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Published on: October 15, 2018
Kinetic parameters of OPT pesticide desulfuration by c-DNA expressed human CYPs
Franca M Buratti1, Maria Teresa Volpe, Laura Fabrizi
1Comparative Toxicology and Ecotoxicology Laboratory, Istituto Superiore di Sanità, Viale Regina Elena 299, I-00161 Rome, Italy.
Abstract:
The role of different cytochrome P450 isoforms (CYPs) in the desulfuration of four organophosphorothionate pesticides (OPTs), namely diazinon (DIA), azinphos-methyl (AZ), chlorpyrifos (CPF) and parathion (PARA), at OPT levels representative of actual human exposure has been investigated. For this purpose c-DNA expressed human CYPs and a method, based on acetylcholinesterase (AChE) inhibition, able to detect nM levels of oxon have been used. Our results indicate that the four tested OPTs at low concentration were mainly desulfurated by CYP2B6, 2C19 and 1A2, showing K(m) values in the range 0.8-5 μM and the highest efficiency (intrinsic clearance (ICL)) values. CYP3A4 was generally endowed with high K(m) and resulted linear up to 25-100 μM OPT, concentrations saturating the most efficient CYPs. The tentative extrapolation of the relative contribution of single CYPs, taking into account the average content of different isoforms in the human liver, indicate that CYP1A2 is the major responsible for oxon formation. Indeed this CYP catalyses the 50-90% of desulfuration reaction, depending on the OPT. As CYP3A4 activity is not completely saturated up to 100 μM OPT, and due to the high hepatic content, its contribution to oxon formation may result relevant in poisoning episodes, when individuals are exposed at high doses of OPTs.
Insights
Cytochrome P450 enzymes, particularly CYP1A2, are key in detoxifying organophosphorothionate pesticides (OPTs) at low exposure levels. CYP3A4 becomes significant at higher doses, relevant in poisoning cases.
Area of Science:
- Biochemistry
- Toxicology
- Pharmacology
Background:
- Organophosphorothionate pesticides (OPTs) are widely used, posing potential human health risks.
- Understanding the metabolic pathways of OPTs is crucial for assessing exposure risks and developing interventions.
Purpose of the Study:
- To investigate the role of specific human cytochrome P450 (CYP) isoforms in the desulfuration of four common OPTs: diazinon, azinphos-methyl, chlorpyrifos, and parathion.
- To determine the contribution of different CYPs at exposure levels relevant to humans.
Main Methods:
- Utilized cDNA-expressed human CYPs to study pesticide metabolism.
- Employed an acetylcholinesterase (AChE) inhibition assay capable of detecting nanomolar levels of the toxic oxon metabolites.
Main Results:
- At low concentrations, CYP2B6, CYP2C19, and CYP1A2 were the primary enzymes responsible for OPT desulfuration, exhibiting high efficiency (intrinsic clearance) and low K(m) values (0.8-5 μM).
- CYP3A4 showed high K(m) values and linearity up to 25-100 μM OPT, indicating saturation of other CYPs at these levels.
- Extrapolation suggests CYP1A2 is the major contributor (50-90%) to oxon formation at typical exposure levels.
- CYP3A4's contribution becomes significant at high OPT doses due to its high hepatic abundance and incomplete saturation.
Conclusions:
- CYP1A2 plays a dominant role in the low-dose metabolism of these OPTs.
- CYP3A4's role in oxon formation increases substantially at high exposure levels, particularly relevant in acute poisoning incidents.
