Human serum paraoxonase (PON1): identification of essential amino acid residues by group-selective labelling and site-directed mutagenesis

Regulation of Human Hydrolases and Its Implications in Pharmacokinetics and Pharmacodynamics

Hydrolases represent an essential class of enzymes indispensable for the metabolism of various clinically essential medications. Individuals exhibit marked differences in the expression and activation of hydrolases, resulting in significant variability in the pharmacokinetics (PK) and pharmacodynamics (PD) of drugs metabolized by these enzymes. The regulation of hydrolase expression and activity involves both genetic polymorphisms and nongenetic factors. This review examines the current understanding of genetic and nongenetic regulators of six clinically significant hydrolases, including carboxylesterase (CES)-1 CES2, arylacetamide deacetylase (AADAC), paraoxonase (PON)-1 PON3, and cathepsin A (CTSA). We explore genetic variants linked to the expression and activity of the hydrolases and their effects on the PK and PD of their substrate drugs. Regarding nongenetic regulators, we focus on the inhibitors and inducers of these enzymes. Additionally, we examine the developmental expression patterns and gender differences in the hydrolases when pertinent information was available. Many genetic and nongenetic regulators were found to be associated with the expression and activity of the hydrolases and PK and PD. However, hydrolases remain generally understudied compared with other drug-metabolizing enzymes, such as cytochrome P450s. The clinical significance of genetic and nongenetic regulators has not yet been firmly established for the majority of hydrolases. Comprehending the mechanisms that underpin the regulation of these enzymes holds the potential to refine therapeutic regimens, thereby enhancing the efficacy and safety of drugs metabolized by the hydrolases. SIGNIFICANCE STATEMENT: Hydrolases play a crucial role in the metabolism of numerous clinically important medications. Genetic polymorphisms and nongenetic regulators can affect hydrolases' expression and activity, consequently influencing the exposure and clinical outcomes of hydrolase substrate drugs. A comprehensive understanding of hydrolase regulation can refine therapeutic regimens, ultimately enhancing the efficacy and safety of drugs metabolized by the enzymes.

Role of paraoxonase 1 in organophosphate G-series nerve agent poisoning and future therapeutic strategies

Chemical warfare nerve agents (CWNA) are neurotoxic chemicals unethically used as agents of mass destruction by terrorist outfits and during war. The available antidote against CWNA-mediated toxicity is not sufficiently effective and possesses several limitations. As a countermeasure, paraoxonase 1 (PON1), a catalytic bioscavenger, is being developed as a prophylactic treatment. However, the catalytic activity and substrate specificity of human PON1 are insufficient to be used as a potential antidote. Several laboratories have made different approaches to enhance the CWNA hydrolytic activity against various nerve agents. This review explores the holistic view of PON1 as a potential prophylactic agent against G-series CWNA poisoning, from its initial development to recent advancements and limitations. Apart from this, the review also provides an overview of all available PON1 variants that could be used as a potential prophylactic agent and discusses several possible ways to counteract immunogenicity.

Ancient convergent losses of Paraoxonase 1 yield potential risks for modern marine mammals

Mammals diversified by colonizing drastically different environments, with each transition yielding numerous molecular changes, including losses of protein function. Though not initially deleterious, these losses could subsequently carry deleterious pleiotropic consequences. We have used phylogenetic methods to identify convergent functional losses across independent marine mammal lineages. In one extreme case, Paraoxonase 1 (PON1) accrued lesions in all marine lineages, while remaining intact in all terrestrial mammals. These lesions coincide with PON1 enzymatic activity loss in marine species' blood plasma. This convergent loss is likely explained by parallel shifts in marine ancestors' lipid metabolism and/or bloodstream oxidative environment affecting PON1's role in fatty acid oxidation. PON1 loss also eliminates marine mammals' main defense against neurotoxicity from specific man-made organophosphorus compounds, implying potential risks in modern environments.

