Oxidase activity, DEHP

Because the metabolism of DEHP was catalyzed by so many fractions of the trout liver homogenate, these fractions were characterized by measurement of marker enzymes to determine which organelles actually were responsible for the observed DEHP metabolism. Succinic dehydrogenase activity was used as a marker for mitochondria, whereas glucose-6-phosphatase was used as a marker for microsomes. The distribution of DEHP oxidase activity (production of polar metabolites 1 and 2 with added NADPH) and of DEHP esterase activity (production of monoester without added NADPH) were also determined. It was found (Figure 2) that the distribution of DEHP oxidase activity parallels the distribution of microsomal activity and the distribution of DEHP esterase activity parallels the distribution of microsomal activity, but is also present in the cytosol fraction. 

In vitro and in vivo data on Cynomolgus monkeys (Table 17.3) and other species of monkeys (i.e., marmoset and Rhesus) indicate that the key events in the PPARa activator MOA are relatively nonresponsive in monkeys. Palmitoyl-CoA oxidase activity was evaluated in monkeys after in vivo exposure to a variety of PPARa activators [e.g., bezafibrate, clofibrate, DEHP, mono-2-ethylhexyl phthalate (MEHP), fenofibrate, nafenopin, and LY171883], and changes were minimal or nonexistent relative to controls (Klaunig et al. 2003). Moreover, Cynomolgus monkeys exposed to DEHP, DINP, or clofibrate failed to exhibit an increase in cell proliferation (Doull et al. 1999 Pugh et al. 2000). Cynomolgus monkeys treated for two weeks with clinically relevant doses of the PPARa activators fenofibrate or ciproflbrate exhibited increases in the number of hepatic peroxisomes (Hoivik et al. 

Figure 2. Distribution of marker enzymes and DEHP-metabolizing enzymes in trout liver homogenate fractions. DEHP esterase and DEHP oxidase were each measured by 1-hr incubations of 0.010 ftmol of UC-DEHP in a total volume of 2 mL. Fraction (A), 2,000 g pellet (B), 10,000 g pellet (C), 100,000 g pellet and (D), 100,000 g supernatant. Relative Specific Activity = percent of total activity/ percent of total protein (14).

Hepatic Effects. No studies were located regarding hepatic effects in humans after oral exposure to DEHP. Limited information on hepatic effects in humans exposed to DEHP is available from studies of dialysis patients and cultured human hepatocytes. In one individual there was an increased number of liver peroxisomes after 1 year, but not after 1 month of treatment (Ganning et al. 1984, 1987). A serious limitation of this observation is that repeat biopsies were not obtained from the same patient, so that an appropriately controlled analysis is not possible. Additionally, analysis of liver biopsies from patients receiving other kinds of hypolipidemic drugs has not yielded any evidence for peroxisomal proliferation (Doull et al. 1999). Recognizing some limitations of using primary hepatocytes in vitro because of their tendency to lose some metabolic capabilities (Reid 1990), in cultured human hepatocytes there were no changes in the activities of peroxisomal palmitoyl-CoA oxidase and/or carnitine acetyltransferase when... 

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