For several years we have researched the kinetics, metabolism and human toxicology of dichloroacetate (DCA), a potentially harmful metabolite of trichloroethylene (TCE). This new proposal continues this research and also integrates it with new human studies of chloral hydrate (CH), which is both a primary metabolite of TCE and a precursor of DCA. Indeed, recent advances in our understanding of the mechanisms of DCA and CH biotransformation in vivo provide new insight into the possible causes of the adverse effects of chronic DCA and CH exposure in humans. Specifically, we have discovered that 1) children metabolize CH to DCA, 2) CH and DCA alter each other's clearance in vivo, 3) DCA is dechlorinated to glyoxylate, 4) dechlorination is mediated by hepatic maleylacetoacetate isomerase (MAAI), an enzyme of tyrosine catabolism, 5) DCA inhibits MAAI expression in animals causing, buildup of potentially toxic tyrosine intermediates, and 6) these metabolites may be responsible for DCA (and CH) toxicity. Humans exhibit polymorphisms of MAAI that may possess different kinetic properties toward DCA. In turn, human haplotype variability may influence DCA's kinetics and toxicology. These hypotheses will be tested by accomplishing the following Specific Aims:
Specific Aim 1 : Quantify the influence of DCA, at exposure levels ranging from environmental (mu g/kg/d) to clinical (mg/kg/d), on human liver MAAI and tyrosine catabolism.
This aim tests the hypotheses that 1) there is a dose-dependent effect of DCA on human hepatocellular tyrosine metabolism in general and on the accumulation of potentially hepatotoxic tyrosine intermediates in particular, 2) DCA inhibits hepatic MAAI expression in vivo in humans in a dose-dependent manner and 3) DCA, tyrosine and/or their metabolites form adducts with MAAI.
Specific Aim 2 : Establish the relationship between human MAAI haplotype and DCA and tyrosine metabolism.
This aim tests the postulates that MAAI haplotype determines, and thus can predict, 1) dose-dependent DCA kinetics and biotransformation and 2) DCA's effects on tyrosine metabolism.
Specific Aim 3 : Determine the in vivo kinetics and biotransformation of CH in healthy adults and the influence of CH and DCA on each other's metabolism.
This specific aim will examine three postulates 1) CH is-metabolized in adults to DCA, 2) CH, via DCA formation, inhibits its own metabolism and that of tyrosine and 3) these effects are dependent upon exposure level but not upon gender.
