Effects of the antioxidants dihydrolipoic acid (DHLA) and probucol on xenobiotic-mediated methaemoglobin formation in diabetic and non-diabetic human erythrocytes in vitro(1)

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.

Effects of Lipoic Acid and Dihydrolipoic Acid on 4-Aminophenol-Mediated Erythrocytic Toxicity in vitro

The effects of the antioxidant lipoic acid and its reduced form, dihydrolipoic acid (DHLA), were studied on the process of the erythrocytic toxicity of 4-aminophenol in human erythrocytes in vitro. 4-Aminophenol alone caused a stepwise increase in methaemoglobin formation, along with a commensurate decrease in total thiols. At 10 min., in the presence of lipoic acid alone and the thiol depletor 1-chloro-2,4-dinitrobenzene (CDNB) alone, 4-aminophenol-mediated methaemoglobin formation was significantly increased, whilst thiol levels were significantly reduced compared with the 4-aminophenol alone. At 10 min., with DHLA and CDNB alone, 4-aminophenol was associated with significantly increased methaemoglobin formation. However, thiol levels were not significantly different in the presence of DHLA compared with 4-aminophenol alone, although thiol levels were different compared with control (4-aminophenol alone) in the incubations with CDNB alone. At 15 min., only CDNB/4-aminophenol methaemoglobin formation differed from control, whilst thiol levels were significantly lower in the presence of CDNB alone compared with 4-aminophenol alone. Lipoic acid enhanced the toxicity of 4-aminophenol in terms of increased methaemoglobin formation coupled with increased thiol depletion, whilst DHLA showed increased 4-aminophenol-mediated methaemoglobin formation without thiol depletion. Lipoic acid, and to a lesser extent its reduced derivative DHLA, acted as a prooxidant in the presence of 4-aminophenol, enhancing the oxidative stress effects of the amine in human erythrocytes.

Effects of dihydrolipoic acid (DHLA), α-lipoic acid. N-acetyl cysteine and ascorbate on xenobiotic-mediated methaemoglobin formation in human erythrocytes in vitro

α-Lipoic acid, dihydrolipoic acid (DHLA), N-acetyl cysteine and ascorbate were compared with methylene blue for their ability to attenuate and/or reduce methaemoglobin formation induced by sodium nitrite, 4-aminophenol and dapsone hydroxylamine in human erythrocytes. Neither α-lipoic acid, DHLA, N-acetyl cysteine nor ascorbate had any significant effects on methaemoglobin formed by nitrite, either from pre-treatment, simultaneous addition or post 30 min addition of the agents up to the 60 min time point, although N-acetyl cysteine did reduce methaemoglobin formation at 120 min (P<0.05). In all three treatment groups at 30, 60 and 120 min, there were no significant effects mediated by DHLA or N-acetyl cysteine on 4-aminophenol (1 mM)-mediated haemoglobin oxidation. Ascorbate caused marked significant reductions in 4-aminophenol methaemoglobin in all treatment groups at 30-120 min except at 30 min in the simultaneous addition group (P<0.0001). Neither α-lipoic acid, nor N-acetyl cysteine showed any effects on hydroxylamine-mediated methaemoglobin formation at 30 and 60 in all treatment groups. In contrast, DHLA significantly reduced hydroxylamine-mediated methaemoglobin formation at all three time points after pre-incubation and simultaneous addition (P<0.001), while ascorbate was ineffective. Compared with methylene blue, which was effective in reducing methaemoglobin formation by all three toxins (P<0.01), ascorbate was only highly effective against 4-aminophenol mediated methaemoglobin, whilst the DHLA-mediated attenuation of dapsone hydroxylamine-mediated methaemoglobin formation indicates a possible clinical application in high-dose dapsone therapy.

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.

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

Diabetes leads to premature organ and system failure and considerably shortens lifespan. Careful control of glucose levels may not be enough to prevent the onset of complications in most diabetics. Compared with non-diabetics, diabetic tissues must not only resist a much greater long-term threat from hyperglycaemia-mediated reactive species but also defend themselves with compromised antioxidant systems. Although antioxidant therapy is a logical step in the prevention of oxidant and carbonyl stresses in the face of intermittent hyperglycaemia, this approach is not yet universally accepted to be effective in preventing complications. Although there are many biochemical indices of oxidant stress, piecemeal elevations of individual markers may not necessarily reflect true diabetic cellular antioxidant status. A dynamic process such as in vitro methaemoglobin generation may provide an opportunity to compare the response of a diabetic erythrocyte with that of a non-diabetic before and after corrective antioxidant therapy. Due to compromised cellular antioxidant capacity, diabetic cells generate less methaemoglobin in the presence of aromatic amine hydroxylamines, 4-aminophenol and nitrite compared with non-diabetics. Agents such as dihydrolipoic acid have been shown to correct methaemoglobin formation-mediated thiol deficits during in vitro studies. It is hoped that the progress of antioxidant supplementation studies in diabetics can be monitored with the aid of in vitro methaemoglobin generation using agents such as hydroxylamines, 4-aminophenol and nitrite. The most appropriate antioxidants and dosages can thus be recommended to diabetics worldwide to attenuate the development of complications.

The therapeutic use of lipoic acid in diabetes: a current perspective

Lipoic acid and its reduced derivative, dihydrolipoic acid (DHLA) are highly promising antioxidant agents, which are potent attenuators of reactive species-mediated damage in vitro and in animal studies. Lipoic acid is a universal antioxidant, effective in lipophilic and aqueous environments. In contrast to an equivalent endogenous agent, such as oxidised glutathione (GSSG), lipoic acid acts as an antioxidant in its oxidised form. Lipoic acid has been evaluated in diabetic polyneuropathy, a condition which is thought to result in part from oxidant damage caused by long-term hyperglycaemia. Diabetic patients are prone to incur enhanced cellular free radical formation and reduced antioxidant defence. Treatment with lipoic acid has improved nerve conduction velocity during studies in diabetic animals. Trials in diabetic patients have often observed some relief of neuropathic symptoms during treatment with lipoic acid, but consistent objective benefits have been difficult to establish. Lipoic acid is now used in Germany for the treatment of diabetic neuropathy and definitive evidence of efficacy should arise from postmarketing surveillance studies. It is possible that lipoic acid may be more effective as a long-term dietary supplement aimed at the prophylactic protection of diabetics from complications.