Serum Paraoxonase-1 Arylesterase Research Articles

Assessment of amino-terminal C-type natriuretic peptide serum level and its correlation with high-density lipoprotein structure and function in patients with end stage renal disease before and after kidney transplantation

We aimed to investigate serum amino-terminal C-type natriuretic peptide (NT-proCNP) and its relationship with quantitative and qualitative HDL-parameters in patients with end-stage renal disease (ESRD) before, then 1 and 6 months after kidney transplantation (TX). Seventy patients (47 males, 23 females, mean age 51.7 ± 12.4 years) were enrolled in a prospective follow-up study. We examined serum creatinine, C-reactive protein, procalcitonin, fasting glucose and lipid parameters before, then 1 and 6 months after TX. High-density lipoprotein- (HDL)-associated paraoxonase-1 (PON1) paraoxonase and arylesterase activities were measured spectrophotometrically. Lipoprotein subfractions were determined by Lipoprint. NT-proCNP and oxidized low-density lipoprotein (oxLDL) levels were measured by ELISA. Mean NT-proCNP was 45.8 ± 21.9 pmol/L before renal transplantation and decreased markedly 1 month and 6 months after transplantation (5.3 ± 2.5 and 7.7 ± 4.9 pmol/L, respectively, P = 1 × 10−4). During the 6 months’ follow-up, PON1 arylesterase, paraoxonase and salt-stimulated paraoxonase activities improved. NT-proCNP positively correlated with procalcitonin and creatinine and negatively with GFR, LDL-cholesterol (LDL-C) and HDL-cholesterol (HDL-C). There was a negative correlation between serum NT-proCNP and PON1 arylesterase activity. According to the multiple regression analysis, the best predicting variables of NT-proCNP were serum procalcitonin, creatinine and PON1 arylesterase activity. NT-proCNP might be a novel link between HDL dysfunction and impaired vascular function in ESRD, but not after kidney transplantation. Further studies in larger populations are needed to clarify the exact role of NT-proCNP in the risk prediction for cardiovascular comorbidities and complications in ESRD.

Paraoxonase 1 and atrial fibrillation: Is there a relationship?

Atrial fibrillation (AF) is associated with oxidative stress and inflammation. Paraoxonase-1 (PON1), circulates in blood bound to high-density lipoproteins and reduces systemic oxidative stress. The aim of this study was to evaluate PON1 serum concentration and PON1 arylesterase activity (AREase) in patients with AF. We studied a group of 67 patients with symptomatic paroxysmal or persistent AF admitted for cardioversion and a control group of 59 patients without AF. Clinical parameters, lipid profile, PON1 concentration and AREase were evaluated. A significant difference in serum PON1 concentration and in AREase was found among the two groups. In a multivariate linear regression model, the presence of AF was associated with low PON1 concentration (P = .022). The body mass index was also independently associated with PON1 values (P < .001). Only the high-density lipoproteins-cholesterol level was independently associated with AREase (P = .002). PON1 serum concentrations and AREase were diminished in patients with AF, and the presence of AF was independently associated with low PON1 values.

Paraoxonase (PON)-1 activity in septic neonates: One more arrow in the quiver of biomarkers of neonatal sepsis?

Paraoxonase-1 (PON-1), a calcium ion-dependent high-density lipoprotein (HDL)-associated enzyme, has been proposed as a negative acute phase reactant biomarker in animal and human adult studies. The aim of this study was to evaluate the value of PON-1 activity in the diagnosis and monitoring of neonatal sepsis. Serum PON-1 activity, as paraoxonase and arylesterase, was prospectively studied in 48 septic neonates and matched controls. PON-1 activity was decreased at the acute phase of sepsis in comparison with values at recovery and values in controls. Paraoxonase or arylesterase at enrollment correlated significantly with serum Amyloid-A, CRP and IL-6 and could also discriminate septic than non-septic neonates. In conclusion, our results are promising regarding the role of PON-1 as a biomarker of neonatal sepsis. Larger studies are needed to validate the clinical utility of PON-1 in neonatal medicine.

