Oxidative inactivation of paraoxonase—implications in diabetes mellitus and atherosclerosis

Lipid radicals and oxidized cholesteryl esters in low- and high-density lipoproteins in patients with β-thalassemia: Effects of iron overload and iron chelation therapy

Iron overload results in lipid peroxidation (LPO) and the oxidative modification of circulating lipoproteins, which contributes to cardiovascular complications in patients with β-thalassemia. Investigating LPO may provide opportunities for the development of novel therapeutic strategies; however, the chemical pathways underlying iron overload-induced LPO in β-thalassemia lipoproteins remain unclear. In this study, we identified various species of lipid radicals (L•), the key mediators of LPO, and oxidized cholesteryl esters (oxCE) derived from the in vitro oxidation of major core lipids, cholesteryl linoleate (CE18:2) and cholesteryl arachidonate (CE20:4); the levels of these radical products in low-density lipoproteins (LDL) and high-density lipoproteins (HDL) were measured and compared between β-thalassemia patients and healthy subjects by using a specific fluorescent probe for L• with a liquid chromatography-tandem mass spectrometric method. Our results demonstrated that iron overload substantially decreased the levels of CE18:2 and CE20:4 substrates and α-tocopherol, resulting in higher levels of full-length and short-chain truncated L• and oxCE products. In particular, CE epoxyallyl radicals (•CE-O) were observed in the lipoproteins of β-thalassemia, revealing the pathological roles of iron overload in the progression of LPO. In addition, we found that intermission for two weeks of iron chelators can increase the production of these oxidized products; therefore, suggesting the beneficial effects of iron chelators in preventing LPO progression. In conclusion, our findings partly revealed the primary chemical pathway by which the LPO of circulating lipoproteins is influenced by iron overload and affected by iron chelation therapy. Moreover, we found that •CE + O shows potential as a sensitive biomarker for monitoring LPO in individuals with β-thalassemia.

High-density lipoprotein in diabetes: Structural and functional relevance

Low levels of high-density lipoprotein-cholesterol (HDL-C) is considered a major cardiovascular risk factor. However, recent studies have suggested a more U-shaped association between HDL-C and cardiovascular disease. It has been shown that the cardioprotective effect of HDL is related to the functions of HDL particles rather than their cholesterol content. HDL particles are highly heterogeneous and have multiple functions relevant to cardiometabolic conditions including cholesterol efflux capacity, anti-oxidative, anti-inflammatory, and vasoactive properties. There are quantitative and qualitative changes in HDL as well as functional abnormalities in both type 1 and type 2 diabetes. Non-enzymatic glycation, carbamylation, oxidative stress, and systemic inflammation can modify the HDL composition and therefore the functions, especially in situations of poor glycemic control. Studies of HDL proteomics and lipidomics have provided further insights into the structure-function relationship of HDL in diabetes. Interestingly, HDL also has a pleiotropic anti-diabetic effect, improving glycemic control through improvement in insulin sensitivity and β-cell function. Given the important role of HDL in cardiometabolic health, HDL-based therapeutics are being developed to enhance HDL functions rather than to increase HDL-C levels. Among these, recombinant HDL and small synthetic apolipoprotein A-I mimetic peptides may hold promise for preventing and treating diabetes and cardiovascular disease.

Hyperhomocysteinemia in Treatment with Atypical Antipsychotics is Independent of Metabolic Syndrome

Strong association between homocysteine (Hcy) and metabolic syndrome (MetS) is documented in individuals with schizophrenia and it is suggested that alterations in Hcy levels might be secondary to metabolic changes induced by atypical antipsychotics (AA). Serum paraoxonase (PON-1) activity, which is negatively affected by increased Hcy concentrations are lower in schizophrenia, and this may impact the development of metabolic side effects. Forty-five subjects with schizophrenia and 43 healthy volunteers, matched according to age, gender, smoking habits, and MetS predictors, were enrolled in this study to examine how Hcy level, PON-1 activity, and MetS indicators influence each other in schizophrenic individuals on AA treatment. Serum Hcy concentrations were significantly higher (15 ± 8 μmol/L vs 12 ± 3 μmol/L), and PON activity tended to be impaired (182±82 U/L vs 216 ± 110 U/L) in schizophrenia. Serum Hcy concentrations were not different between subjects with and without metabolic syndrome in study (14±4 μmol/L and 16±9 μmol/L) and control groups (12±3 μmol/L and 13±7 μmol/L), respectively. Similarly, PON and aryl esterase (AE) activities were not different between subjects with and without metabolic syndrome in study (PON: 185±100 U/L and 181±76 U/L; AE: 84±34 kU/L and 89±20 kU/L) and control (PON: 215±111 U/L and 216±113 U/L; AE: 83±27 kU/L and 88±33 kU/L) groups, respectively. . Hcy levels and MetS predictors were not statistically correlated. Results indicate that schizophrenic subjects on AA treatment have increased levels of Hcy compared to healthy controls and this is not influenced by the presence of MetS.

Genetic polymorphisms in the angiotensin converting enzyme, actinin 3 and paraoxonase 1 genes in women with diabetes and hypertension

Objective

To study associations between polymorphisms in the angiotensin converting enzyme (ACE I/D), actinin 3 (ACTN3 R577X) and paraoxonase 1 (PON1 T(-107)C) genes and chronic diseases (diabetes and hypertension) in women.

Materials and methods

Genomic DNA was extracted from saliva samples of 78 women between 18 and 59 years old used for genetic polymorphism screening. Biochemical data were collected from the medical records in Basic Health Units from Southern Brazil. Questionnaires about food consumption, physical activity level and socioeconomic status were applied.

Results

The XX genotype of ACTN3 was associated with low HDL levels and high triglycerides, total cholesterol and glucose levels. Additionally, high triglycerides and LDL levels were observed in carriers of the TT genotype of PON1, and lower total cholesterol levels were associated to the CC genotype. As expected, women with diabetes/hypertense had increased body weight, BMI (p = 0.02), waist circumference (p = 0.01), body fat percentage, blood pressure (p = 0.02), cholesterol, triglycerides (p = 0.02), and blood glucose (p = 0.01), when compared to the control group.

Conclusion

Both ACTN3 R577X and PON1 T(-107)C polymorphisms are associated with nutritional status and blood glucose and lipid levels in women with diabetes/hypertense. These results contribute to genetic knowledge about predisposition to obesity-related diseases.

