Removal of Chlorpyrifos and Diazinon from Retail Sales
Superfund Research Program
The collective body of the research carried out by Drs. Lucio Costa and Clement Furlong on the importance of genetic variability in the human paraoxonase (PON1) gene in determining sensitivity to specific organophosphate exposures has provided regulators with data that was important in the Environmental Protection Agency's (EPA's) negotiating removal of chlorpyrifos and diazinon from retail sales and home use. One of the important considerations was the demonstration that it required from 6 months to two years for infants to develop mature levels of plasma PON1. The studies also demonstrated a sub-class of adults that are predicted to manifest high sensitivity to exposure to these two organophosphate (OP) compounds.
Another important spin-off from these studies was the demonstration that individuals with low PON1 levels have increased risk for carotid artery disease. The extension of these studies identified more than 150 new polymorphisms in the PON1 gene and the identification of a major polymorphism in the promoter region that had a significant affect on plasma PON1 levels (C-108T).
A more recent development from these studies was the demonstration that all three members of the PON1 family of enzymes are capable of inactivating a quorum sensing factor of Pseudomonas that is important in regulating biofilm formation and pathogenesis genes.
More recent experiments on mixed exposures indicates that a prior or concurrent exposure to chlorpyrifos oxon potentiates the toxicity of malaoxon, the toxic metabolite of malathion. The best explanation for this observation is that chlorpyrifos oxon inactivates carboxylesterases that are important in detoxifying malaoxon/malathion.
Taken together, these studies have pointed out the importance of using the two-substrate, high throughput assay to establish an individual's PON1 status (phenotype or PON1 level and position 192 genotype). Many studies have been carried out that have ignored the importance of determining PON1 levels which vary by 15-fold among individuals. These studies analyzed only PON1 single-nucleotide polymorphisms (SNPs), which cannot predict PON1 levels. In the PON1 project, we have developed a new two-substrate protocol for determination of PON1 status that does not use toxic substrates and will be suitable for transfer to clinical laboratories. This assay will be useful for establishing an individual's risk for exposure to diazoxon/diazinon and chlorpyrifos/chlorpyrifos oxon. It will also be useful in examining an important risk factor for carotid artery disease, understanding individual pharmacokinetic differences and most likely for understanding differences in risk of Pseudomonas infection in cystic fibrosis (CF) patients. Studies are underway to determine if PON1 status is a risk factor for Parkinson's disease in collaboration with other SRP researchers.