Monitoring diabetic antioxidant status: a role for in vitro methaemoglobin formation

Monitoring antioxidant effects using methaemoglobin formation in diabetic erythrocytes †

Deficiencies in the reducing power of diabetic erythrocytes can be studied using the sulphone metabolite, monoacetyl dapsone hydroxylamine (MADDS-NHOH) to generate methaemoglobin, which is a form of oxidative stress. The effects of three antioxidants, dyhydrolipoic acid (DHLA), lipoic acid and ascorbate were compared using this method in erythrocytes of type 1 diabetic patients. During 2-hour incubations, DHLA and lipoic acid significantly reduced MADDS-NHOH-mediated methaemoglobin formation to the same extent in diabetic and non-diabetic erythrocytes. In contrast, ascorbate markedly increased MADDS-NHOH-mediated methaemoglobin formation at all time points up to 2 hours for diabetic cells and 15—60 minutes in non-diabetic erythrocytes. Ascorbate abolished the deficiency in methaemoglobin sensitivity normally seen in diabetic erythrocytes compared with non-diabetic erythrocytes. During a 1-hour pre-incubation period, neither DHLA, lipoic acid nor ascorbate significantly altered total thiol concentrations. After a 2-hour incubation with MADDS-NHOH, thiol concentrations did not significantly change in the non-diabetic erythrocytes, but they fell significantly in the diabetic cells. Diabetic and non-diabetic erythrocytes pre-incubated with ascorbate, DHLA and lipoic acid showed no changes in thiol levels in the presence of MADDS-NHOH. In summary, ascorbate, DHLA and lipoic acid showed contrasting effects on methaemoglobin generation, although they each abolished the diabetic erythrocytic deficit in total thiol status caused by hydroxylamine-mediated methaemoglobin formation. This work provides evidence for the potential future use of antioxidant supplements in diabetic management.

Therapeutic potential of some plant extracts used in Turkish traditional medicine on streptozocin-induced type 1 diabetes mellitus in rats

Diabetes mellitus (DM) is known to impair many physiological functions. Some reports claim that medicinal plants can reduce these alterations caused by DM. The aim of this study was to investigate the therapeutic potential of aqueous-methanol extracts of Urtica dioica, Thymus vulgaris (TV), Myrtus communis (MC), Scolymus hispanicus (SH) and Cinnamomun zeylanicum (CZ) on streptozotocin (STZ)-induced type 1 DM in rats. Diabetes was induced via a single i.p. injection of STZ (65 mg/kg body weight). After 1 week to allow for development of diabetes, each plant extract was administered to diabetic rats separately at a dose of 100 mg/kg body weight daily for 28 days. The results showed that only SH extract significantly (P < 0.05) amended fasting blood glucose level. The lipid profile was ameliorated especially by supplementations of TV, MC and CZ extracts. Almost all plant extract treatments markedly (P < 0.05) increased reduced glutathione content and decreased lipid peroxidation levels of erythrocyte, plasma, retina and lens tissues. They also significantly (P < 0.05) amended erythrocyte catalase activity, levels of marker serum enzymes (except amylase), urea and blood urea nitrogen when compared to diabetic rats treated with nothing. Furthermore, none of the plant extracts counteracted body weight loss of diabetic rats. Our data revealed that the aforementioned plant extracts have remarkable potential to counteract DM-caused alterations, probably through their antioxidant and free radical-defusing effects.

Antioxidant and pancreas-protective effect of red mold fermented products on streptozotocin-induced diabetic rats

BACKGROUND

Red mold fermented products contain various antioxidants, and were mentioned in an ancient Chinese pharmacopoeia of medicinal food and herbs. They were considered useful for the treatment of diabetes. The objective of this study was to determine the antioxidant and pancreas-protective function of red mold fermented products on streptozotocin (STZ)-induced diabetic rats.