[Decontamination of organophosphorus compounds: Towards new alternatives]

Organophosphorus coumpounds (OP) are toxic chemicals mainly used for agricultural purpose such as insecticides and were also developed and used as warfare nerve agents. OP are inhibitors of acetylcholinesterase, a key enzyme involved in the regulation of the central nervous system. Chemical, physical and biological approaches have been considered to decontaminate OP. This review summarizes the current and emerging strategies that are investigated to tackle this issue with a special emphasis on enzymatic remediation methods. During the last decade, many studies have been dedicated to the development of biocatalysts for OP removal. Among these, recent reports have pointed out the promising enzyme SsoPox isolated from the archaea Sulfolobus solfataricus. Considering both its intrinsic stability and activity, this hyperthermostable enzyme is highly appealing for the decontamination of OP.

Current and emerging strategies for organophosphate decontamination: special focus on hyperstable enzymes

Organophosphorus chemicals are highly toxic molecules mainly used as pesticides. Some of them are banned warfare nerve agents. These compounds are covalent inhibitors of acetylcholinesterase, a key enzyme in central and peripheral nervous systems. Numerous approaches, including chemical, physical, and biological decontamination, have been considered for developing decontamination methods against organophosphates (OPs). This work is an overview of both validated and emerging strategies for the protection against OP pollution with special attention to the use of decontaminating enzymes. Considerable efforts have been dedicated during the past decades to the development of efficient OP degrading biocatalysts. Among these, the promising biocatalyst SsoPox isolated from the archaeon Sulfolobus solfataricus is emphasized in the light of recently published results. This hyperthermostable enzyme appears to be particularly attractive for external decontamination purposes with regard to both its catalytic and stability properties.

The human paraoxonase gene cluster as a target in the treatment of atherosclerosis

The paraoxonase (PON) gene cluster contains three adjacent gene members, PON1, PON2, and PON3. Originating from the same fungus lactonase precursor, all of the three PON genes share high sequence identity and a similar β propeller protein structure. PON1 and PON3 are primarily expressed in the liver and secreted into the serum upon expression, whereas PON2 is ubiquitously expressed and remains inside the cell. Each PON member has high catalytic activity toward corresponding artificial organophosphate, and all exhibit activities to lactones. Therefore, all three members of the family are regarded as lactonases. Under physiological conditions, they act to degrade metabolites of polyunsaturated fatty acids and homocysteine (Hcy) thiolactone, among other compounds. By detoxifying both oxidized low-density lipoprotein and Hcy thiolactone, PONs protect against atherosclerosis and coronary artery diseases, as has been illustrated by many types of in vitro and in vivo experimental evidence. Clinical observations focusing on gene polymorphisms also indicate that PON1, PON2, and PON3 are protective against coronary artery disease. Many other conditions, such as diabetes, metabolic syndrome, and aging, have been shown to relate to PONs. The abundance and/or activity of PONs can be regulated by lipoproteins and their metabolites, biological macromolecules, pharmacological treatments, dietary factors, and lifestyle. In conclusion, both previous results and ongoing studies provide evidence, making the PON cluster a prospective target for the treatment of atherosclerosis.

Pesticide exposure and suicidal ideation in rural communities in Zhejiang province, China

OBJECTIVE

To investigate the association between pesticide exposure and suicidal ideation in rural areas of China.

METHODS

The analysis involved data from a survey of a representative sample of 9811 rural residents in Zhejiang province who had been asked about the storage of pesticides at home and about whether or not they had considered suicide within the 2 years before the interview. The Chinese version of the 12-item General Health Questionnaire (GHQ) was administered to screen for mental disorder.

FINDINGS

The unadjusted odds ratio (OR) for the association between pesticide storage at home and suicidal ideation over the prior 2 years was 2.12 (95% confidence interval, CI: 1.54-2.93). After adjusting for gender, age, education, socioeconomic status, marital status, physical health, family history of suicidal behaviour, GHQ caseness and study design effects, the OR was 1.63 (95% CI: 1.13-2.35).

