Role of arginine 216 in catalytic activity of murine Alpha class glutathione transferases mGSTAl-1 and mGSTA2-2 toward carcinogenic diol epoxides of polycyclic aromatic hydrocarbons

Murine class Alpha glutathione (GSH) transferase A1-1 (mGSTA1-1) is unique among mammalian Alpha class GSTs due to its exceptionally high catalytic ac tivity toward (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo [a ]pyrene [(+)-anti-BPDE], which is the activated metabolite of an environmen tally relevant carcinogen, benzo[a] pyrene (BP). However, the molecular bas is for high catalytic activity of mGSTA1-1 toward (+)-anti-BPDE is not clea r. In the present study, we demonstrate that an arginine residue at positio n 216, which is conserved in some but not all mammalian class Alpha GSTs, p lays an important role in catalytic activity of mGSTA1-1 toward (+)-anti-BP DE and carcinogenic diol epoxides of other environmentally relevant polycyc lic aromatic hydrocarbons (PAHs). The catalytic efficiency (k(cat)/K-m) of mGSTA1-1 for the GSH conjugation of (+)-anti-BPDE (108/mM/s) was reduced by about 58 % upon replacement of arginine 216 with alanine (R216A). This was mainly due to a significantly lower Vma for the R216A mutant of mGSTA1-1 c ompared with wildtype mGSTA1-1. The R216A mutation also resulted in a stati stically significant reduction (> 70%) in specific activity of mGSTA1-1 tow ard racemic anti-diol epoxides of chrysene and benzo[c]phenanthrene (anti-C DE and anti-B[c]PDE, respectively). The catalytic activity of mGSTA2-2, whi ch is a close structural homologue of mGSTA1-1, was also reduced upon R216A mutation. The results of the present study clearly indicate that an argini ne residue at position 216 is critical for catalytic activity of mGSTA1-1 a nd mGSTA2-2 toward carcinogenic diol epoxide metabolites of various PAHs th at are abundant in the environment and suspected human carcinogens.