Modulatory Effect of Lifestyle-Related, Environmental and Genetic Factors on Paraoxonase-1 Activity: A Review

Paraoxonase-1 (PON1) is a calcium-dependent, HDL-bound serum hydrolase active toward a wide variety of substrates. PON1 displays three types of activities, among which lactonase, paraoxonase, arylesterase and phosphotriesterase can be distinguished. Not only is this enzyme a major organophosphate compound detoxifier, but it is also an important constituent of the cellular antioxidant system and has anti-inflammatory and antiatherogenic functions. The concentration and activity of PON1 is highly variable among individuals, and these differences can be both of genetic origin and be a subject of epigenetic regulation. Owing to the fact that, in recent decades, the exposure of humans to an increasing number of different xenobiotics has been continuously rising, the issues concerning the role and activity of PON1 shall be reconsidered with particular attention to growing pharmaceuticals intake, dietary habits and environmental awareness. In the following manuscript, the current state of knowledge concerning the influence of certain modifiable and unmodifiable factors, including smoking, alcohol intake, gender, age and genotype variation on PON1 activity, along with pathways through which these could interfere with the enzyme's protective functions, is presented and discussed. Since exposure to certain xenobiotics plays a key role in PON1 activity, the influence of organophosphates, heavy metals and several pharmaceutical agents is also specified.

Comparison of Plasma Exosome Proteomes Between Obese and Non-Obese Patients with Type 2 Diabetes Mellitus

PURPOSE

Obesity is considered a promoter of type 2 diabetes mellitus (T2DM). However, the underlying mechanism remains unclear. This study aimed to identify plasma exosome differentially expressed proteins (DEPs) that are potentially involved in the development of obesity-related T2DM.

METHODS

Exosomes were isolated from the plasma of obese and non-obese T2DM patients (n = 10 for each group). A label-free quantitative mass spectrometry analysis was applied to identify plasma exosome DEPs in obese patients compared with non-obese patients, followed by bioinformatics analysis including GO annotation, KEGG analysis, subcellular localization prediction, transcription factor analysis, and protein-protein interaction (PPI) prediction.

RESULTS

We identified 2 significantly upregulated proteins (C9 and PON1) and 5 significantly downregulated proteins (HPX, A1BG, CFHR1, ANG, and CALM) in obese patients compared with those in non-obese patients. KEGG analysis demonstrated that the insulin signaling pathway was one of the pathways that significantly correlated with the DEPs. The DEPs were primarily localized in the extracellular space (5 out of 7). HMG-box and NF-Y beta might regulate the transcription of the DEPs. C9, PON1, HPX, and CFHR1 were present in the PPI network.

CONCLUSION

The plasma exosome DEPs are potentially responsible for the development of obesity-related T2DM possibly through the insulin signaling pathway and the interaction with other proteins. Our study may guide future research direction toward the pathogenesis of obesity-related T2DM.

Glucose-Related Traits and Risk of Migraine—A Potential Mechanism and Treatment Consideration

Migraine and glucose-related (glycaemic) traits (fasting glucose, fasting insulin, and type 2 diabetes) are common and complex comorbid disorders that cause major economic and social burdens on patients and their families. Studies on the relationship between migraine and glucose-related traits have yielded inconsistent results. The purpose of this review is to synthesise and discuss the information from the available literature on the relationship between fasting glucose, fasting insulin, and type 2 diabetes (T2D) with migraine. Publications on migraine and fasting glucose, migraine and fasting insulin, and migraine and T2D were identified from a PubMed and Google Scholar database search and reviewed for this article. Multiple publications have suggested that the comorbidity of migraine and glucose-related traits may have a similar complex pathogenic mechanism, including impaired glucose homeostasis, insulin resistance, reduced cerebrovascular reactivity, abnormal brain metabolism, shared genetic factors, neurotransmitters, and sex hormones. Furthermore, several studies have found a bi-directional link between migraine with insulin resistance and T2D. There is strong evidence for a biological association between migraine headache and glucose-related traits, and burgeoning evidence for shared genetic influences. Therefore, genetic research into these comorbid traits has the potential to identify new biomarkers and therapeutic targets and provide biological insight into their relationships. We encourage healthcare professionals to consider the co-occurrence of migraine with glucose-related traits in the evaluation and treatment of their patients.

Concentration/activity of superoxide dismutase isozymes and the pro-/antioxidative status, in context of type 2 diabetes and selected single nucleotide polymorphisms (genes: INS, SOD1, SOD2, SOD3) - Preliminary findings

The alterations in concentration/activity of superoxide dismutase isozymes in the context of type 2 diabetes or obesity are well-described. Moreover, many hereditary factors, including single-nucleotide polymorphisms (SNPs) of genes for coding insulin, insulin receptors, or insulin receptor substrates (INS, INSR, IRS1, IRS2) or superoxide dismutase isozymes (SOD1, SOD2, SOD3), have been linked with the incidence of obesity and diabetes. However, the underlying changes in the plasma concentration/activity of superoxide dismutase isozymes and their potential connection with the said hereditary factors remain unexplored. Previously, we have observed that the plasma concentration/activity of superoxide dismutase isozymes differs in the context of obesity and/or rs2234694 (SOD1) and rs4880 (SOD2) and that the concentrations of SOD1, SOD2, SOD3 are correlated with each other. Intersexual variability of SOD1 concentration was detected regardless of obesity. In this study, the variability of concentration/activity of superoxide dismutase isozymes in plasma is considered in the context of type 2 diabetes and/or SNPs: rs2234694 (SOD1), rs5746105 (SOD2), rs4880 (SOD2), rs927450 (SOD2), rs8192287 (SOD3). Genotypic variability of SNP rs3842729 (INS), previously studied in the context of insulin-dependent diabetes, is investigated in terms of selected clinical parameters associated with type 2 diabetes. This study revealed higher SOD1 concentration in diabetic men compared to women, and extremely high SOD1 concentration, higher total superoxide dismutase, and copper-zinc superoxide dismutase activity, and lower superoxide dismutase and copper-zinc superoxide dismutase activity (when adjusted for the concentration of SODs) in the diabetic group regardless of sex. Multiple logistic regression, applied to explore possible links between the studied SNPs and other factors with the odds of type 2 diabetes or obesity, revealed that the genotypic variability of rs4880 (SOD2) could affect these odds, supporting the findings of several other studies.

