DIFFERENCES IN THE CATALYTIC EFFICIENCIES OF ALLELIC VARIANTS OF GLUTATHIONE TRANSFERASE P1-1 TOWARDS CARCINOGENIC DIOL EPOXIDES OF POLYCYCLIC AROMATIC-HYDROCARBONS

Previous studies have identified allelic variants of the human glutath ione transferase (GST) Pi gene and showed that the two different encod ed proteins with isoleucine (GSTP1-1/I-105) or valine (GSTP1-1/V-105) at position 105, respectively, differ significantly in their catalytic activities with model substrates. Moreover, recent epidemiological st udies have demonstrated that individuals differing in the expression o f these allelic variants also differ in susceptibility to tumour forma tion in certain organs, including such in which polycyclic aromatic hy drocarbons (PAH) may be etiological factors, In the present study the catalytic efficiencies (k(cat)/K-m) of these GSTP1-1 variants were det ermined with a number of stereoisomeric bay-region diol epoxides, know n as the ultimate mutagenic and carcinogenic metabolites of PAH, inclu ding those from chrysene, benzo[a]pyrene and dibenz[a,h]anthracene. In addition, GSTP1-1 mutants in which amino residue 105 is alanine (GSTP 1-1/A-105) or tryptophan (GSTP1-1/W-105) have been constructed and cha racterized, GSTP1-1/V-105 was found to be more active than GSTP1-1/I-1 05 in conjugation reactions with the bulky diol epoxides of PAH, being up to 3-fold as active towards the anti-and syn-diol epoxide enantiom ers with R-absolute configuration at the benzylic oxiranyl carbon. Com paring the four enzyme variants, GSTP1-1/A-105 generally demonstrated the highest k(cat)/K-m value and GSTP1-1/W-105 the lowest with the ant i-diol epoxides. A close correlation was observed between the volume o ccupied by the amino acid residue at position 105 and the value of k(c at)/K-m. With the syn-diol epoxides, such a correlation was observed w ith alanine, valine and isoleucine, whereas tryptophan was associated with increased k(cat)/K-m values. The mutational replacement of isoleu cine with alanine or tryptophan at position 105 did not alter the enan tio selectivity of the GSTP1-1 variants compared with the naturally oc curring allelic variants GSTP1-1/I-105 and GSTP1-1/V-105. Since the am ino acid at position 105 forms part of the substrate binding site (H-s ite) the effect of increasing bulkiness is expected to cause restricte d access of the diol epoxide and proper alignment of the two reactants for efficient glutathionylation. In conclusion, the present study ind icates that individuals who are homozygous for the allele GSTP1 B (c oding for GSTP1-1/V-105) display a higher susceptibility to malignancy because of other factors than a decreased catalytic efficiency of GST P1-1/V-105 in the detoxication of carcinogenic diol epoxides of benzo[ a]pyrene or structurally related PAH.