The role of paraoxonase (PON1) in the detoxication of organophosphates and its human polymorphism.

In human populations, serum paraoxonase (PON1) exhibits a substrate dependent polymorphism. The Arg192 isoform hydrolyzes paraoxon rapidly but diazoxon, soman and especially sarin slowly. On the other hand, the Gln192 isoform hydrolyzes paraoxon slowly, but diazoxon, soman and sarin more rapidly than the Arg192 isoform. Our experiments with a mouse model system have convincingly shown that PON1 plays a major role in the detoxication of organophosphate (OP) compounds processed through the P450/PON1 pathway. Recent studies have also shown that PON1 plays an important role in the metabolism of oxidized lipid compounds. Currently, there is an effort underway to identify genes and polymorphisms that play an important role in 'environmental susceptibility'. The PON1 polymorphism has been cited as a prime example of such a genetic polymorphism. The advent of the polymerase chain reaction (PCR) for DNA amplification with improvements, modifications and automation has provided a very convenient way to do individual genotyping. It is tempting to set up large scale PCR analyses of populations to determine individuals at risk for environmental exposures affected by the PON1 polymorphism. In fact, a number of such studies have already been carried out in examining the relationship of the PON1 polymorphism to vascular disease. We advocate the use of a high throughput two-dimensional enzyme assay that provides both PON1 genotype and phenotype (PON1 status). The high level of variation of gene expression within each genetic class in humans, together with our animal model studies indicate that it is very important to determine PON status as opposed to PON1 genotype alone. Experiments in rats and mice have shown that injection of PON1 purified from rabbit serum by the i.v., i.p. or i.m. route, significantly increases PON1 activities in rodents' plasma. Under these conditions, the acute toxicity (assessed by the degree of acetylcholinesterase inhibition) of paraoxon and chlorpyrifos oxon is significantly decreased, compared to control animals. Protection is maximal when PON1 is administered before the OPs, but still occurs when PON1 is utilized as a post-exposure treatment. Furthermore, protection by PON1 is also provided toward the parent compound chlorpyrifos. Pon1-knockout mice display a much greater sensitivity to chlorpyrifos oxon toxicity than wild mice. However, the acute toxicity of guthion, which is not a substrate for PON1, does not differ between knockout and wild mice. These observations underline the importance of considering both genetic variability of enzyme isoform as well as enzyme level (PON1 status) and the developmental time course of appearance of PON1 in developing risk assessment models.