Evaluation of Relationship Between Arylesterase-Based Activity and Genetic Variants of Paraoxonase1 in T2DM Patients within Golestan Province

Arylesterase activity of Paraoxonase-1 (PON1) enzyme may be play important role in initiation and progression of several diseases. Activity or serum level of Arylesterase can be affected by many genetic alterations such as SNPs. The reduction in the activity and serum level of Arylesterase could be involved in Type2 diabetes mellitus (T2DM). The aim of this investigation is to examine the association between Arylesterase activity and promoter polymorphism (- 108C > T) of PON1gene in patients with T2DM in Golestan Province, northern area of Iran. Achievement of this purpose was due to DNA obtaining from blood then SNP determination using PCR-RFLP and Arylesterase activity measurement in the serum of 90 normal individuals and 90patients suffering diabetes. Data was processed by SPSS software version 16. The significant association was observed between the Arylesterase activity and three genotypes of PON1 gene such as CC, CT, and TT in subjects with T2DM. In the present study, it has been shown level of Arylestrase activity of PON1 in patients (117.33 ± 63.96) is lower than it in control group (289.33 ± 68.38); P < 0.05. Our results declared that activity of Arylesterase in diabetic patients was significantly lower than the healthy individuals.

Cladribine Treatment Improved Homocysteine Metabolism and Increased Total Serum Antioxidant Activity in Secondary Progressive Multiple Sclerosis Patients

Hyperhomocysteinemia plays a crucial role in the pathogenesis of many diseases of the central nervous system (CNS). The nervous system is particularly sensitive to high homocysteine (Hcy) level mainly due to its prooxidative and cytotoxic effects. Cladribine, a drug recently registered for the treatment of multiple sclerosis (MS), possesses additionally neuroprotective effects which are independent of its peripheral immunosuppressant action. Accumulating evidence suggests that oxidative stress and homocysteine thiolactone-mediated protein homocysteinylation play a causal role in MS. Both of these processes may be attenuated by paraoxonase 1 (PON1). Therefore, in the present study, we aimed to examine whether the beneficial effects of the drug in MS patients with a secondary progressive (SP) clinical course, treated with cladribine subcutaneously (s.c.), may be related to its ability to modify serum PON1 activity, Hcy concentration, and protein homocysteinylation, as well as to correct total antioxidant status. A total of 118 subjects were enrolled into the study: (1) patients with a SP type of MS, SP-MS (n = 40); (2) patients with a relapsing-remitting (RR) type of MS, RR-MS (n = 30); and (3) healthy people (n = 48). Patients with SP-MS were treated with cladribine. The drug was given in SP-SM patients s.c. six times every 6 weeks up to a total mean cumulative dose of 1.8 mg/kg. PON1 activity was assessed spectrophotometrically. The level of Hcy, homocysteine thiolactone (HTL) attached to plasma proteins (N-Hcy-protein), and antibodies against homocysteinylated proteins was assessed with an enzyme immunoassay. The total antioxidant activity of the serum was assessed with the ferric-reducing activity of plasma (FRAP) method. Basically, there was no difference in PON1 activity between untreated SP-MS, RR-MS, and control subjects. Serum Hcy was significantly higher in RR-MS patients (p < 0.001) and in SP-MS patients (p < 0.01) compared to the control group. The N-Hcy protein level was higher in RR-MS patients (p < 0.05) in comparison to the control group. Moreover, the elevated level of antibodies against homocysteinylated proteins was observed in the serum of patients with SP-MS. The total antioxidant capacity of serum was lower in MS patients vs. the control group (p < 0.001). After cladribine treatment, the activity of PON1 did not change in SP-MS patients, whereas cladribine treatment decreased the level of total Hcy (p < 0.05). Treatment with cladribine increased the total serum antioxidant activity in SP-MS patients (p < 0.01). The Expanded Disability Status Scale (EDSS) score did not change in SP-MS patients. Cladribine treatment in the SP-MS group attenuates hyperhomocysteinemia-induced protein homocysteinylation (n.s.). It also stabilises the neurological condition of SP-MS patients. The stabilisation of a neurological condition observed in SP-MS patients after cladribine treatment may be partially related to its ability to reduce elevated Hcy level and to improve serum antioxidant potential.

Serum paraoxonase-1 activity is inversely related to free thyroxine in euthyroid subjects: The PREVEND Cohort Study

BACKGROUND

Low-normal thyroid function within the euthyroid range has been suggested to enhance atherosclerosis susceptibility. Paraoxonase-1 (PON-1) may protect against atherosclerotic cardiovascular disease development by attenuating oxidative stress. We evaluated relationships of PON-1 with thyroid stimulating hormone (TSH), free T₄ , free T₃ , lipids and apolipoprotein (apo)A-I in euthyroid subjects, and assessed whether such relationships are modified in the context of the metabolic syndrome (MetS).

MATERIALS AND METHODS

Serum PON-1 activity (arylesterase activity), TSH, free T₄ , free T₃ , lipids and apoA-I was measured in 2206 euthyroid subjects (aged 28-75 years; 1138 men (age 49 ± 13 years) and 1068 women (age 46 ± 12 years), recruited from the general population (PREVEND cohort).

RESULTS

In age- and sex-adjusted analysis, PON-1 activity (divided into tertiles) was positively related to TSH (β = -0.045, P = .036) and inversely to free T₄ (β = -0.042, P = .050) but not to free T₃ (β = -0.027, P = .20). PON-1 activity was positively related to total cholesterol, non-HDL cholesterol and triglycerides, as well as to HDL cholesterol and apoA-I (P < .01 to <.001). The inverse relationship of PON-1 activity with free T₄ remained present after adjustment for lipids and other potential confounders (β = -0.066, P = .002), but the positive relationship with TSH lost significance (β = 0.034, P = .11). The inverse relationship of PON-1 activity with free T₄ was not different in subjects with vs without MetS (P = .94), nor modified by the presence of its individual components (P ≥ .22 for each).

CONCLUSIONS

Serum PON-1 activity is inversely associated with free T₄ in euthyroid subjects, suggesting that low-normal thyroid function may affect PON-1 regulation.

The role and function of HDL in patients with diabetes mellitus and the related cardiovascular risk

BACKGROUND

Diabetes mellitus (DM) is a major public health problem which prevalence is constantly raising, particularly in low- and middle-income countries. Both diabetes mellitus types (DMT1 and DMT2) are associated with high risk of developing chronic complications, such as retinopathy, nephropathy, neuropathy, endothelial dysfunction, and atherosclerosis.

METHODS

This is a review of available articles concerning HDL subfractions profile in diabetes mellitus and the related cardiovascular risk. In this review, HDL dysfunction in diabetes, the impact of HDL alterations on the risk diabetes development as well as the association between disturbed HDL particle in DM and cardiovascular risk is discussed.

