Lipid peroxidation in hemodialysis patients: Effect of vitamin C supplementation
Introduction
Chronic renal failure is a disease caused by damage to renal parenchyma. Despite significant progress in dialysis technology, atherosclerosis and cardiovascular diseases are the leading causes of morbidity and mortality in patients [1], [2] receiving renal replacement therapy.
Alterations of plasma lipoprotein levels and a higher susceptibility of low density lipoproteins (LDL) to lipid peroxidation have been previously demonstrated in hemodialysis (HD) patients [3]. These alterations, the overproduction of reactive oxygen species (ROS) and the increase in proatherogenic cytokine levels in hemodialysis patients may contribute to the accelerated HD-induced atherogenesis [4], [5]. Production of oxidants and/or free radicals can facilitate damage to cell macromolecules, can activate signalling events that mediate expression of inflammatory genes and apoptosis [6].
The susceptibility to oxidative stress in patients is mediated by lower antioxidant defences. Previous studies have demonstrated several deficiencies in hydrophilic antioxidant molecules in plasma of HD patients with a lower concentration of vitamin C [7]. As far as concerns cell antioxidant enzymes, decreased superoxide dismutase (SOD) and glutathione peroxidase (GPx) activity has been demonstrated in erythrocytes isolated from HD patients [8]. A decrease of the activity of the HDL-associated enzyme paraoxonase (PON1) has also been observed in hemodialysis patients [9], [10], [11]. PON1 is a HDL-bound enzyme whose physiological substrate has not been elucidated. An esterase, peroxidase and thiolactonase activity have been attributed to PON1 [12], [13], [14] and we have previously demonstrated that high density lipoproteins isolated from subjects with higher paraoxonase activity have a lower sensibility to atherogenic modifications of HDL such as oxidation, non-enzymatic glycation and homocysteinylation [15], [16], [17].
The decrease of PON1 activity in HD patients has been correlated with the effectiveness of dialysis to clear creatinine and urea and with the clearance of advanced glycation end products (AGEs) of low molecular weight [10], therefore it has been hypothesized that PON1 could represent a useful biochemical index to check new therapy and dialysis approaches in HD patients.
A decrease of biochemical markers of oxidative damage has been demonstrated in HD patients supplemented with ascorbic acid [18], [19]; however contrasting results likely related to duration of therapy and administration, have been reported [20].
Aim of the study was to further investigate the relationship between oxidative stress and chronic renal failure; we compared the paraoxonase activity and levels of lipid hydroperoxides and total AGEs in plasma of HD patients before and after intravenous supplementation with vitamin C for 6 months. The correlations between changes of paraoxonase activity and markers of inflammation (C reactive protein-CRP and ferritin) in HD patients have been also included.
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Materials
Phosphate buffer solution (PBS), xylenol orange [o-cresolsulfonephthalein-3′,3′-bis(methyliminodiacetic acid sodium salt)], ammonium iron (II) sulfate hexahydrate [Fe(NH4)2(SO4)2], sulphuric acid (H2SO4), butylated hydroxytoluene (BHT), Tris [(Hydroxymethyl)aminomethane], sodium chloride (NaCl), magnesium chloride (MgCl2) calcium chloride (CaCl2), paraoxon (dietyl-p-nitrophenyl phosphate), t-butyl hydroperoxide were purchased from Sigma Chemical Company (St. Louis, MO).
Patients
42 patients affected by
Biochemical data in controls and unsupplemented patients
Table 1 summarizes the biochemical profiles of healthy subjects and unsupplemented patients. Ferritin, free iron and CRP (C reactive protein) levels were significantly higher in patients with respect to controls. The mean level of triglycerides (TG) was significantly higher in unsupplemented patients with respect to controls (p < 0.001); while the levels of total cholesterol (TC) and LDL cholesterol (LDL-C) were significantly lower. No significant differences were observed in the levels of HDL
Discussion
Oxidative stress, lipid and lipoprotein peroxidation, hyperhomocysteinemia and inflammation have been described in an number of diseases associated with uremia and hemodialysis [3], [4], [5], [23] with an increase of production of H2O2 [6], HOCl [24], lipid peroxidation and AGEs [10] with respect to healthy subjects. Dialysis treatment appears to contribute to oxidative stress and to alterations of the balance between free radicals generation and antioxidant systems. In fact, previous studies
Acknowledgment
This work was supported by a grant from MURST 60% (GF).
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