Glutathione S-transferase-catalyzed conjugation of bioactivated aflatoxin B-1 in human lung: differential cellular distribution and lack of significance of the GSTM1 genetic polymorphism

Epidemiological studies suggest that aflatoxin B-1 (AFB(1)), a mycotoxin pr oduced by certain Aspergillus species, may play a role in human respiratory cancers in occupationally-exposed individuals. AFB1 requires bioactivation to the corresponding exo-8,9-epoxide for carcinogenicity, and glutathione S-transferase (GST)-catalyzed conjugation of the epoxide with glutathione ( GSH) is a critical determinant of susceptibility to AFB1, Of the purified h uman GST enzymes studied, the polymorphic hGSTM1-1 has the highest activity towards AFB1 exo-epoxide, The influence of the GSTM1 polymorphism on AFB(1 )-GSH formation, as well as the abilities of cytosols from preparations enr iched in different isolated lung cell types to conjugate AFB(1)-epoxides, w ere examined. In whole-lung cytosols from patients undergoing clinically in dicated lobectomy, GSTM1 genotype correlated with GSTM1 phenotype as determ ined by [H-3]trans-stilbene oxide conjugation: GSTM1-positive = 295 +/- 31 pmol/mg/h (n = 6); GSTM1-negative = 92.8 +/- 23.3 pmol/mg/h (n = 4) (P < 0. 05), In contrast, conjugation of microsome-generated [3H]AFB1-epoxides with GSH was low and variable between patients, and did not correlate with GSTM 1 genotype: GSTM1-positive 11.9 +/- 8.1, 111 +/- 66 and 510 +/- 248 fmol/mg /h (n = 6); GSTM1-negative = 15.3 +/- 16.7, 167 +/- 225 and 540 +/- 618 fmo l/mg/h (n = 4) (for 1, 10 and 100 mu M [3H]AFB1, respectively). GSH conjuga tes of AFB1 exo-epoxide and the much less mutagenic stereoisomer AFB1 endo- epoxide were produced in a ratio of similar to 1:1 in cytosols from both wh ole lung and isolated cells. Total cytosolic AFB(1)-epoxide conjugation was significantly higher in fractions enriched in alveolar type II cells (3.07 +/- 1.61 pmol/mg/h) than in unseparated lung cells (0.143 +/- 0.055 pmol/m g/h) or fractions enriched in alveolar macrophages (0.904 +/- 0.319 pmol/mg /h; n = 4) (P < 0.05), Furthermore, AFB1-GSH formation and percentage of al veolar type II cells in different cell fractions were correlated (r = 0.78, P < 0.05). These results demonstrate that human lung GSTs exhibit very low conjugation activity for both AFB(1)-8,9-epoxide stereoisomers, and that t his activity is heterogeneously distributed among cell types, with alveolar type II cells exhibiting relatively high activity. Of the GSTs present in human peripheral lung which contribute to AFB(1) exo- and endo-epoxide deto xification, hGSTM1-1 appears to play at most only a minor role.