RESULTS

Changes in the amount of circulation lipids, including triglycerides and LDL cholesterol as well as the HDL are frequent also in the course of DMT1 and DMT2. In normal state HDL exerts various antiatherogenic properties, including reverse cholesterol transport, antioxidative and anti-inflammatory capacities. However, it has been suggested that in pathological state HDL becomes "dysfunctional" which means that relative composition of lipids and proteins in HDL, as well as enzymatic activities associated to HDL, such as paraoxonase 1 (PON1) and lipoprotein-associated phospholipase 11 (Lp-PLA2) are altered. HDL properties are compromised in patients with diabetes mellitus (DM), due to oxidative modification and glycation of the HDL protein as well as the transformation of the HDL proteome into a proinflammatory protein. Numerous studies confirm that the ability of HDL to suppress inflammatory signals is significantly reduced in this group of patients. However, the exact underlying mechanisms remains to be unravelled in vivo.

CONCLUSIONS

The understanding of pathological mechanisms underlying HDL dysfunction may enable the development of therapies targeted at specific subpopulations and focusing at the diminishing of cardiovascular risk.

Association between promoter polymorphism (-108C>T) of paraoxonase1 gene and it's paraoxonase activity in patients with Type2 diabetes in northern Iran

Type 2 diabetes mellitus (T2DM) is one of the diseases which depend on the obesity associated with insulin resistance. Various factors, such as a reduction in the activity of paraoxonase-1 enzyme (PON1), could affect T2DM. PON1 is a multifunctional enzyme with paraoxonase and arylesterase activities. The Activity of PON1 is influenced by various SNPs. The aim of presence study is to investigate the association between promoter polymorphism (-108C>T) of PON1 gene and its paraoxonase activity in patients suffering from Type 2 diabetes in Golestan Province, north of Iran. To this end, genomic DNA was extracted from whole blood then genotyping was carried out using PCR-RFLP and enzymatic activity of paraoxonase was measured in the serum of 90 healthy subjects and 90 diabetic patients. Data was analyzed using SPSS version 16. The relationship between the level of paraoxonase activity with polymorphisms of CC, CT, and TT was statistically significant in patients with T2DM. There was a significant association between polymorphism C-108>T of PON1 and type2 diabetes mellitus, with 24.4% of control group subjects and 15.5% of patients having CC genotype; p<0.05. The ratio for CT genotype in target and control groups was 51.1% and 61.1% respectively; p<0.05. TT genotype was 33.3% in patients and 14.4% in the control group; p<0.05. In the present study, the highest frequency belonged to CT genotype in both the control and the target groups, followed by CC genotype in control group and TT genotype in target group. Our findings revealed that paraoxonase activity of PON1 in the control group was significantly higher in comparison with diabetic patients.

The inverse association of HDL-cholesterol with future risk of hypertension is not modified by its antioxidant constituent, paraoxonase-1: The PREVEND prospective cohort study

BACKGROUND AND AIMS

High-density lipoprotein cholesterol (HDL-C), an established risk marker for atherosclerotic cardiovascular disease (CVD), has been shown to be inversely and independently associated with incident hypertension. Paraoxonase-1 (PON-1) is an HDL-bound esterase enzyme associated with CVD, but its relationship with incident hypertension has not been previously investigated. We aimed at evaluating the prospective association between PON-1 and hypertension risk.

METHODS

PON-1 arylesterase activity was measured in serum at baseline in 3988 participants without pre-existing hypertension in the Prevention of Renal and Vascular End-stage Disease (PREVEND) prospective population-based study. During a median follow-up of 10.7 years, 1206 participants developed hypertension.

RESULTS

In age- and sex-adjusted analysis, the hazard ratio (95% CI) for incident hypertension per 1 standard deviation increase in PON-1 was 1.01 (0.96-1.07; p = 0.656), which remained non-significant after adjustment for several established hypertension risk factors and other potential confounders (0.99, 0.93 to 1.05; p = 0.764). The association was also non-existent on further adjustment for HDL-C (1.00 (0.94-1.06; p = 0.936)) and did not importantly vary across several clinical subgroups. In analyses in the same set of participants, HDL-C was continuously inversely and independently associated with hypertension risk; the association persisted after further adjustment for PON-1 activity and was not modified by PON-1 activity.

CONCLUSIONS

In this Caucasian cohort of men and women, HDL-C, but not its anti-oxidant constituent - PON-1, is inversely, continuously and independently associated with future risk of hypertension. The association is independent of and not modified by PON-1.

Water-Soluble Components of Sesame Oil Reduce Inflammation and Atherosclerosis

Atherosclerosis, a major form of cardiovascular disease, is now recognized as a chronic inflammatory disease. Nonpharmacological means of treating chronic diseases have gained attention recently. We previously reported that sesame oil aqueous extract (SOAE) has anti-inflammatory properties, both in vitro and in vivo. In this study, we have investigated the antiatherosclerotic properties of SOAE, and the mechanisms, through genes and inflammatory markers, by which SOAE might modulate atherosclerosis. Low-density lipoprotein receptor (LDL-R) knockout female mice were fed with either a high-fat (HF) diet or an HF diet supplemented with SOAE. Plasma lipids and atherosclerotic lesions were quantified after 3 months of feeding. Plasma samples were used for global cytokine array. RNA was extracted from both liver tissue and the aorta, and used for gene analysis. The high-fat diet supplemented with SOAE significantly reduced atherosclerotic lesions, plasma cholesterol, and LDL cholesterol levels in LDL-R(-/-) mice. Plasma inflammatory cytokines were reduced in the SOAE diet-fed animals, but not significantly, demonstrating potential anti-inflammatory properties of SOAE. Gene analysis showed the HF diet supplemented with SOAE reduced gene expression involved in inflammation and induced genes involved in cholesterol metabolism and reverse cholesterol transport, an anti-inflammatory process. Our studies suggest that a SOAE-enriched diet could be an effective nonpharmacological treatment for atherosclerosis by controlling inflammation and regulating lipid metabolism.

