Profenofos metabolism and estimates of profenofos exposure in Egyptian cotton field workers
Dadson, Oswald Appiasam
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A pesticide is a substance or a mixture of any substances intended for preventing, destroying, repelling or mitigating any pests. Organophosphate (OP) pesticides are some of the most widely used pesticides in the United States and throughout the world so human exposure to these compounds is a growing concern. Profenofos is a thiophosphate organophosphorus (OP) pesticide capable of inhibiting B-esterases such as acetylcholinesterase, butyrylcholinesterase, and carboxylesterase. Detoxification of profenofos is known to be mediated by cytochrome P450s (CYPs) to form the biologically inactive metabolite 4-bromo-2-chlorophenol (BCP), which is excreted in the urine. The goals of this thesis were to investigate the metabolism of profenofos by human liver microsomes and recombinant human CYPs as well as determine the concentration of BCP, a profenofos specific metabolite, in urine from Egyptian cotton field workers involved in profenofos application. Human liver microsomes and recombinant CYPs were incubated with profenofos and the rate of BCP formation was assessed. A preliminary screen with 9 human CYPs found that only CYPs 3A4, 2B6, and 2C19 were able to metabolize profenofos. The K m provides a measure of the relative affinity of these CYPs for profenofos metabolism, with CYP 2C19 having the highest affinity (lowest K m ) (0.516 μM) followed by 2B6 (1.02 μM) and 3A4 (18.9 μM). The relative rate of BCP formation as measured by V max is 2B6 (47.9 nmol/min/nmol CYP) > 2C19 (25.1 nmol/min/nmol CYP) > 3A4 (19.2 nmol/min/nmol CYP). The relative efficiency of each CYP for profenofos metabolism, as measured by intrinsic clearance (V max /K m ) is 2C19 (48.64 ml/min/nmol CYP) > 2B6 (46.96 ml/min/nmol CYP) > 3A4 (1.02 ml/min/nmol CYP). In vitro metabolism studies demonstrate that human CYP 2C19 and CYP 2B6 are the primary enzymes that mediate the detoxification of profenofos to BCP. Daily urinary BCP concentrations were also utilized as a biomarker of profenofos exposure in Egyptian cotton field workers during 8–10 consecutive days of profenofos application to cotton fields. Substantial interindividual variability was observed in urinary BCP concentrations which ranged from 0.9–8,053μg/g creatinine. From these BCP levels, estimates of daily absorbed profenofos were also calculated and ranged from 0.037–327μg/kg/day. This is the first study to estimate the internal dose of profenofos associated with occupational use of this pesticide.