Interactions between plasticizers and fatty acid metabolism in the perfused rat liver and in vivo: Inhibition of ketogenesis by 2-ethylhexanol

Abstract

Rates of ketone body (β-hydroxybutyrate plus acetoacetate) production by perfused livers from starved rats were decreased about 60% from 39 ± 2 to 17 ± 3 μmol/g/hr by 2-ethylhexanol (200,μM), a primary metabolite of the plasticizer diethylhexyl phthalate. Inhibition of ketogenesis by ethylhexanol was dose dependent (half-maximal inhibition occurred with 25 μM) in the presence or absence of 4-methylpyrazole, an inhibitor of alcohol dehydrogenase. Concentrations of β-hydroxbutyrate relative to acetoacetate (B/A) increased in a step-wise manner from 0.32 to 0.75 in the effluent perfusate when ethylhexanol was infused. In contrast, the B/A ratio decreased in parallel with inhibition of ketone body production when alcohol dehydrogenase was inhibited. Pretreatment of rats with phenobarbital, an inducer of ω and ω-1 hydroxylases, diminished inhibition of ketone body production by low (<50 μM) but not high concentrations (>50 μM) of ethylhexanol. Thus, ethylhexanol is oxidized via phenobarbitalinducible pathways to metabolites which do not inhibit ketogenesis. Studies were conducted to determine the site of inhibition of fatty acid oxidation by ethylhexanol. Rates of ketone body production in the presence of oleate (250 μM), which requires transport of the corresponding CoA compound into mitochondria, were reduced from 80 ± 6 to 58 ± 8 μmol/g/hr by ethylhexanol. In contrast, ketone body production from hexanoate, which is activated in the mitochondria, was not affected by ethylhexanol. Basal and oleate-stimulated rates of H 2O 2 production were not affected by ethylhexanol, indicating that peroxisomal β-oxidation was not altered by the compound. Based on these data it is concluded that 2-ethylhexanol inhibits β-oxidation of fatty acids in mitochondria but not in peroxisomes. Treatment of rats with ethylhexanol (0.32 g/kg, i.p.) decreased plasma ketone bodies from 1.6 to 0.8 mM, increased hepatic triglycerides and increased lipid predominantly in periportal regions of the liver lobule. These data indicate that alterations in hepatic fatty acid metabolism in periportal regions of the liver lobule may be early events in peroxisome proliferation.