ReviewMeasurement of paraoxonase (PON1) status as a potential biomarker of susceptibility to organophosphate toxicity
Section snippets
Introduction: organophosphates and their toxicity
Organophosphorus (OP) compounds are triesters of phosphoric acid. All OPs share a general structure characterized by the presence of a phosphorus atom linked with a double bond to a sulfur or oxygen atom (Fig. 1). Two alkyl chains, either methyl, ethyl or isopropyl, esterify the phosphorus atom. The remaining bond of the pentavalent phosphorus is to a chemical moiety, the so-called “leaving group” that varies greatly among OPs (Fig. 1) [1]. Approximately 80 OPs are currently used as
Biomarkers for OP insecticides
The term biomarker is used to mean biological, biochemical and molecular markers that can be measured by chemical, biochemical or molecular techniques [16]. In humans, biomarkers must be present in easily and ethically obtainable tissues such as blood, buccal cells or urine. Biomarkers are usually divided in three categories: biomarkers of exposure, of effect and of susceptibility [17]. For OPs, a number of well-established measurements exist to assess exposure and early biological effects.
OP metabolites in urine
Determination of metabolite levels in urine is the most practical method to estimate exposure to OPs. Such metabolites include alkylphosphate derivatives, as well as a chemical residue (the so-called “leaving group”) specific for each compound. The alkylphosphates or alkyl-(di)-thiophosphates are the result of metabolism of parent compounds or their oxygen analogs by cytochromes P450 or esterases. They are not specific for a certain OP, but are useful to assess exposure (or internal dose) to
BuChE, AChE and NTE
AChE, the main target for OP cholinergic toxicity, is also present in red blood cells (RBC). Additionally, BuChE, also known as pseudo-cholinesterase or butyrylcholinesterase, is found in plasma. The physiological functions of these enzymes in blood are yet to be discovered. Nevertheless, since activity of both enzymes is usually inhibited upon exposure to OPs, their measurement is widely used as an indication of exposure and/or biological effect of OPs. The validity, specificity and usefulness
Biomarkers of susceptibility to OP toxicity
Genetic differences in the enzymes involved in the bioactivation and detoxification of OPs and in the enzymes that are targeted by these compounds can greatly influence their toxicity [22]. The focus of this review is on paraoxonase (PON1), but polymorphisms of cytochromes P450 (CYP), glutathione-S-transferases (GST), as well as of BuChE and AChE, are first briefly discussed. CYPs catalyze the oxidative desulfuration of organothiophosphates to the corresponding oxons (see Fig. 2) and also
Paraoxonase (PON1) and its human polymorphisms
Paraoxonase (PON1) is a member of a family of proteins that also includes PON2 and PON3, the genes of which are clustered on the long arm of human chromosome 7 (q21.22). PON1 is synthesized primarily in the liver and a portion is secreted into the plasma, where it is associated with high-density lipoprotein (HDL) particles [31], [32]. PON1 received its name from paraoxon, its first and one of its most studied substrates. PON1 hydrolyzes the active metabolites of several other OP insecticides
PON1 and its role in OP toxicity
The ability of PON1 to hydrolyze several OPs in vitro has long been taken as an indication that it may modulate OP toxicity in vivo. Furthermore, the presence of polymorphisms in PON1, which confer different enzyme levels and catalytic efficiency, have suggested that certain individuals may be more susceptible to the toxic effect of OP exposure. Over the past 15 years, a series of studies in rodents has provided important evidence to ascertain the role of PON1 in modulating OP toxicity. In
Determining an individual's PON1 status
Most studies investigating the association of PON1 polymorphisms with sensitivity to OP toxicity or with diseases (particularly cardiovascular diseases) have examined only the nucleotide polymorphisms (Q192R, L55M, C-108T) with polymerase chain reaction (PCR)-based assays. A functional genomic analysis, however, provides a much more informative approach, as direct measurement of an individual's PON1 function (plasma activity) takes into account all polymorphisms that might affect activity. This
Clinical evidence for a role of PON1 in OP toxicity
The studies summarized in the previous sections have (1) characterized the PON1 polymorphisms responsible for different catalytic activities and levels of expression, (2) demonstrated the relevance of PON1 in modulating OP toxicity in various animal models, and (3) indicated the importance of an individual's PON1 status. Direct confirmation in humans of the relevance of PON1 status in determining relative sensitivity to OP toxicity is, however, still elusive, though it can be inferred from all
Should we screen for PON1 status?
Occupational illness resulting from workplace chemicals is an important public health problem, and genetic testing has the potential to evaluate potential risk for those workers who are predisposed to react adversely to certain chemicals. Given the strong evidence from animal studies indicating a role for PON1 polymorphisms in modulating the toxicity of certain OPs, and the still limited human evidence, should we recommend that workers using OPs such as chlorpyrifos or diazinon be tested for
Conclusion
As OPs remain the most widely used class of insecticides worldwide [72], the probability for human exposure is still high. While approaches to determine exposure to OPs and early biological effects through measurements of urinary metabolites and of blood BuChE and AChE are well established, recent interest has been focusing on possible genetic susceptibility to OP toxicity. The studies summarized in this review provide strong evidence that the polymorphic enzyme PON1 plays a relevant role in
Acknowledgements
Research by the authors was supported by grants from the National Institutes of Health (ES04696, ES07033, ES09883, ES11397, ES09601/EPA-R826886, T32-AG00057).
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