Correlation Between Serum PON1 Arylesterase Activity and Rs 854573 &amp;lt;i&amp;gt;PON1 A &amp;lt; G&amp;lt;/i&amp;gt; Polymorphism with Type 2 Diabetes in an Eastern Indian Cohort

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder of glucose and lipids and characterized by defect in insulin secretion or action. Oxidative imbalance has also been implicated in the etiology of diabetes. Paraoxonase-1 (PON1) is an esterase and lactonase which is found in the circulation bound to high-density lipoproteins (HDL). Alterations and associations of circulating PON1 levels with a variety of diseases including diabetes encourages us to investigate the possible association between PON1 A/G rs854573 polymorphism and serum PON1 activity with T2DM. The study essentially follows a population based case-control format with 101 diabetic and 102 healthy controls. The findings revealed association of polymorphism with the diseased status (p value 0.0002, OR 3.49, 95% CI 1.77 to 6.9). With significantly higher range of mean serum PON1 Arylesterase (AREase) activity in control (9.99 – 0.96 kU/L) than in diabetic patients (5.25 - 0.508 kU/L) (p value,<0.001), a large difference between common diabetic AA genotype and combined diabetic heterozygous and homozygous genotypes (AG+GG) for risk allele G (assymptometic p value,<0.001), or in between two AA genotypes (Diabetic/Non diabetic, p<0.001), was explored by parametric and non parametric statistical pairwise comparison. Serum PON1 activity was found to be independent of other clinical factors such as plasma glucose levels. Western blot analysis of serum samples detected a significant difference of PON1 proteins in diabetic patients and control subjects (p value 0.008). In conclusion serum PON1 AREase activity which to an extent correlated with PON1 promoter polymorphism might be a good predictor of the disease risk.

Paraoxonase 1 Q192R genotype and activity affect homocysteine thiolactone levels in humans.

Genetic or nutritional deficiencies in 1 carbon and homocysteine (Hcy) metabolism elevate Hcy-thiolactone levels and are associated with cardiovascular and neurologic diseases. Hcy-thiolactone causes protein damage, cellular toxicity, and proatherogenic changes in gene expression in human cells and tissues. A polymorphic cardio-protective enzyme, paraoxonase 1 (PON1), hydrolyzes Hcy-thiolactone in vitro. However, whether Hcy-thiolactone hydrolysis is a physiologic function of the PON1 protein and whether polymorphisms in the PON1 gene affect Hcy-thiolactone levels in humans was unknown. Here we show that the PON1-192 genotype, which affects the enzymatic activity of the PON1 protein, also affected urinary Hcy-thiolactone levels, normalized to creatinine. Carriers of the PON1-192R allele had significantly lower Hcy-thiolactone/creatinine levels than individuals carrying the PON1-192Q allele. Individuals with low serum PON1 paraoxonase activity had significantly higher Hcy-thiolactone/creatinine levels compared with individuals with high paraoxonase activity. In contrast, Hcy-thiolactone/creatinine levels were unaffected by serum PON1 arylesterase activity or by PON1 protein levels. Taken together, these findings suggest that PON1 hydrolyzes Hcy-thiolactone in humans and that the interindividual variations in PON1 genotype/activity can modulate the pathology of hyperhomocysteinemia.-Perła-Kaján, J., Borowczyk, K., Głowacki, R., Nygård, O., Jakubowski, H. Paraoxonase 1 Q192r genotype and activity affect homocysteine thiolactone levels in humans.

Decreased serum PON1 arylesterase activity in familial hypercholesterolemia patients with a mutated LDLR gene.