Serum paraoxonase 1 (PON1) activity and lipid metabolism parameters changes in different production cycle periods of Holstein-Friesian, Polish Red and Norwegian breeds

We investigated the measurement of paraoxonase 1 (PON1), as a potential marker of redox state changes in dairy cows, its involvement in lipid metabolism and compared it with superoxide dismutase (SOD) activity changes. We also evaluated lipid metabolism parameters associated with dairy production. PON1 paraoxonase and arylesterase acitvities, SOD activity, beta-hydroxybutyrate (BHB), uric acid (UA), high density lipoprotein (HDL), low density lipoprotein (LDL), cholesterol and triglyceride concentrations were measured in Holstein-Friesian, Polish Red and Norwegian breeds serum in two production cycles. Our data showed a significant postpartum depletion in PON1 activity and lipoprotein and lipid products concentrations, with elevated BHB values. However, there were no significant changes in SOD activity and uric acid concentrations in Holstein-Friesian and Polish Red breeds after calving. At lactation peak there was a significant SOD activity decrease correlated with standardized PON1 activity depletion in all examined breeds. The results suggest that PON1 might be a better parameter for minimal redox state changes in serum, shortly after labour in the examined breeds.

A review of paraoxonase 1 properties and diagnostic applications

Paraoxonase 1 (PON1) is an arylesterase associated with serum high density lipoprotein particles. Its name is derived from hydrolyzing one of several organophosphate compounds, namely paraoxon. Recent studies have shown that PON1 plays a protective role in diseases associated with oxidative stress such as atherosclerosis and diabetes mellitus. Studies have demonstrated reduction-oxidative state changes involving PON1 in humans and laboratory animal models. Although there is less information about the role of this enzyme in veterinary medicine, new data suggest that PON1 might be a new oxidative stress marker in animal patients, similarly to humans.

Urotensin-II level and its association with oxidative stress in early diabetic nephropathy

OBJECTIVE

Diabetic nephropathy is the most common cause of end stage renal failure. Early treatment of diabetic nephropathy depends on understanding the underlying mechanisms of the disease. In this study we investigated the role of U-II in early nephropathy and ıts association with oxidative stress, paraoxonase (PON)-1 and arylesterase.

RESEARCH DESIGN AND METHODS

Twenty-three diabetic patients with microalbuminuria, 23 diabetic patients with normoalbuminuria and 25 healthy individuals were enrolled in the study. Serum total antioxidant status (TAS), total oxidant status (TOS), PON-1, arylesterase, and urotensin-II (U-II) levels were measured. Oxidative stress index (OSI) percent ratio of TOS to TAS level was accepted as OSI.

RESULTS

Serum U-II levels were higher in the microalbuminuric diabetes group compared to the normoalbuminuric diabetic group and the healthy control group (p=0.009 and p=0.0001, respectively). Normoalbuminuric diabetic group's U-II levels were significantly higher compared to those of the healthy control group (p=0.0001). Correlation analysis yielded that plasma U-II levels are negatively correlated to TAS, arylesterase, and PON-1 levels (r=-0.395, p=0.001; r=-0.291, p=0.014; and r=-0.279, p=0.018, respectively) and that they had a positive correlation with OSI levels (r=0.312, p=0.008). These associations were confirmed in the multiple regression analysis. The results of multiple logistic regression analysis showed that oxidative stress is important in the development of microalbuminuria.

CONCLUSION

The data of this study reveal that increased serum U-II has a role in the development of diabetic nephropathy. This effect of U-II may be related to high levels oxidative stress parameters.

HDL3 stimulates paraoxonase 1 antiatherogenic catalytic and biological activities in a macrophage model system: in vivo and in vitro studies

We analyzed in-vivo and in-vitro high density lipoprotein (HDL) effects on paraoxonase 1 (PON1) antiatherogenic properties in serum and in macrophages. Intraperitoneal injection to C57BL/6 mice of recombinant PON1 (rePON1) + HDL, in comparison to HDL or to rePON1 alone, significantly increased serum PON1 arylesterase activity (by 20%), and serum-mediated cholesterol efflux from J774A.1 macrophages (by 18%). Similarly, in peritoneal macrophages (MPM) harvested from mice injected with HDL + rePON1 versus rePON1 alone, we observed reduction in oxidative stress (by 11%), increase in cellular PON1 activity (by 14%) and in HDL-mediated cholesterol efflux (by 38%). Incubation of serum or HDL with rePON1, substantially increased PON1 arylesterase activity, two-fold more than the expected additive values. HDL2 and HDL3 increased PON1 activity by 199% or 274%, respectively. Macrophage (J774A.1) cholesterol efflux rate significantly increased by HDL3 + rePON1 versus HDL3 alone (by 19%), but not by HDL2 + rePON1 versus HDL2 alone. Oxidation of HDL3 reduced its ability to induce macrophage cholesterol efflux, and abolished HDL3 stimulatory effects on rePON1. Addition of exogenous polyphenol quercetin (60 µM), but not phosphatidylcholine or apolipoprotein A1, to HDL + rePON1 increased PON1 activity (by 404%), increased the ability to reduce oxidative stress in J774A.1 macrophages (by 53%) and to stimulate macrophage cholesterol efflux (by 14%). Upon adding the hypocholesterolemic drug simvastatin (15 µg/mL) to HDL + rePON1, PON1 activity and the ability to induce macrophage cholesterol efflux increased, in comparison to HDL + rePON1. We thus concluded that HDL (mostly HDL3), stimulates PON1 antiatherogenic activities in macrophages, and these PON1 activities were further stimulated by quercetin, or by simvastatin.

Paraoxonase lactonase activity, inflammation and antioxidant status in plasma of patients with type 1 diabetes mellitus

OBJECTIVES

To investigate paraoxonase-1 (PON1) lactonase activity, myeloperoxidase (MPO) activity (as a marker of inflammation) and antioxidant status in plasma of patients with type 1 diabetes mellitus.

METHODS

Whole blood and plasma samples were collected from patients with diabetes and healthy control subjects. PON1 lactonase and MPO activities and total antioxidant capacity (TEAC) were determined in plasma. Glycosylated haemoglobin (HbA1c) was quantified in whole blood.

RESULTS

Plasma PON1 lactonase and MPO activities were significantly higher and TEAC was significantly lower in patients with diabetes (n = 18) compared with healthy control subjects (n = 20). There were significant positive correlations between PON1 lactonase activity and MPO activity and HbA1c level, and plasma MPO and HbA1c. There were significant negative correlations between PON1 lactonase activity and TEAC, and MPO activity and TEAC.

CONCLUSIONS

Increased lactonase activity may inefficiently compensate for the high level of chronic inflammation and low antioxidant capacity in the plasma of patients with type 1 diabetes mellitus.