CONCLUSION

A potential marker of chronic pesticide exposure was found to be associated with suicidal ideation, which supports findings from previous studies. Given the high level of pesticide exposure and the high suicide risk in rural China, clarification of the causal mechanisms underlying this association and the development of appropriate interventions are priorities for public health and health policy.

Interethnic variability of plasma paraoxonase (PON1) activity towards organophosphates and PON1 polymorphisms among Asian populations--a short review

Organophosphate (OP) poisoning is a progressively worrying phenomenon as worldwide pesticide production and consumption has doubled. On average, WHO estimates that 3% of agricultural workers in developing Asian countries suffer an episode of pesticide poisoning every year. Furthermore, the threat of OP usage in terrorism is existent, as seen by the subway tragedy in Tokyo in 1995 where sarin was used. Despite these alarming facts, there is currently no global system to track poisonings related to pesticide use. Human serum paraoxonase (PON1) is the enzyme that hydrolyses OP compounds. Serum PON1 levels and activity vary widely among different ethnic populations. Two commonly studied polymorphisms of PON1 are PON1Q192R and PON1L55M. PON1R192 hydrolyses paraoxon faster than PON1Q192 but hydrolyses diazoxon, sarin and soman eight times slower, and vice versa. PON1M55 has lower plasma levels of PON1 than PON1L55. As the prevalence of the different alleles and genotypic distribution vary between the Asian populations we studied, we propose the necessity to study PON1 polymorphisms and its role in OP toxicity in Asian populations. This would help safeguard the proper care of agricultural workers who might be affected by OP poisoning, and alert relevant anti biological terrorism agencies on possible risks involved in the event of an OP attack and provide effective counter measures.

The current state of serum biomarkers of hepatotoxicity

The level of serum alanine aminotransferase (ALT) activity reflects damage to hepatocytes and is considered to be a highly sensitive and fairly specific preclinical and clinical biomarker of hepatotoxicity. However, an increase in serum ALT activity level has also been associated with other organ toxicities, thus, indicating that the enzyme has specificity beyond liver in the absence of correlative histomorphologic alteration in liver. Thus, unidentified non-hepatic sources of serum ALT activity may inadvertently influence the decision of whether to continue development of a novel pharmaceutical compound. To assess the risk of false positives due to extraneous sources of serum ALT activity, additional biomarkers are sought with improved specificity for liver function compared to serum ALT activity alone. Current published biomarker candidates are reviewed herein and compared with ALT performance in preclinical and on occasion, clinical studies. An examination of the current state of hepatotoxic biomarkers indicates that serum F protein, arginase I, and glutathione-S-transferase alpha (GSTalpha) levels, all measured by ELISA, may show utility, however, antibody availability and high cost per run may present limitations to widespread applicability in preclinical safety studies. In contrast, the enzymatic markers sorbitol dehydrogenase, glutamate dehydrogenase, paraxonase, malate dehydrogenase, and purine nucleoside phosphorylase are all readily measured by photometric methods and use reagents that work across preclinical species and humans and are commercially available. The published literature suggests that these markers, once examined collectively in a large qualification study, could provide additional information relative to serum ALT and aspartate aminotransferase (AST) values. Since these biomarkers are found in the serum/plasma of treated humans and rats, they have potential to be utilized as bridging markers to monitor acute drug-induced liver injury in early clinical trials.

Comparative hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate and paraoxon in different tissues of vertebrates

The Ca(2+)-dependent and EDTA-resistant hydrolysis of O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) and paraoxon was studied in serum and subcellular fractions of liver, kidney and brain of hen, rat and rabbit. HDCP was the best substrate among all the tissues studied, except that of rabbit serum which showed the highest Ca(2+)-dependent paraoxon hydrolysing activity (paraoxonase). High HDCP hydrolysing activity (HDCPase) was detected in the brain tissue of the three species studied, whereas low or no paraoxonase was found. The HDCPase/paraoxonase ratio of Ca(2+)-dependent hydrolysing activities ranged from 0.5 to 83 for tissues of the same species. EDTA-resistant HDCPase activity was more than 50% of the total activities in hen tissues, with an almost undetectable Ca(2+)-dependent paraoxonase activity in most organs. The same response was observed in rat tissues, except for serum where the Ca(2+)-dependent HDCPase and paraoxonase activities were higher (70 and 25% of total activities, respectively). EDTA-resistant HDCPase and paraoxonase activities represented less than 25% of all activities in rabbit tissues. Paraoxon has traditionally been the substrate for measuring organophosphorus hydrolysing activities. However, HDCP could be a good substrate in addition to paraoxon for monitoring other phosphotriesterases in biological tissues.

