Catalytic activities of human alpha class glutathione transferases toward carcinogenic dibenzo[a,l]pyrene diol epoxides

Chem Res Toxicol. 2002 Jun;15(6):825-31. doi: 10.1021/tx025519i.

Abstract

In this study, human glutathione transferases (GSTs) of alpha class have been assayed with the ultimate carcinogenic (-)-anti- and (+)-syn-diol epoxides (DEs) derived from the nonplanar dibenzo[a,l]pyrene (DBPDE) and the (+)-anti-diol epoxide of the planar benzo[a]pyrene [(+)-anti-BPDE] in the presence of glutathione (GSH). In all DEs, the benzylic oxirane carbon reacting with GSH, possess R-absolute configuration. GSTA1-1 demonstrated activity with all DEs tested whereas A2-2 and A3-3 only were active with the DBPDE enantiomers. With GSTA4-4, no detectable activity was observed. GSTA1-1 was found to be the most efficient enzyme and demonstrated a catalytic efficiency (k(cat)/K(m)) of 464 mM(-)(1) s(-)(1) with (+)-syn-DBPDE. This activity was about 7-fold higher than that observed with (-)-anti-DBPDE and more than 65-fold higher than previously observed with less complex fjord-region DEs. GSTA3-3 also demonstrated high k(cat)/K(m) with the DEs of DBP and a high preference for the (+)-syn-DBPDE enantiomer [190 vs 16.2 mM(-)(1) s(-)(1) for (-)-anti-DBPDE]. Lowest k(cat)/K(m) value of the active enzymes was observed with GSTA2-2. In this case, 30.4 mM(-)(1) s(-)(1) was estimated for (+)-syn-DBPDE and 3.4 mM(-)(1) s(-)(1) with (-)-anti-DBPDE. Comparing the activity of the alpha class GSTs with (-)-anti-DBPDE and (+)-anti-BPDE revealed that GSTA1-1 was considerable more active with the former substrate (about 25-fold). Molecular modeling studies showed that the H-site of GSTA1-1 is deeper and wider than that of GSTA4-4. This is mainly due to the changes of Ser212-->Tyr212 and Ala216-->Val216, which cause a shallower active site, which cannot accommodate large substrates such as DBPDE. The higher activity of GSTA1-1 with (+)-syn-DBPDE relative to (-)-anti-DBPDE is explained by the formation of more favorable interactions between the substrate and the enzyme-GSH complex. The presence of GSTA1-1 in significant amounts in human lung, a primary target tissue for PAH carcinogenesis, may be an important factor for the protection against the harmful action of this type of potent carcinogenic intermediates.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / chemistry
  • 7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / pharmacokinetics*
  • Benzopyrenes / chemistry
  • Benzopyrenes / pharmacokinetics*
  • Carcinogens / chemistry
  • Carcinogens / pharmacokinetics*
  • Epoxy Compounds / chemistry
  • Epoxy Compounds / pharmacokinetics*
  • Glutathione / chemistry
  • Glutathione / metabolism
  • Glutathione Transferase / chemistry
  • Glutathione Transferase / genetics
  • Glutathione Transferase / metabolism*
  • Humans
  • Inactivation, Metabolic
  • Isoenzymes / chemistry
  • Isoenzymes / genetics
  • Isoenzymes / metabolism*
  • Kinetics
  • Liver / enzymology
  • Lung / enzymology
  • Models, Molecular
  • Molecular Conformation
  • Stereoisomerism
  • Substrate Specificity
  • Thermodynamics

Substances

  • Benzopyrenes
  • Carcinogens
  • Epoxy Compounds
  • Isoenzymes
  • 11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo(a,l)pyrene
  • 7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide
  • Glutathione Transferase
  • glutathione S-transferase alpha
  • Glutathione