title: Catalytic activities of human alpha class glutathione transferases toward carcinogenic dibenzo[a,l]pyrene diol epoxides.
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.
authors.affiliation: The Institute of Environmental Medicine, Division of Biochemical Toxicology, Karolinska Institutet, Box 210, SE-17177 Stockholm, Sweden.
authors.forename: Kristian
authors.initials: K
authors.lastname: Dreij
authors.forename: Kathrin
authors.initials: K
authors.lastname: Sundberg
authors.forename: Ann-Sofie
authors.initials: AS
authors.lastname: Johansson
authors.forename: Erik
authors.initials: E
authors.lastname: Nordling
authors.forename: Albrecht
authors.initials: A
authors.lastname: Seidel
authors.forename: Bengt
authors.initials: B
authors.lastname: Persson
authors.forename: Bengt
authors.initials: B
authors.lastname: Mannervik
authors.forename: Bengt
authors.initials: B
authors.lastname: Jernström
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000737:chemistry / Q000493:pharmacokinetics*; D001580:Benzopyrenes / Q000737:chemistry / Q000493:pharmacokinetics*; D002273:Carcinogens / Q000737:chemistry / Q000493:pharmacokinetics*; D004852:Epoxy Compounds / Q000737:chemistry / Q000493:pharmacokinetics*; D005978:Glutathione / Q000737:chemistry / Q000378:metabolism; D005982:Glutathione Transferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008658:Inactivation, Metabolic; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008168:Lung / Q000201:enzymology; D008958:Models, Molecular; D008968:Molecular Conformation; D013237:Stereoisomerism; D013379:Substrate Specificity; D013816:Thermodynamics
chemical_list: D001580:Benzopyrenes; D002273:Carcinogens; D004852:Epoxy Compounds; D007527:Isoenzymes; C087596:11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo(a,l)pyrene; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; D005982:Glutathione Transferase; C119995:glutathione S-transferase alpha; D005978:Glutathione