Stacpoole an association between TCE exposure and renal cancer in humans (Brüning et al. 2003; Charbotel et al. 2006), but no experimental or clinical evidence is currently available to implicate DCA as a cause of this malignancy. Indeed, no epidemiological investigations have studied the carcinogenicity of DCA in humans. Lee et al. (2003) found a much weaker epide- miological association between environmental exposure to TCE and liver cancer. In this case, however, the relationship between TCE and DCA becomes highly significant, because DCA is unquestionably a liver carcinogen in certain inbred strains of mice and rats (reviewed by IARC 2004; Stacpoole et al. 1998b). Most of the studies that have investigated the carcino- genic potential of DCA have used drinking water or, occasionally, gavage to administer the compound for weeks or months at effective doses usually of 75 mg/kg/day or more [see Supplemental Material, Text A (doi:10.1289/ ehp.1002554)]. Regardless of the precise mechanism(s) that underlies DCA’s carcino- genic potential, it is clear that hepatotoxicity in rodents from DCA is obtainable only at doses thousands of times greater than those to which humans are normally exposed from the com- bined effects of the atmosphere and chlorinated drinking water. This fact, together with current data from population studies, does not sup- port the postulate that exposure to DCA from environmental sources represents a general haz- ard to humans. Whether chronic exposure to DCA at concentrations encountered in the environment or in the clinic is problematic to select human populations is addressed below. Are certain populations at increased health risk or benefit? As a medicinal agent, DCA has been administered as the sodium salt in doses ranging from 10 to > 100 mg/kg/day by intravenous and oral routes for a few days to > 10 years [see Supplemental Material, Text B (doi:10.1289/ehp.1002554)]. These clinical studies have reported occasional asymptomatic increases in serum concentrations of alanine aminotransferase and aspartate aminotrans- ferase that seldom exceeded two to three times the pretreatment values of these enzymes and that returned to pretreatment levels within a few weeks after the DCA dose was reduced or stopped (Barshop et al. 2004; Berendzen et al. 2006; Stacpoole et al. 1998b). A more worri- some toxicity of DCA is reversible peripheral neuropathy, an effect replicated in rodents, dogs, and humans (Stacpoole 1989). Clinical features include tingling of the fingers and toes and weakness of the facial and distal muscles of the extremities, accompanied by reduc- tion in nerve conduction velocity in the sural and tibial nerves [see Supplemental Material, Text C (doi:10.1289/ehp.1002554)]. O n e p o s s i b l e e x p l a n a t i o n f o r t h e hepatic and neurological toxicity of DCA relates to the unusual consequences of its 156 biotransformation. DCA is dehalogenated to glyoxylate by the cytosolic zeta-1 family isoform of glutathione transferase (GSTz1), which is identical to maleylacetoacetate isomerase (MAAI), the penultimate enzyme in the phenylalanine/tyrosine catabolic path- way [see Supplemental Material, Figure 2 (doi:10.1289/ehp.1002554)]. DCA causes a mechanism-based inhibition of GSTz1/MAAI (Anderson et al. 1999), an effect so profound as to significantly delay the plasma clearance of DCA at both environmental (micrograms per kilogram per day) and clinical (micro- grams per kilogram per day) exposure levels in humans (Henderson et al. 1997; Jia et al. 2006; Schultz and Shangraw 2006; Stacpoole et al. 1998a) [see Supplemental Material, Text D (doi:10.1289/ehp.1002554)]. Genetic or DCA-induced ablation of GSTz1/MAAI causes accumulation of the MAAI substrates maleylacetone and, presum- ably, maleylacetoacetate (MAA) (Ammini et al. 2003; Cornett et al. 1999; Lim et al. 2004; Tzeng et al. 2000), although the latter compound is too unstable to be detected by gas chromatography–mass spectrometry. Depending on exposure level, recovery of basal GSTz1/MAAI enzyme activity occurs over days to months after the withdrawal of DCA (Guo et al. 2006; Saghir and Schultz 2002). The toxicological significance of inhi- bition