All experiments in this study were performed at
All experiments in this study were performed at pD 6.1 for reasons described above. This leaves a question about the implications of stereochemistry in vivo. At physiological pH the GSTP1-1 catalyzed rate of EASG formation is only 1.1-fold higher than the non-enzymatic rate . However, when human melanoma cells were exposed to ethacrynic Berberine Sulfate for 2 h, diastereoisomer A was the principal conjugate present in the medium. This might be the result of either the stereoselective formation of EASG by GSTP1-1, present in the cells, and/or the stereoselective transport out of the cells by transport pumps, i.e. the multidrug resistance associated protein (MRP). Results from a recent study describing non-stereoselective transport of prostaglandin A2-glutathione conjugates by MRP indicate that very likely no stereoselective transport will occur for the EASG diastereoisomers. Stereoselectivity thus plays a role in cellular systems and further research on this topic would be very valuable.
Introduction Brazil is the largest sugarcane producer in the world with about 4.5 ×106 ha of sugarcane fields and 320 ethanol production plants (http://www.portalunica.com.br/index.jsp). A common procedure in this country is to burn the leaves before harvesting the sugarcane. The burning leads to production of soot, suspected to increase respiratory problems in the exposed workers (Zamperlini et al., 1997). Sugarcane workers are exposed to various genotoxic compounds, including polycyclic aromatic hydrocarbons (PAHs) derived from an incomplete organic combustion process (Zamperlini et al., 1997). PAHs undergo metabolic activation by phase I enzymes, mainly cytochrome P4501A1 (CYP1A1), to diol epoxides which are capable of binding covalently to DNA, potentially initiating a carcinogenic process (Hall and Grover, 1990). Activated PAH metabolites can be detoxified by phase II enzymes, such as glutathione S-transferases (GSTs) and uridine diphosphoglucuronosyltransferase (UDP glucuronyl-transferase), which catalyze conjugative reactions of oxidative products (Nerurkar et al., 2000). In humans, a substantial variability in the biological response to PAHs is expected, since there are interindividual differences in the rate and metabolism pathways of these compounds, determined by genetic polymorphisms of phases I and II enzymes (Alexandrie et al., 2000). The CYP1A1 gene is characterized by several polymorphisms. Two of them, one in the 3′-noncoding region (CYP1A1⁎2A, T3801C) and the other within exon 7 (CYP1A1⁎2B, A2455G), are of interest in the biotransformation of PAHs and have been extensively studied to evaluate their possible role in DNA damage and cancer promotion (Grzybowska et al., 2000, Gaspar et al., 2004). The CYP1A1⁎4 (C2453A) polymorphism also has shown to produce a variant protein that possesses different catalytic activities when compared to the wild-type protein (Schwarz et al., 2001) and was associated with an increased risk of endometrial cancer (Esteller et al., 1997). The GSTM1, GSTT1 and GSTP1 genes produce important biotransformation enzymes (Seidegard, 1990, Pemble et al., 1994, Harries et al., 1997). The lack of GSTM1 activity, caused by an inherited homozygous deletion of the GSTM1 gene (null genotype), has been associated with an increased risk of lung cancer (Pinarbasi et al., 2003, Mohr et al., 2003). The GSTT1 polymorphism is also caused by a deletion that results in total lack of gene product. The null genotype of GSTT1 was reported to be associated with an increased risk of oral and lung cancer (Sreelekha et al., 2001, Sorensen et al., 2004). The polymorphism of the GSTP1 gene, which consists in an A→G transition at nucleotide 313 in exon 5, leads to an Ile→Val substitution in the substrate-binding active site of the enzyme. This substitution has been associated with a reduced conjugating activity of the enzyme (Taningher et al., 1999) and also with higher levels of polycyclic aromatic hydrocarbon (PAH)-DNA adducts in human lymphocytes (Miller et al., 2003).