Paraoxonase 1 (PON1) is a serum enzyme associated with high density lipoprotein (HDL) regulation through its paraoxonase and arylesterase activity. PON1 inhibits the oxidation of HDL and low density lipoprotein (LDL), and is involved in the pathogenesis of a variety of diseases including atherosclerosis. Conversely, mutations in the low density lipoprotein receptor (LDLR) result in failure of receptor mediated endocytosis of LDL leading to its elevated plasma levels and onset of familial hypercholesterolemia (FH). In the current study we investigated the role of PON1 polymorphisms rs662; c.575A > G (p.Gln192Arg) and rs854560; c.163T > A (p.Leu55Met) in a large family having FH patients harboring a functional mutation in LDLR. Genotypes were revealed by RFLP, followed by confirmation through Sanger sequencing. PON1 activity was measure by spectrophotometry. Our results show significantly reduced serum paraoxonase and arylesterase activities in FH patients compared with the healthy individuals of the family (p < 0.05). PON1 QQ192 genotype showed a significantly higher association with FH (p=0.0002). PON1 Q192 isoform was associated with reduced serum paraoxonase activity by in silico analysis and PON1 R192 exhibited higher serum paraoxonase and arylesterase activity than the other polymorphs. Our results highlight that the combination of LDLR mutations and PON1 MMQQ genotypes may lead to severe cardiac events.

Association between paraoxonase 1 (PON1) enzyme activity, PON1 C(−107)T polymorphism, nutritional status, and lipid profile in children

Abstract Background Paraoxonase 1 (PON1) is an enzyme that possesses anti-atherogenic and anti-inflammatory properties with serum levels determined by genetic and exogenous factors. Lower serum PON1 arylesterase activity is associated to metabolic alterations related to childhood overweight and onset and/or development of diabetes and CVD later in life. However, data on the relationship between genetic PON1 polymorphisms and nutritional status as well as lipid profile in children are limited. To investigate the distribution of the C(−107)T PON1 gene polymorphism and its relation with serum PON1 enzyme activity, nutritional status and lipid profile in children. Methods A cross-sectional study was performed including 73 children aged 5 to 7 years who attended public pediatric clinics. PON1 C(−107)T, arylesterase activity, body mass index for the age, and serum lipid profile were evaluated. Results PON1 activity was higher in overweight children compared to the normal weight ones (p = 0.02). The genotypic frequency did not differ between the two groups (p &gt; 0.05). Carriers of CC genotype had higher enzyme activity than T allele carriers, and this difference was greater among normal weight children. HDL levels were higher among normal weight children carrying CC genotype, compared to those carrying the T allele (p &lt; 0.01). Conclusion The PON1 C(−107)T polymorphism is associated with higher serum enzyme activity in children, as observed previously in adults. In addition, this polymorphism also shows association to higher high density lipoprotein (HDL) levels and serum PON1 arylesterase activity in the normal weight children studied.

Paraoxonase-1 activities in children and adolescents with type 1 diabetes mellitus.

Paraoxonase-1 is an HDL-associated esterase that acts as an anti-atherogenic agent by protecting LDL from oxidation. This study investigates paraoxonase-1 activities in children and adolescents with type 1 diabetes mellitus and possible associations with other biochemical markers. The study enrolled 82 children and adolescents with type 1 diabetes mellitus and 41 controls with similar age and gender distribution. Serum paraoxonase-1 arylesterase and salt-stimulated paraoxonase activities were assessed by measuring the rates of phenyl acetate and paraoxon hydrolysis, respectively; paraoxonase-1 lactonase activity and oxidized LDL were assessed by a pH-sensitive colorimetric assay and ELISA, respectively. Glycated haemoglobin HbA1c and lipid profile were assayed with an immunoturbidimetric method and commercially available kits, respectively. We found lower paraoxonase-1 activities in diabetics when compared to controls. The decrease was statistically significant only for the lactonase activity, the difference being higher when referring to the subgroup with poor glycaemic control. The lactonase activity/HDL ratio was also lower in diabetics vs. controls, but without statistical significance. Both lactonase and arylesterase activities were negatively correlated with HbA1c in diabetics, but only the latter was statistically significant (ρ = -0.21, P = 0.055; ρ = -0.24, P = 0.03, respectively). A correlation coefficient of ρ = 0.196 (P = 0.078) was found between oxidized LDL and HbA1c. All paraoxonase-1 activities were lower in diabetic children and adolescents, but only the decrease in the lactonase activity was statistically significant. Although lipid profile and glycaemic control were altered in diabetics, no differences were observed between groups regarding oxidized LDL level.