of MAAI is highlighted by the con- sequences of loss of function mutations in fumarylacetoacetate (FAA) hydrolase, the ter- minal enzyme of phenylalanine and tyrosine catabolism. Deficiency of this enzyme causes heredity tyrosinemia type 1 (hepatorenal tyrosinemia), an autosomal recessive disease that has an estimated prevalence of 100,000 persons worldwide (Mitchell et al. 2001). In this disease, accumulation of the substrates for both the hydrolase and MAAI-catalyzed reactions are thought to be responsible for the dramatically increased risk of hepatocellular carcinoma in affected young children, because of the ability of MAA and FAA to form adducts (Lantum et al. 2002). This property is also shared by glyoxylate (Anderson et al. 2004), the product of the GSTz1/MAAI- mediated dehalogenation of DCA. Another important consequence of tyrosinemia type 1 is the diversion of carbon precursors to succinylacetone, high urinary levels of which are diagnostic for the disease. Succinylacetone is a competitive inhibitor of δ-aminolevulinate dehydratase (enzyme– inhibitor dissociation constant Ki = 0.3 µM; Sassa and Kappas 1983), which catalyzes an early step in heme biosynthesis. Inhibition of the dehydratase causes accumulation of δ-aminolevulinate, a neurotoxin that has been implicated in several peripheral neuropa- thies, including lead poisoning (see Felitsyn et al. 2008). Increased concentrations of δ-aminolevulinate or subsequent perturba- tions in heme metabolism or both are thought to be the cause of the neurological mani- festations of tyrosinemia type 1, including peripheral neuropathy (Lindberg et al. 1999; Mitchell et al. 2001). Given the effects of DCA on GSTz1/MAAI function, it is note- worthy that increased urinary concentrations of maleylacetone and δ-aminolevulinate have been measured in children who were treated for months or years with DCA. For some of these children, maleylacetone levels approached or exceeded those found in children with hereditary tyrosinemia type 1 (Shroads AL, Langaee T, Coats BS, Kurtz TL, Bullock JR, Weithorn D, Gong Y, Wagner DA, Ostrov DA, Johnson JA, Stacpoole PW, unpublished observations) [see Supplemental Material, Text E (doi:10.1289/ ehp.1002554)]. Recently, age and GSTz1/MAAI geno- type have emerged as important determinants of clinical toxicity from DCA. For example, in a randomized clinical trial of 43 children (mean age at entry, 5.6 years) with congen- ital forms of lactic acidosis, Stacpoole et al. (2006) found no difference in clinical or biochemical adverse effects, including hema- tological, hepatic, and peripheral nerve func- tion, between those children who received 25 mg/kg/day of sodium DCA and those who received placebo for 6 months. In contrast, a controlled clinical trial of 30 patients with mitochondrial encephalomyopathy, lactic acedosis, and stroke-like episodes (MELAS) whose mean age at entry was 30 years, ended prematurely because of drug-associated neurotoxicity (Kaufmann et al. 2006). In exam- ining the plasma kinetics of DCA in a repre- sentative sample of children and adults from both trials, a striking age-dependent decrease was discovered in plasma DCA clearance that was replicated in rats (Shroads et al. 2008); this finding confirmed and extended earlier obser- vations in rodents (James et al. 1998; Schultz et al. 2002). Furthermore, delayed clearance of DCA in rats was associated with increased urinary excretion of DCA, maleylacetone, and monochloroacetate (a putative neurotoxin and a very minor product of DCA metabo- lism) and with a decrease in urinary oxalate, an end product of DCA biotransformation [see Supplemental Material, Figure 1 and Text F (doi:10.1289/ehp.1002554)]. Age alone does not account entirely for variability in the kinetics and biotransforma- tion of DCA. The human GSTz1/MAAI gene contains four nonsynonymous single- nucleotide polymorphisms, with products that show different activities toward DCA and variable frequencies among ethnic and racial groups (Lim et al. 2004; Stacpoole et al. 2008; Shroads AL, Langaee T, Coats BS, Kurtz TL, Bullock JR, Weithorn D, Gong Y, ovlume 119 | number 2 | February 2011 • Environmental Health Perspectives