BINDING OF 2-HYDROXY-5-NITROBENZYL ALCOHOL TO RAT ALPHA-CLASS GLUTATHIONE S-TRANSFERASES - EVIDENCE FOR BINDING AT TRYPTOPHAN-21

2-Hydroxy-5-nitrobenzyl alcohol (HNB) was prepared from dimethyl(2-hyd roxyl-5-nitrobenzyl)sulfonium bromide (HNBB). HNB binds to glutathione S-transferases (GSTs) 1-2 and 2-2 with moderate affinity at a site se parate from 1-anilino-8-naphthalenesulfonate (ANS). Intrinsic fluoresc ence due to Trp-21 is strongly quenched by HNB binding but there is no effect on catalytic activity. There appear to be two HNB binding site s per dimer in each GST isoenzyme. We suggest that HNB binds directly at Trp-21 of each subunit and that previously reported quenching of in trinsic fluorescence in these proteins upon ligand binding may be due to indirect structural effects rather than direct binding at this resi due.