Formation and disposition of diethylphosphoryl-obidoxime, a potent anticholinesterase that is hydrolyzed by human paraoxonase (PON1)

The potential of pyridinium-4-aldoximes, such as obidoxime, to reactivate diethylphosphorylated acetylcholinesterases is not fully exploited due to the inevitable formation of phosphoryloximes (POX) with high anticholinesterase activity. Mono(diethylphosphoryl) obidoxime (DEP-obidoxime) was isolated for the first time showing remarkable stability under physiological conditions (half-life 13.5min; pH 7.1; 37 degrees C). The half-life was considerably extended to 20h at 0 degrees C, which facilitated the preparation and allowed isolation by HPLC. The structure was confirmed by mass spectrometry and the degradation pattern. DEP-obidoxime decomposed by an elimination reaction forming the intermediate nitrile that hydrolyzed mainly into the pyridone and cyanide. The intermediates were prepared and confirmed by mass spectroscopy. DEP-Obidoxime was an extremely potent inhibitor of human acetylcholinesterase approaching a second-order rate constant of 10(9)M(-1)min(-1) (pH 7.4; 37 degrees C). The nitrile and the pyridone were still good reactivators. In the presence of human plasma DEP-obidoxime was hydrolyzed into parent obidoxime. Calcium-dependence and sensitivity towards chelators, substitution pattern by other divalent cations and protein-modifying agents all pointed to human paraoxonase (hPON1) as the responsible protein with POX-hydrolase activity. Subjects, probably belonging to the homozygous (192)arginine subtype, were virtually devoid of POX-hydrolase activity while a highly purified hPON1 of the homozygous (192)glutamine subtype exhibited particularly high POX-hydrolase activity. Two parathion-poisoned patients with high and low POX-hydrolase activity responded well and poorly, respectively, to obidoxime treatment although the former patient had higher plasma paraoxon levels than the poor responder. Hence, the POX-hydrolase associated PON1 subtype may be another contributor that modulates pyridinium-4-aldoxime effectiveness.

Analysis of active-site amino-acid residues of human serum paraoxonase using competitive substrates

Serum paraoxonase (PON1) is a calcium-dependent six-fold beta-propeller protein structurally similar to the di-isopropylfluorophosphatase (DFPase) found in the squid Loligo vulgaris. Human serum paraoxonase (HuPON1) has been shown to hydrolyze an array of substrates even though relatively little is known about its physiological role(s) or its catalytic mechanism. Through site-directed mutagenesis studies, designed from a DFPase-like homology model, and from a crystal structure of a hybrid PON1 molecule, amino-acid residues essential for enzyme function, including H115 and F222, have been identified. It was shown previously that, when H115 is replaced with tryptophan, the resulting enzyme hydrolyzes paraoxon but not phenyl acetate. This study shows that, when present simultaneously, phenyl acetate competitively inhibits paraoxon hydrolysis by H115W. Conversely, when F222 is replaced with tyrosine, mutant F222Y can hydrolyze phenyl acetate but not paraoxon. The presence of DFP, an inhibitor of both arylesterase and paraoxonase activities of wild-type HuPON1 (mean Ki=0.48+/-0.15 mM), has no effect on the ability of F222Y to catalyze the hydrolysis of phenyl acetate, suggesting that the F222Y mutant is unable to bind DFP. Together, the results suggest that, in wild-type HuPON1, H115 and F222 are important in determining substrate binding and specificity, but are not likely to be directly involved in substrate hydrolysis.