The metabolism of di(2-ethylhexyl) phthalate (DEHP) and mono-(2-ethylhexyl) phthalate (MEHP) in rats: in vivo and in vitro dose and time dependency of metabolism

Abstract

This study investigated the in vivo metabolism of di(2-ethylhexyl) phthalate (DEHP) and mono(2-ethylhexyl) phthalate (MEHP) in rats after multiple dosing, the metabolism of MEHP in primary rat hepatocyte cultures for periods of up to 3 days, and the biotransformation of some major metabolites of MEHP. Rats were orally administered [14C]DEHP or [14C]MEHP at doses of 50 and 500 mg/kg body wt for three consecutive days. Urine was collected at 24-hr intervals, and metabolite profiles were determined. After a single dose of either compound, urinary metabolite profiles were similar to those previously reported. However, after multiple administration of both DEHP and MEHP at 500 mg/kg, increases in omega-/beta-oxidation products [metabolites I and V, mono(3-carboxy-2-ethylpropyl) phthalate and mono(5-carboxy-2-ethylpentyl) phthalate, respectively] and decreases in omega - 1-oxidation products [metabolites VI and IX, mono(2-ethyl-5-oxohexyl) phthalate and mono(2-ethyl-5-hydroxyhexyl) phthalate, respectively] were seen. At the low dose of 50 mg/kg little or no alteration in urinary metabolite profiles was observed. At 500 mg/kg of MEHP a 4-fold stimulation of CN- -insensitive palmitoyl-CoA oxidation (a peroxisomal beta-oxidation marker) was seen after three consecutive daily doses. At the low dose of 50 mg/kg only a 1.8-fold increase was noted. Similar observations were made with rat hepatocyte cultures. MEHP at concentrations of 50 and 500 microM was extensively metabolized in the rat hepatocyte cultures. Similar metabolic profiles to those seen after in vivo administration of MEHP were observed. At the high (500 microM) concentration of MEHP, changes in the relative proportions of omega- and omega- 1-oxidized metabolites were seen. Over the 3-day experimental period, omega-/beta-oxidation products increased in a time-dependent manner at the expense of omega - 1-oxidation products. At a concentration of 500 microM MEHP, a 12-fold increase of CN- -insensitive palmitoyl CoA oxidation (a peroxisomal beta-oxidation marker) was observed. At the low concentration of MEHP (50 microM) only a 3-fold increase in CN- -insensitive palmitoyl-CoA oxidation was noted and little alteration in the metabolite profile of MEHP was observed with time. Biotransformation studies of the metabolites of MEHP confirmed the postulated metabolic pathways. Metabolites I and VI appeared to be endpoints of metabolism, while metabolite V was converted to metabolite I, and metabolite IX to metabolite VI. It was also possible to reduce the transformation of metabolite X [mono(2-ethyl-6-hydroxyhexyl) phthalate] to metabolite V.(ABSTRACT TRUNCATED AT 400 WORDS)