In Vitro Biotransformation of Methyl Parathion By Human Placental and Fetal Liver Glutathione S-Transferase (Hplc, Organophosphates).

Methyl parathion (MeP), a commonly used organophosphorus pesticide, is fetotoxic and teratogenic in animals. The two major biochemical pathways of MeP metabolism in mammals are cytochrome P-450 (P-450) and glutathione S-transferase (GSHTr). Since P-450 content of human placenta and fetal liver is minimal, GSHTr-mediated metabolism of MeP is likely to be the predominant metabolic pathway in vivo. Data on the biotransformation of MeP by the human feto-placental unit are lacking. This study was undertaken to purify and characterize human placental and fetal liver GSHTr and to evaluate their role in the metabolism of MeP. Term placental GSHTr was purified by affinity chromatography and High Performance Liquid Chromatography (HPLC). Pre-term and fetal liver GSHTrs were partially purified by affinity chromatography. Two interconvertible forms of term placental GSHTr were quantitatively separated by HPLC. The isolation of two conformers was dependent upon the concentration of reduced glutathione (GSH). Kinetic studies using ethacrynic acid, a highly specific substrate for anionic forms of GSHTr, were conducted. These studies demonstrated that term placental GSHTr can exist as two catalytically distinct conformers, a high affinity-low activity and a low affinity-high activity form for GSH. Since the average tissue concentration of GSH for pre-term and term placenta is low (0.11-0.19 mM), the results suggest that the high affinity-low activity conformer would predominate in vivo. HPLC analysis of placental and fetal liver GSHTr-mediated biotransformation of MeP revealed the presence of desmethyl parathion (DesMeP) as the sole metabolite. Term placental activity (mean (+OR-) S.E.) was 2.97 (+OR-) 0.14 nmoles DesMeP formed/min/mg, while an activity of 0.82 (+OR-) 0.07 nmoles DesMeP formed/min/mg was observed with pre-term. Fetal liver activity was 73 (+OR-) 11 nmoles MeP formed/min/mg and is approximately 7-40 fold greater than the specific activities reported for purified forms of rat liver GSHTr. The ability of human placental and fetal liver GSHTr to metabolize MeP exclusively to DesMeP via O-dealkylation represents a major biochemical difference from that of the neutral/acidic isozymes of adult rat liver, which were shown to catalyze both O-dealkylation and O-dearylation, and is considered a detoxification reaction.PhDPharmacologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161151/1/8621357.pd