Effects of rosiglitazone on serum paraoxonase activity and metabolic parameters in patients with type 2 diabetes mellitus

Human serum paraoxonase contributes to the anti-atherogenic effect of high-density lipoprotein cholesterol (HDL-C) and has been shown to protect both low-density lipoprotein cholesterol (LDL-C) and HDL-C against lipid peroxidation. We investigated the effects of rosiglitazone on paraoxonase activity and metabolic parameters in patients with type 2 diabetes mellitus [50 patients (30 males, 20 females); mean±SD age: 58.7±9.2 years, body mass index: 28.2±4.1'kg/m²], in whom glucose control could not be achieved despite treatment with metformin, sulphonylurea, and/or insulin. The patients were given 4'mg/day rosiglitazone for 3 months in addition to their usual treatment. Serum paraoxonase activity, malondialdehyde, homocysteine, and lipid profile were measured at the time of initiation and at the end of therapy with rosiglitazone. After rosiglitazone therapy, serum levels of HDL-C, apolipoprotein A-1, and paraoxonase activity increased significantly (P<0.05) and malondialdehyde, homocysteine, lipoprotein(a), and glucose levels decreased significantly (P<0.05), but no significant changes in levels of total cholesterol and apolipoprotein B were observed. Triglyceride levels also increased significantly (P<0.05). Rosiglitazone treatment led to an improvement in glycemic control and to an increase in paraoxonase activity and HDL-C levels. Although rosiglitazone showed favorable effects on oxidant/antioxidant balance and lipid profile, further studies are needed to determine the effect of rosiglitazone on cardiovascular risk factors and cardiovascular morbidity and mortality.

Implications of serum paraoxonase activity in obesity, diabetes mellitus, and dyslipidemia

Human serum paraoxonase 1 (PON1) is an enzyme with esterase activity, and is physically bound to high-density lipoproteins (HDL). It plays a key role in the action of HDL toward protection of lipoprotein and biological membrane against oxidative damage. It may have a protective role against atherosclerosis by virtue of its action on hydrolyzing lipid peroxides and preventing accumulation of phospholipids in oxidized low-density lipoprotein (LDL). PON1 is hypothesized to be an indicator of the risk of atherosclerosis and coronary artery disease development. Numerous studies have implicated PON1 activity in relation to various endocrine disorders. The current article reviews the clinical perspectives of PON1 activity with regards to obesity, diabetes mellitus with its complications, and dyslipidemia.

HDL dysfunction in diabetes: causes and possible treatments

HDL is known to be inversely correlated with cardiovascular disease due to its diverse antiatherogenic functions. These functions include cholesterol efflux and reverse cholesterol transport, antioxidative and anti-inflammatory activities. However, HDL has been shown to undergo a loss of function in several pathophysiological states, as in the acute phase response, obesity and chronic inflammatory diseases. Some of these diseases were also shown to be associated with increased risk for cardiovascular disease. One such disease that is associated with HDL dysfunction and accelerated atherosclerosis is diabetes mellitus, a disease in which the HDL particle undergoes diverse structural modifications that result in significant changes in its function. This review will summarize the changes that occur in HDL in diabetes mellitus and how these changes lead to HDL dysfunction. Possible treatments for HDL dysfunction are also briefly described.

The antioxidant paradox in diabetes mellitus

There is ample empiric evidence to indicate that oxidative stress contributes to the pathogenesis of coronary artery disease and has a key role in the onset and progression of diabetes and its complications. Diabetes leads to depletion of the cellular antioxidant defense system and is associated with an increase in the production of free radicals. Oxidative stress can be the result of multiple pathways. Some of these are related to substrate-driven overproduction of mitochondrial reactive oxygen species, advanced glycation end product formation, glucose autoxidation, and depletion of micronutrients and cellular elements with antioxidative properties. There are numerous observational studies in the literature showing a beneficial outcome of the consumption of antioxidant vitamins. However, the interventional trials portray a different picture. The divide between the robust experimental evidence of the pathogenetic role of increased oxidative load in diabetes and the overwhelming failure of antioxidants to show any health benefits in clinical trials may well be characterized as the "antioxidant paradox."

The relationship of PON1 QR 192 and LM 55 polymorphisms with serum paraoxonase activities of Turkish diabetic patients

Paraoxonase (PON1) is a serum esterase responsible for the protection against xenobiotics toxicity such as paraoxon. Alterations in PON1 concentrations have been reported in a variety of diseases including diabetes mellitus (DM). It has been shown that the serum PON1 concentration and activity are decreased in patients with both type 1 and type 2 DM. This study aimed to investigate the lipid profiles and the relationship between PON1 activity and PON1, QR192 and LM55 polymorphisms in Turkish type 2 diabetic patients and non-diabetic control subjects. According to our results, RR variant had significantly higher PON activity than QQ and QR variants (p < 0.01) and LL variant had significantly higher PON activity than MM variant in both control and patient groups (p < 0.05). In conclusion, we found that PON1 192RR and 55LL genotypes are associated with higher PON activity than QQ and MM genotypes. This may be more protective to lipid peroxidation.

The three-gene paraoxonase family: physiologic roles, actions and regulation

The paraoxonase (PON) gene family is composed of three members (PON1, PON2, PON3) that share considerable structural homology and are located adjacently on chromosome 7 in humans. By far the most-studied member is PON1, a high-density lipoprotein-associated esterase/lactonase, also endowed with the capacity to hydrolyze organophosphates, but all the three proteins prevent oxidative stress and fight inflammation. They therefore seem central to a wide variety of human illnesses, including atherosclerosis, diabetes mellitus, mental disorders and inflammatory bowel disease. The major goal of this review is to highlight the regulation of each of the paraoxonase components by diverse nutritional molecules and pharmacological agents as well as a number of pathophysiological events, such as oxidative stress and inflammation. Considerable and detailed cell-based studies and animal model experiments have been provided to allow a thorough scrutiny of PON modulation, which will increase our understanding and ability to target these genes in order to efficiently increase their transcriptional activity and decrease the risks of developing different disorders.

The differentiating effect of glimepiride and glibenclamide on paraoxonase 1 and platelet-activating factor acetylohydrolase activity

AIMS

The present study was designed to examine the effect of sulphonylureas, glimepiride (GM) and glibenclamide (GB), on paraoxonase 1 (PON1) and platelet activating factor acetylohydrolase (PAF-AH) activity in normal and streptozotocin (STZ)-induced (50 mg/kg) diabetic rats.

MAIN METHODS

In treated groups, glimepiride (0.1 mg/kg) or glibenclamide (2 mg/kg) was given orally for 4 weeks. A PON1 and PAF-AH activity were estimated by spectrophotometric method.