Postprandial Paraoxonase 1 Activity Following Consumption of Recommended Amounts of Mixed Meals in Healthy Males.

Postprandial lipid level increases induce oxidative stress, which is involved in atherogenesis. The antioxidant properties of paraoxonase 1 (PON1) have attracted attention. However, changes in postprandial PON1 levels differ across prior studies, and changes in PON1 lactonase activity, potentially relevant to PON1 physiology, after the consumption of ordinary meals are unknown. Herein we evaluated postprandial serum lipid levels and PON1 changes following mixed-meal consumption of the amounts recommended for ordinary meals. Nine healthy male volunteers consumed three different meals in a randomized cross-over design. The test meals were as follows: S, white rice; SMF, S with fat-containing protein-rich main dishes; and SMFV: SMF with vegetable dishes. The serum lipid concentrations and PON1 lactonase and arylesterase activities were determined during a three-hour period after the consumption of these meals. The postprandial triglyceride levels were higher after consuming the SMF and SMFV meals than after consuming the S meal. Despite postprandial high-density lipoprotein cholesterol being unchanged, PON1 lactonase activity was decreased, while PON1 arylesterase activity was increased in the postprandial state after all test meals. Postprandial changes in lactonase and arylesterase activities did not differ among the test meals. Inverse changes in PON1 lactonase and arylesterase activities were observed after consuming recommended ordinary meals. This observation provides useful information for choosing PON1 species as postprandial markers.

Serum paraoxonase and arylesterase activities in patients with chronic otitis media

Objectives: Paraoxonase-1 (PON1) prevents oxidative stress by inhibiting the oxidation of cell membrane lipids by the reactive oxygen species that form during acute and chronic inflammation. The aim of this study was to investigate serum PON1 activity and oxidative stress in patients with chronic otitis media (COM).Methods: Fifty consecutive patients with COM and 55 controls were enrolled in the present study. The patients were divided into two groups according to the presence of cholesteatoma. The serum PON1 arylesterase activities and lipid hydroperoxide (LOOH) levels were determined.Results: Serum paraoxonase and arylesterase activities were significantly lower in the COM patients than in the controls (P < 0.001 for all comparisons), whereas the LOOH levels were significantly higher (P < 0.001).Discussion: These results indicated that a lower level of PON1 activity was associated with an oxidant–antioxidant imbalance. In addition, decreased PON1 activity may play an important role in the pathophysiology of COM.

Altered lipid subfraction profile and impaired antioxidant defense of high-density lipoprotein in Smith-Lemli-Opitz syndrome.

Smith-Lemli-Opitz syndrome (SLOS) is a rare disease caused by biallelic mutation in the 7-dehydrocholesterol (7DHC) reductase gene. High oxidizability of 7DHC and the appearance of small-sized low-density lipoprotein (LDL) subfractions indicate increased endogenous oxidative stress that is counterbalanced by natural antioxidant defense mechanisms including the high-density lipoprotein (HDL)-associated paraoxonase-1 (PON1) enzyme. PON1 prevents lipoproteins from oxidative modifications; however, PON1 activity and the distribution of lipoprotein subfractions have not been studied in SLOS. 7DHC levels and PON1 arylesterase activities were measured spectrophotometrically in 11 SLOS patients and 10 healthy children. Lipoprotein subfractions were detected by polyacrylamide gel electrophoresis. Compared to controls, there was a shift towards the small-dense LDL subfraction and the large HDL subfraction in SLOS. PON1 arylesterase activity was significantly decreased in SLOS patients and correlated negatively with the proportion of small-dense LDL subfraction and the proportion of large HDL subfraction. Significant positive correlations were detected between PON1 arylesterase activity and the ratios of intermediate and small HDL subfractions. Decreased PON1 activity and the deleterious shift in the distribution of lipoprotein subfractions may contribute to the impaired antioxidant status observed in SLOS. Monitoring of serum PON1 arylesterase activity may be a complementary biomarker in SLOS.

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.

