Channel catfish glutathione S-transferase isoenzyme activity toward (+/-)-anti-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide

The glutathione S-transferases (GSTs) detoxify a number of environmental carcinogens and epoxide intermediates. In the present study, we examined the capacity of channel catfish GSTs to detoxify environmental epoxides, in particular, (+/-)-anti-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide (BPDE), the highly mutagenic and carcinogenic epoxide intermediates of benzo[a]pyrene metabolism. Channel catfish hepatic cytosolic GSTs catalyzed the conjugation of BPDE with GSH, but not aflatoxin B-1-8,9-epoxide (AFBO), the carcinogenic epoxide intermediate produced during aflatoxin B-1 (AFB(1)) metabolism. Channel catfish cytosolic GSTs were also active toward 1-chloro-2,4-dinitrobenzene (CDNB, a non-specific GST substrate), delta(5)-androstene-3,17-dione (delta(5)-ADI, a predominantly alpha class GST substrate) 1,2-dichloro-4-nitrobenzene (DCNB, a predominantly mu-class GST substrate), and ethacrynic acid (ECA, a predominantly pi-class GST substrate). Catfish cytosolic GST(s) showed moderate immunological cross-reactivity to polyclonal antibodies against rat alpha and pi class GST isozymes, and limited immunological cross-reactivity to a rat mu class polyclonal antibody. Isoenzymes of GST were purified to electrophoretic homogeneity from channel catfish hepatic cytosol by affinity chromatography and chromatofocusing. The major isoenzymes were tentatively identified as CC-1, CC-2 and CC-4, and a minor GST was identified as CC-5, based in order of decreasing apparent isoelectric points of chromatofocusing. In addition, an acidic fraction of the affinity-purified GSTs was isolated but not analyzed. When the GST isoenzyme fractions were characterized in terms of catalytic activity toward BPDE, all GST isoforms tested exhibited detectable GST-BPDE activity. However, the specific activity of CC-4 toward BPDE was 7- and 17-fold greater than that of CC-1 and CC-2, respectively. CC-4 also displayed the highest activity toward CDNB. The results of this study indicate that the GST-BPDE activity in channel catfish liver cytosol is apparently due to the presence of one or more GST isoenzymes with high specific catalytic activity toward BPDE. The expression of GSTs with high BPDE activity may play a contributing role in the apparent resistance of channel catfish to environmental hepatocarcinogenesis.