RESULTS

Throughout the experimental period (8 weeks), significantly (P < 0.05) lowered plasma glucose, amylase, triglyceride and cholesterol levels were observed in groups treated with red mold fermented products. Diabetic rats showed higher reactive oxygen species (P < 0.05) and lower activities of glutathione disulfide reductase, glutathione reductase and catalase (P < 0.05) in pancreas as compared to normal rats and those treated with red mold fermented products. There were some necrotic cells observed in the pancreas of diabetic rats and pancreatic tissue damaged by STZ in the red mold fermented products supplemented groups was ameliorated.

CONCLUSION

These results indicate that red mold fermented products not only regulate hyperglycemia but also provide prevention against the effects of STZ-induced pancreatic damage.

Acute diphenyl diselenide treatment reduces hyperglycemia but does not change delta-aminolevulinate dehydratase activity in alloxan-induced diabetes in rats

This study was designed to evaluate the effect of diphenyl diselenide in a classical model of alloxan-induced diabetes in rats. Oxidative stress is involved in alloxan toxic effects and we have hypothesized that diphenyl diselenide via its antioxidant properties could confer protection against alloxan pancreatic toxicity. Diabetes was induced by administration of alloxan (150 mg/kg, intravenously). Diphenyl diselenide (10 mg/kg, subcutaneously) was administered for 6 d before (prevention group) or for 6 d after (remediation group) diabetes induction. Diphenyl diselenide treatment reduced the blood glucose and fructosamine levels, which were increased in alloxan-treated rats. However, the delta-aminolevulinate dehydratase (delta-ALA-D) activity inhibited by alloxan was not restored by diphenyl diselenide. Moreover, diphenyl diselenide caused by itself an inhibition in hepatic and renal delta-ALA-D activity. Our findings suggest that the acute treatment with diphenyl diselenide reduces the hyperglycemia but does not improve delta-ALA-D activity decreased by alloxan. Although the dose of diphenyl diselenide used here for treating diabetic animals has been relatively high and produced toxic effects, the compound or analogous molecules might not be rejected as a promising anti-hyperglycemic agent.

Uptake and reduction of alpha-lipoic acid by human erythrocytes

OBJECTIVES

The reducing capacity of erythrocytes has been used clinically as to estimate resistance to oxidant stress. In this work we targeted the antioxidant capacity of pyridine nucleotide disulfide reductases of these cells by measuring their ability to reduce the disulfide alpha-lipoic acid.

METHODS

Erythrocyte reduction of alpha-lipoic acid and related disulfides was measured as reduction of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) outside the cells.

RESULTS

Lipoic acid-dependent DTNB reduction by human erythrocytes required d-glucose and consumed NADPH, but not NADH. This activity was inhibited by carmustine and phenylarsine oxide, as expected if alpha-lipoic acid is reduced by the glutathione and thioredoxin reductase systems. Reduction of hydroxyethyl disulfide, which provides an estimate of total erythrocyte disulfide reduction capacity, was similar to that of alpha-lipoic acid. Erythrocytes incubated with alpha-lipoic acid also reduced extracellular ferricyanide, although rates of dehydroascorbate reduction were several-fold greater, probably because intracellular GSH can recycle ascorbate but not alpha-lipoic acid in erythrocytes.

CONCLUSION

These results show that alpha-lipoic acid-dependent DTNB reduction provides a simple method to selectively assess the capacity of pyridine nucleotide disulfide reductases of human erythrocytes. When coupled with other non-destructive assays, such as reduction of hydroxyethyl disulfide and ferricyanide, this assay provides a comprehensive approach to assessing erythrocyte reducing capacity in a variety of clinical conditions associated with oxidant stress.