Low paraoxonase 1 arylesterase activity and high von Willebrand factor levels are associated with severe coronary atherosclerosis in patients with non-diabetic stable coronary artery disease.

BackgroundParaoxonase 1 (PON1) activity and von Willebrand factor (VWF) release are associated with lesion initiation in atherosclerosis. Diabetes can complicate coronary artery disease (CAD) due to the production of advanced glycation end products. This study evaluated PON1 activity and VWF levels in non-post-acute coronary syndrome, stable CAD (SCAD) patients without diabetes.Material/MethodsNon-diabetic SCAD patients and patients experiencing acute stress periods were selected (n=130). Forty-seven cases with normal coronary angiography and 50 healthy individuals served as controls. The non-diabetic SCAD group was then stratified into single-vessel lesions, multiple-vessel lesions, and mild or severe luminal stenosis according to the number and the degree of luminal stenoses. Serum PON1 paraoxonase and arylesterase activities, and plasma VWF levels were measured, as well as serum total cholesterol, total triglycerides, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and apolipoprotein A1. PON1 arylesterase activity was detected with an ordinary chemistry system using a novel phenylacetate derivative.ResultsBoth PON1 paraoxonase and arylesterase were lower in the non-diabetic SCAD group, but VWF levels were higher (versus controls, all P<0.001). PON1 paraoxonase activity (OR=0.991), PON1 arylesterase activity (OR=0.981), and VWF (OR 2.854) influenced SCAD in multiple logistic regression. Decreased PON1 arylesterase activity and increased VWF levels were associated with severe atherosclerosis in non-diabetic SCAD patients. We also observed a slight negative correlation between VWF and PON1 paraoxonase/arylesterase.ConclusionsPON1 and VWF are detectable markers that may predict the severity of stenoses, ideally facilitating a non-diabetic SCAD diagnosis before the sudden onset of life-threatening symptoms.

Serum paraoxonase-1 as biomarker for improved diagnosis of fatty liver in dairy cows

BackgroundFatty liver is a major metabolic disorder in dairy cows and is believed to result in major economic losses in dairy farming due to decreased health status, reproductive performance and fertility. Currently, the definitive means for diagnosing fatty liver is determining the fat content of hepatic tissue by liver biopsy, which is an invasive and costly procedure, making it poorly suited to dairy farms. Therefore, the key aim of this study was to investigate the measurement of serum paraoxonase-1 (PON1), an enzyme exclusively synthesized by the liver, as a sensitive noninvasive biomarker for diagnosis of fatty liver in dairy cows.ResultsA comparative cohort study using serum specimens from Holstein–Friesian dairy cows (46 healthy and 46 fatty liver cases) was conducted. Serum PON1 (paraoxonase, lactonase and arylesterase) activity and other biochemical and hematological parameters were measured. We found that serum PON1 activity was lower (P<0.001) in cows suffering from fatty liver. The area under the receiver operating characteristic curve (AUC-ROC) of PON1 activity for diagnosis of fatty liver was 0.973–0.989 [95% confidence interval (CI) 0.941, 1.000] which was higher than the AUC-ROC of aspartate aminotransferase (AST), lecithin-cholesterol acyltransferase (LCAT), alkaline phosphatase (ALP), non-esterified fatty acids (NEFA), beta-hydroxybutyrate (BHBA), total cholesterol, high-density lipoprotein (HDL) and low-density lipoprotein (LDL). We found that adding serum PON1 measurement to different batteries of serum diagnostic panels showed a combination of high sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio (+LR), negative likelihood ratio (−LR), diagnostic odd ratio (DOR) and overall diagnostic accuracy in diagnosing fatty liver.ConclusionsThe present results indicate that addition of serum PON1 activity measurement to the biochemical profile could improve the diagnosis of fatty liver in dairy cows, which would have a considerable clinical impact and lead to greater profitability in the dairy industry.

