Project title: The Functional Significance of Genetic Polymorphisms in Human Glutathione S-transferases
Completed in: 2003
Significant time, energy and resources have been expended in recent years o eluidate the functional significance of human biotransformation gene polymorphisms toward mediating disease susceptibility upon exposure to environmental toxicants. The supposition underlying these studies is that if a genetic variation results in altered biotransformation activity toward a biologically plausible toxicant, then the genotype may be predictive of disease suceptibility phenotype. This supposition relies upon several factors holding true. First, in order for a direct relationship between genetic variation and intoxication susceptibility to be established, the polymorphism must change gene expression or enzyme activity or must otherwise influence the gene product's intoxication-mediating effects. Secondly, in order to predict a direct genotype-phenotype relationship, a role for the wild-type gene product in mediating intoxication must be established; the gene of interest should biotransform or otherwise affect the toxicant of interest. Finally, no in vivo factors should mask alterations in the gene product. Increased understanding of these factors will guide efforts to study biotransformation gene ecogenetic relationships, which have often been unfruitful.
To explore these factors, three studies of human glutathione S-transferases (GSTs) have been undertaken. Typically, the human GST gene family facilitates xenobiotic clearance by catalyzing glutathione conjugation of a variety of electrophilic substrates. To begin to determine whether hGST polymorphisms sould be predicted to affect pesticide clearance, the role of GSTs in biotransformation of the organophosphate pesticide, methyl parathion, was investigated. Biotransformation by two polymorphic hGSTs, hGSTT1-1 and hGSTA1-1, was demonstrated. However, no effect of the polymorphisms on human hepatic cytosolic biotransformation was detected. Secondly, glutathione-dependent biotransformation of the chloro-S-triazine pesticide, atrazine, was explored. Highly specific biotransformation of atrazine by pi class GST was revealed. However, purified recombinant protien corresponding to each of the four described hGSTP1 genotypes demonstrated similar atraxine conjugation capability. Finally, metabolites of estradiol were investigated as GST inhibitors; inhibition of GST couild potentially serve to mask the relationship between hGST genotype and biotransformation phenotype. Glutathione-conjugated estradiol metabolites did indeed inhibit hGSTM1-1 and hGSTA1-1 at concentrations in the nanomolar range. However, given the low endogenous concentration of estradiol, hepatic hGST inhibition by such metabolites is unlikely to occur.