Treating insulin resistance: future prospects

Insulin resistance typically reflects multiple defects of insulin receptor and post-receptor signalling that impair a diverse range of metabolic and vascular actions. Many potential intervention targets and compounds with therapeutic activity have been described. Proof of principle for a non-peptide insulin mimetic has been demonstrated by specific activation of the intracellular B-subunit of the insulin receptor. Potentiation of insulin action has been achieved with agents that enhance phosphorylation and prolong the tyrosine kinase activity of the insulin receptor and its protein substrates after activation by insulin. These include inhibitors of phosphatases and serine kinases that normally prevent or terminate tyrosine kinase signalling. Additional approaches involve increasing the activity of phosphatidylinositol 3-kinase and other downstream components of the insulin signalling pathways. Experimental interventions to remove signalling defects caused by cytokines, certain adipocyte hormones, excess fatty acids, glucotoxicity and negative feedback by distal signalling steps have also indicated therapeutic possibilities. Several hormones, metabolic enzymes, minerals, co-factors and transcription co-activators have shown insulin-sensitising potential. Since insulin resistance affects many metabolic and cardiovascular diseases, it provides an opportunity for simultaneous therapeutic attack on a broad front.

A preliminary evaluation of a novel method to monitor a triple antioxidant combination (vitamins E, C and α-lipoic acid) in diabetic volunteers using in vitro methaemoglobin formation

Eight otherwise healthy diabetic volunteers took a daily antioxidant supplement consisting of vitamin E (200 IU), vitamin C (250 mg) and α-lipoic acid (90 mg) for a period of 6 weeks. Diabetic dapsone hydroxylamine-mediated methaemoglobin formation and resistance to erythrocytic thiol depletion was compared with age and sex-matched non-diabetic subjects. At time zero, methaemoglobin formation in the non-diabetic subjects was greater at all four time points compared with that of the diabetic subjects. Resistance to glutathione depletion was initially greater in non-diabetic compared with diabetic samples. Half-way through the study (3 weeks), there were no differences between the two groups in methaemoglobin formation and thiol depletion in the diabetic samples was now lower than the non-diabetic samples at 10 and 20 min. At 6 weeks, diabetic erythrocytic thiol levels remained greater than those of non-diabetics. HbA(1c) values were significantly reduced in the diabetic subjects at 6 weeks compared with time zero values. At 10 weeks, 4 weeks after the end of supplementation, the diabetic HbA1(c) values significantly increased to the point where they were not significantly different from the time zero values. Total antioxidant status measurement (TAS) indicated that diabetic plasma antioxidant capacity was significantly improved during antioxidant supplementation. Conversion of α-lipoic acid to dihydrolipoic acid (DHLA) in vivo led to potent interference in a standard fructosamine assay kit, negating its use in this study. This report suggests that triple antioxidant therapy in diabetic volunteers attenuates the in vitro experimental oxidative stress of methaemoglobin formation and reduces haemoglobin glycation in vivo.

The use of xenobiotic-mediated methaemoglobin formation to assess the effects of thyroid hormones on diabetic and non-diabetic human erythrocytic oxidant defence mechanisms in vitro

Diabetes is associated with an abnormal incidence of hypothyroidism, which exacerbates hyperglycaemia, so further damaging already compromised erythrocytic defence mechanisms. Methaemoglobin formation is a useful measure of the health of these mechanisms, as it determines the resistance of diabetic erythrocytes to sustained oxidative stress. The effect of l-tri-iodothyronine (T(3)) was, therefore, studied on nitrite and monoacetyldapsone hydroxylamine (MADDS-NHOH) mediated methaemoglobin formation in diabetic and non-diabetic human erythrocytes. Diabetic erythrocytes showed less sensitivity compared with non-diabetics to methaemoglobin formation mediated by both compounds. A 30 min pre-incubation with T(3) at 3 and 30 nM did not affect nitrite-mediated methaemoglobin formation compared with control observations in both cell types. In diabetic erythrocytes incubated with T(3) at 30 nM, there were significant increases in MADDS-NHOH-mediated methaemoglobin formation compared with control in both diabetic and non-diabetic cells. Studies comparing blood isolated from diabetic patients stabilised on thyroxine (T(4); 50 μG/day), T(4)-free diabetics and non-diabetics, showed that T(4) supplementation significantly increased MADDS-NHOH-mediated methaemoglobin formation compared with T(4)-free diabetic cells so that for two time points, T(4)-treated diabetic erythrocytic methaemoglobin formation was indistinguishable from that of non-diabetics. These studies indicate that T(4) supplementation improves some erythrocytic oxidant defence mechanisms in a time dependent manner.