KEY FINDINGS

Hyperglycemia was accompanied by a significant decrease in plasma PON1 activity toward paraoxon (P < 0.001) and phenyl acetate (P < 0.01) and increase in plasma PAF-AH activity (P < 0.01). In STZ-induced diabetic rats the administration of both GM and GB had no effect on plasma PON1 activity but reversed elevated plasma PAF-AH activity (GM: P < 0.05, GB: P < 0.01). In non-diabetic rats after either GM or GB administration the decreased PON1 activity in the plasma was observed (GM: P < 0.001, GB: P < 0.05), but plasma PAF-AH activity remained unchanged. Both GM and GB had no effect on total plasma antioxidant capacity in diabetic and control treated groups. Additionally, both drugs increased PON1 activity toward phenyl acetate in the liver, in diabetic rats (GM: P < 0.05, GB:ns) as well as in non-diabetic rats (GM: P < 0.001, GB: P < 0.001), and reduced lipid peroxidation in the liver.

SIGNIFICANCE

These results demonstrate that in streptozotocin-induced diabetic rats as well as in normal rats glimepiride and glibenclamide have no beneficial effects on circulating PON1 and PAF-AH activities, but both drugs increase PON1 activity in the liver.

Effects of acetylsalicylic acid on serum paraoxonase activity, Ox-LDL, coenzyme Q10 and other oxidative stress markers in healthy volunteers

OBJECTIVES

The aim of the study was to examine the effects of acetylsalicylic acid (ASA) on oxidative stress in healthy volunteers.

DESIGN AND METHODS

30 volunteers of which 17 received ASA as 100 mg/day (Group I) and 13 received ASA as 150 mg/day (Group II) for 2 months. Serum paraoxonase 1 (PON1), arylesterase, total antioxidant status (TAS), total oxidant status (TOS), oxidized LDL (Ox-LDL) and coenzyme Q(10) (CoQ(10)) levels were measured before and 1 and 2 months after treatment.

RESULTS

There was no significant differences between the measured parameters of the groups. However, TOS and Ox-LDL levels of group II were significantly reduced after 2 months of treatment (p<0.05).

CONCLUSIONS

Significantly inhibition of LDL oxidation and significantly reduction in TOS levels of group II after 2 months of ASA treatment shows that ASA treatment may contribute to the prevention of atherosclerosis, a beneficial effect which is dose and time dependent.

Paraoxonase: a multifaceted biomolecule

BACKGROUND

Paraoxonase enzyme was first identified as a protective barrier against organophosphorus poisoning. After painstaking research spanning the last three decades, the knowledge about this enzyme has increased immensely. The present review attempts to elaborate the role of paraoxonase enzyme in normal physiology as well as provide an overview of the various disorders in which the enzyme may have a role in etiopathogenesis.

METHODS

The literature was searched from the websites of the National Library of Medicine (http://www.ncbi.nlm.nih.gov/) and Pub Med Central, the U.S. National Library of Medicine's digital archive of life sciences journal literature.

RESULTS

Paraoxonase acts as an important antioxidant enzyme against oxidative stress. The enzyme has been implicated in the pathogenesis of a number of disorders including cardiovascular disorders, cancers etc.

CONCLUSIONS

A better understanding of the molecular mechanism of the enzyme along with the regulatory circuits will help us to utilize agonists to potentiate the anti oxidant actions of the enzyme.

Paraoxonases role in the prevention of cardiovascular diseases

Increased oxidative stress is a characteristic of patients with high risk for atherosclerosis development (hypercholesterolemic, hypertensive, diabetic), and the above phenomenon was shown to be associated with attenuated antioxidative status. The increased oxidative stress in atherosclerotic patients is present in their blood, as well as in their arterial wall cells, including macrophages, the hallmark of foam cells formation during early atherogenesis. Serum high density lipoprotein (HDL)-associated paraoxonase 1 (PON1) reduces oxidative stress in lipoproteins, in macrophages, and in the atherosclerotic lesion, whereas paraoxonase 2 (PON2, which is present in tissues, but not in serum) acts as an antioxidant at the cellular and not humoral level. Both PON1 and PON2 protect against atherosclerosis development, and this phenomenon could be related to their antioxidative properties. The use of nutritional antioxidants such as vitamin E, carotenoids (lycopene and beta-carotene), and mainly polyphenols (such as those present in red wine, licorice root ethanolic extract, or in pomegranate) by atherosclerotic animals and also by cardiovascular patients, leads to a reduction in oxidative stress and to the attenuation of atherosclerosis development. These latter phenomena could be related to the nutritional antioxidants-induced increase in HDL PON1 activity (effects on gene expression, on preventing enzyme inactivation, and on increasing PON1 stability through its binding to HDL), as well as an increase in macrophage PON2 activation (at the gene expression level).

High glucose induces transactivation of the human paraoxonase 1 gene in hepatocytes

Human serum paraoxonase 1 (PON1) is associated with high-density lipoprotein and inhibits oxidative modification of low-density lipoprotein in vitro. Therefore, PON1 is expected to protect against atherosclerosis in vivo. We and other investigators have shown that PON1 enzymatic activity is decreased in diabetic patients; however, an alteration in hepatic PON1 synthesis under hyperglycemic conditions remains unclear. We previously demonstrated that Sp1 is a positive regulator of PON1 transcription and that an interaction between Sp1 and protein kinase C (PKC) is a crucial mechanism for the effect of Sp1 on PON1 transcription in cultured HepG2 cells. Because several PKC isoforms are activated under hyperglycemic conditions, we examined the effect of d-glucose, which can activate the diacylglycerol-PKC pathway, on the transcription and expression of PON1. For a reporter gene assay, Huh7 human hepatocyte cell line incorporated with PON1 (-1232/-6)-luciferase expression vector was established using a cationic lipid method. d-Glucose dose dependently enhanced PON1 promoter activity. d-Glucose also enhanced both messenger RNA and protein expression of PON1. Increased PON1 expression was also detected in primary human hepatocytes treated with high d-glucose concentrations. Bisindolylmaleimide, a PKC inhibitor, significantly inhibited d-glucose-induced transactivation of PON1; and mithramycin, an inhibitor of Sp1, completely abrogated the transactivation. Our data suggest that high glucose concentrations transactivate the PON1 gene through Sp1 activation by PKC in cultured hepatocytes. Up-regulated hepatic PON1 expression under high glucose conditions may be a compensatory mechanism in diabetes in which antioxidant capacity, including PON1 enzymatic activity, is attenuated.

