Platelet antioxidant enzymes in insulin-dependent diabetes mellitus

Abstract

Background: The measurement of the peroxidase scavenging system represented by the activities of superoxide dismutase (SOD), catalase and glutathione peroxidase (GSH-Px) in blood cells of diabetic patients has, in the past, given equivocal results. Likewise, the role of these intracellular enzymatic scavengers against the oxidative stress of diabetes-associated microangiopathic complications is unknown. Methods: Choosing platelets as cell model (as commonly done in previous studies), the aim of this study was to relate the platelet content of SOD, catalase and GSH-Px to the presence of diabetes, as well as to the presence of nephropathy and retinopathy in 35 insulin-dependent diabetic patients, as compared to 10 age-matched control subjects. Results: The enzymatic activities were not changed in diabetic patients in comparison with healthy controls. After stratifying patients according to presence of nephropathy (24-h urinary albumin excretion rate persistently ≥20 μg min⁻¹) or retinopathy, the group of albuminuric patients was characterized by a significant decrease in SOD activity as compared to those in the normoalbuminuric range (4.36±1.06 vs. 6.81±2.26 mU 10⁻⁹ platelets; p=0.01). Catalase and GSH-Px did not change. No modification in platelet enzyme activities has been found in diabetic subjects with retinopathy. Conclusions: These results suggest that diabetic nephropathy, at least in its early stage, may be related to an altered redox state of platelets, as tested by the reduction in SOD activity, thus, indicating that the renal damage in these patients may be associated to a selective increase in platelet susceptibility to variation in the redox state.

Introduction

Evidence is accruing that abnormalities in tissue antioxidant–oxidant status may be important in the pathogenesis of diabetic complications [1]. Platelets of diabetic patients are exposed to increased oxidative stress as shown by their raised levels in malondialdehyde [2]. Moreover, in patients affected with insulin dependent diabetes mellitus (IDDM) platelet levels of low-molecular weight antioxidants such as reduced glutathione (GSH), ascorbic acid or taurine have been found reduced, while platelet levels of ascorbic acid have been found raised [2], [3], [4].

As to the antioxidant enzymes, little is known about their activities in platelets obtained from diabetic patients. The measurements of superoxide dismutase (SOD) activity in blood cells have given equivocal reports: in the red cells of diabetic subjects, it was found lower while in lymphocytes, it was unchanged [5]. On the contrary, Dominiguez et al. [6] found that SOD activity was higher in erythrocytes of children and adolescents with IDDM in comparison to healthy controls. Finally, Vucic et al. [7] found that SOD was lower in lymphocytes and polymorphonuclear cells of diabetic patients. Although the role of SOD in platelet function is not clearly understood, it has been shown that SOD triggers activation of human platelets exposed to subthreshold concentrations of arachidonic acid and collagen [8].

Moreover, it has been shown quite recently that collagen induced platelet aggregation is mediated by a burst of hydrogen peroxide and it is inhibited by catalase [9]. Furthermore, platelet activation triggers the production of biologically active eicosanoids, including hydroperoxides, all of which could be candidate substrates of the platelet glutathione redox cycle in which glutathione peroxidase (GSH-Px) plays a key role [10]. While the activity of the latter has been reported as not altered in platelets of IDDM patients [2], [10], little is known about other enzymes involved in the scavenging processes against hydroperoxides. As a consequence of the above-mentioned uncertainties about cell enzymatic defence against peroxides in IDDM, the aim of this study was to measure the activities of SOD, catalase and GSH-Px in platelets of IDDM patients with or without microangiopathic complications as compared to healthy controls. In conformity with previous work by us, as well as by other authors, we chose platelets as the ‘cell model’; and since the defense against oxidative stress depends primarily on an orchestrated synergism among antioxidants [11], we simultaneously measured the activity of SOD, catalase and GSH-Px in the platelets of IDDM patients and of controls.