VLDL triglycerides inhibit HDL-associated paraoxonase 1 (PON1) activity: in vitro and in vivo studies

We analyzed, for the first time, both in vitro and in vivo, the effect of very low density lipoprotein (VLDL), or of pure triglycerides, on high-density lipoprotein (HDL)-associated paraoxonase1 (PON1) catalytic activities. Incubation of serum or HDL from healthy subjects with VLDL (0-330 μg protein/mL) significantly decreased serum PON1 lactonase or arylesterase activities by up to 11% or 24%, and HDL-associated PON1 lactonase or arylesterase activities by up to 32% or 46%, respectively, in a VLDL dose-dependent manner. VLDL (0-660 μg protein/mL) also inhibited recombinant PON1 (rePON1) lactonase or arylesterase activities by up to 20% or 42%, respectively. Similar inhibitory effect was noted upon rePON1 incubation with pure triglyceride emulsion. Bezafibrate therapy to three hypertriglyceridemic patients (400 mg/day, for one month) significantly decreased serum triglyceride concentration by 67%, and increased serum HDL cholesterol levels by 48%. PON1 arylesterase or paraoxonase activities in the patients' HDL fractions after drug therapy were significantly increased by 86-88%, as compared to PON1 activities before treatment. Similarly, HDL-PON1 protein levels significantly increased after bezafibrate therapy. Finally, bezafibrate therapy improved HDL biological activity, as HDL obtained after drug therapy showed increased ability to induce cholesterol efflux from J774A.1 macrophages, by 19%, as compared to HDL derived before therapy. We thus conclude that VLDL triglycerides inhibit PON1 catalytic activities, and bezafibrate therapy significantly improved HDL-PON1 catalytic and biological activities.

Hypochlorous acid and low serum paraoxonase activity in haemodialysis patients: an in vitro study.

Serum paraoxonase 1 (PON1) is an oxidant-sensitive enzyme associated with high-density lipoprotein (HDL) that inhibits the atherogenic oxidation of low-density lipoprotein (LDL). In haemodialysis patients, production of reactive oxygen species, such as hypochlorous acid (HOCl) and hydrogen peroxide, is increased and serum PON1 arylesterase is abnormally low. We have examined the effect of HOCl and the uraemic milieu on serum PON1 arylesterase activity and the ability of HDL to inhibit LDL oxidation in vitro. Serum was incubated with HOCl, hydrogen peroxide and products of HOCl reaction with excess cysteine, lysine and taurine and then serum PON1 arylesterase and serum protein tryptophan fluorescence were measured. The ability of plasma HDL fractions isolated by a dextran-sulphate method, to protect LDL from mild oxidation in air, was determined by a fluorimetric method using oxidation of 2,7-dichlorofluorescein (DCFH). Incubation of healthy serum with HOCl in the range 6.5-32.9 mmol/l resulted in a linear decrease in serum PON1 arylesterase activity to 40% of that without HOCl and a parallel decrease in protein tryptophan fluorescence. The HOCl-induced decrease in serum PON1 activity was completely removed by reaction of HOCl with a 2.7-fold excess of alpha-amino acids but not taurine. In serum incubated for 1 week, the decrease in serum PON1 activity was significantly (P = 0.04) less while the increase in protein fluorescent advanced glycation end-products was significantly larger (P = 0.01) in haemodialysis patients compared with healthy subjects. The mean decrease in mild oxidation of LDL was not significantly different on addition of HDL-rich fractions from haemodialysis patients (100 +/- 6%, n = 7) and healthy subjects (95 +/- 6%, n = 7) or on addition of the HDL-rich fraction from plasma treated with 0.95 mmol/l HOCl (95%) and control HDL (96%). The fraction rich in HDL and other high molecular weight compounds from plasma that had been incubated with increasing HOCl concentrations up to 1.9 mmol/l significantly (P = 0.001) increased (471%) the oxidation of DCFH. These results suggest that high concentrations of HOCl that severely oxidize serum proteins and tryptophan residues in the active site of PON1 are required to decrease PON1 arylesterase activity in serum. In haemodialysis patients, overproduction of HOCl that leads to high concentrations of severely oxidized proteins and increased oxidants in plasma might also contribute to low serum PON1 arylesterase activity, but does not appear to impair the ability of an HDL molecule to protect LDL from mild oxidation.