respectively. These results suggest that organotins may aggravate the toxicity of other environmental contaminants in fish and other aquatic organisms. Moreover, highly sensitive cytosolic carboxylesterases of S. canaliculatus liver may serve as molecular biomarkers of organotin pollution.