Induction of paraoxonase 1 and apolipoprotein A-I gene expression by aspirin

Low-dose aspirin therapy has become a standard in the treatment of cardiovascular diseases. Aspirin has been shown to inhibit atherosclerosis in mouse models. To determine the mechanisms by which aspirin might inhibit atherosclerosis, we incubated HEPG2 cells and rat primary hepatocytes with aspirin or salicylic acid and noted an increase in paraoxonase 1(PON1) activity in the medium, together with an induction of PON1 and apolipoprotein A-I (apoA-I) gene expression. Mice treated with aspirin also showed a 2-fold increase in plasma PON1 activity and a significant induction of both PON1 and apoA-I gene expression in the liver. The induction of the PON1 gene in cell culture was accompanied by an increase in arylhydrocarbon receptor (AhR) gene expression. Accordingly, aspirin treatment of AhR(-/-) animals failed to induce PON1 gene expression. We previously suggested that aspirin might be hydrolyzed by serum PON1, which could account for its short plasma half-life of 10 min. Taken together with the current studies, we suggest that the antiatherosclerotic effects of aspirin might be mediated by its hydrolytic product salicylate and that the induction of PON1 and apoA-I might be important in the cardioprotective effects of aspirin.

Paraoxonase 1 (PON1) attenuates diabetes development in mice through its antioxidative properties

Paraoxonase 1 (PON1) is a lipo-lactonase which is associated with HDL and possesses antioxidative properties. Diabetes is characterized by increased oxidative stress and by decreased PON1 activity. We aimed to analyze whether oxidative status and PON1 levels in mouse sera and macrophages could affect streptozotocin (STZ)-induced diabetes development. We have used two models of mice under low oxidative stress: STZ-injected apolipoprotein E-deficient mice supplemented with the antioxidant vitamin E, and P47(phox) knockout mice. In both mice models the decreased serum basal oxidative stress, was associated with a decreased rate of diabetes development, compared with control STZ-injected apolipoprotein E-deficient mice or with C57BL mice respectively. These data suggest that oxidative stress accelerates diabetes development. Next, we analyzed the effect of PON1 on macrophage oxidative stress and on diabetes development in STZ-injected C57BL mice, PON1 knockout mice, and PON1 transgenic mice. PON1 overexpression was associated with decreased diabetes-induced macrophage oxidative stress, decreased diabetes development, and decreased mortality, in comparison to C57BL mice, and even more so when compared to PON1KO mice. We thus concluded that on increasing PON1 expression in mice, diabetes development is attenuated, a phenomenon which could be attributed to the antioxidative properties of PON1, as decrement of oxidative stress significantly attenuated STZ-induced diabetes development.

Why is HDL functionally deficient in type 2 diabetes?

High-density lipoprotein (HDL) particles exert a spectrum of atheroprotective activities that can be deficient in type 2 diabetes. Key mechanisms leading to the formation of functionally deficient HDL involve 1) HDL enrichment in triglycerides and depletion in cholesteryl esters with conformational alterations of apolipoprotein A-I; 2) glycation of apolipoproteins and/or HDL-associated enzymes; and 3) oxidative modification of HDL lipids, apolipoproteins, and/or enzymes. Available data identify hypertriglyceridemia, with concomitant compositional modification of the HDL lipid core and conformational change of apolipoprotein A-I, as a driving force in functional alteration of HDL particles in type 2 diabetes. Therapeutic options for correcting HDL functional deficiency should target hypertriglyceridemia by normalizing circulating levels of triglyceride-rich lipoproteins.

Oxidation–reduction potential and paraoxonase–arylesterase activity in trauma patients

The amount of oxidative stress in severely traumatized patients is usually based on various individual parameters such as total antioxidants and lipid peroxidation. Serial measurements of plasma oxidation-reduction potential (ORP) in severely traumatized patients as a simple mean of assessing overall oxidative stress is described. Serial whole blood samples were obtained from multi-trauma patients (N=39) and healthy individuals (N=10). Plasma ORP in multi-trauma patients increased during the first few days of hospitalization and approached normal ORP levels upon discharge. On the ORP maxima day (5.8 days+/-0.5 SEM), a statistically significant decrease (p<0.05) was observed for negative acute phase reactants such as plasma paraoxonase-arylesterase (PON-AE) activity and total plasma protein in comparison with admission plasma levels. Monitoring ORP could be a useful tool for assessing the degree of oxidative stress, inflammation, severity of injury, and potential efficacy of treatment.

A biphasic U-shape effect of cellular oxidative stress on the macrophage anti-oxidant paraoxonase 2 (PON2) enzymatic activity

Expression of macrophage paraoxonase 2 (PON2), a cellular lactonase with anti-oxidant and anti-atherogenic properties, was shown to be upregulated under high oxidative stress. The aim of the present study was to analyze the relationship between the extent of cellular oxidative stress in J774A.1 macrophage and PON2 lactonase activity under various levels of oxidation, obtained by cell incubation with either anti-oxidants or oxidants. PON2 activity exhibited a U-shape response curve. In the oxidative stress range below that of control untreated cells, PON2 activity decreased upon increasing macrophage oxidative state, whereas in the range over that of control untreated cells, PON2 activity increased. The biphasic effect of oxidative stress on macrophage PON2 activity could be related to PON2 inactivation (decreased enzymatic activity) under oxidative stress induction at its low range, whereas at high range of oxidative stress, macrophage anti-oxidant compensatory mechanism up-regulates PON2 (increased protein expression), in order to cope with oxidative burden.

Vitamin C preserves the cardio-protective paraoxonase activity of high-density lipoprotein during oxidant stress

HDL-associated paraoxonase (PON) antioxidant enzyme activity is cardio-protective. We investigated whether vitamin C prevented loss of PON activity from HDL during oxidant stress. HDL was incubated with either hydrophilic or lipophilic peroxyl radical initiators in the absence (control) or presence of vitamin C (50 and 100 micromol/L). Regardless of the type of radical, accumulation of lipid oxidation products in HDL was similar in incubations lacking vitamin C. Loss of PON activity was greater in HDL exposed to hydrophilic, in contrast to lipophilic, radicals, but addition of vitamin C maintained enzyme activity. Vitamin C's capacity to attenuate loss of the HDL ability to prevent atherogenic modification of LDL (assessed as electrophoretic mobility) was, however, modest, and appeared limited only to those incubations in which HDL was exposed to lipophilic radicals. Our results indicate that vitamin C may, under some conditions, prevent loss of cardio-protective function from HDL during oxidant stress.