myo-Inositol and sorbitol in erythrocytes from diabetic patients before and after sorbinil treatment

A sensitive assay of red blood cell sorbitol level by high performance liquid chromatography: potential for diagnostic evaluation of diabetes

BACKGROUND

The level of sorbitol within erythrocyte or other tissue is increased during diabetic complications such as neuropathy, retinopathy, and nephropathy and is thus considered as an indicator for the diagnosis and monitoring the diabetic status. A highly sensitive method of estimation of RBC sorbitol may be very useful in determining the state of diabetes. In this study a high performance liquid chromatography method was developed to analyze RBC sorbitol level.

METHOD

Benzoylation was used for the precolumn derivatization of RBC polyol prior to analyze by HPLC using C-18 reversed phase column and UV detector.

RESULTS

The sensitivity was 5 ng/ml using standard sorbitol. The linearity was highly satisfactory up to 2.5 mug/ml which was adequate to measure RBC sorbitol in both diabetic and normal subjects. The values were significantly higher for the diabetic patient compared to normal. Within-run assay CVs were <10% and <14% for between-run.

CONCLUSION

This method could be useful in estimating RBC sorbitol content for the clinical evaluation of diabetic patients.

Diagnostic performance of red cell sorbitol assay in a Nigerian teaching hospital

The analytical linearity, detection limit, precision and accuracy, as well as diagnostic sensitivity, specificity and predictive values of red cell sorbitol (RCS) assay were evaluated using the fluorimetric enzymatic technique with the available laboratory facilities in a Nigerian Teaching Hospital. The assay was linear up to 40.0 nmol/ml. The detection limit was 0.55 nmol/ml. The within-assay coefficients of variation (CV) from low to high concentrations were in the range of 2.82% to 5.48% while the corresponding values for the between-assay variation were 3.60% to 6.05%. The mean recovery was 90.4%. High concentrations of some common blood constituents and drugs did not cause significant interference. An evaluation of the diagnostic sensitivity and specificity of RCS in 102 out-patients revealed a sensitivity of 94.4%, specificity of 98.8% and efficiency of 98.0%. The predictive value of a positive test was 94.4% while that of a negative test was 98.8%. The mean value of red cell sorbitol in patients confirmed to have diabetes mellitus (23.1 +/- 1.13 SE nmol/g Hb) was significantly higher (P < 0.001) than the corresponding value (11.0 +/- 0.0.27 SE nmol/g Hb) for those without diabetes and for control subject. A similar observation was also made for plasma glucose and fructosamine. These observations demonstrate that RCS assay could be adopted in a third world laboratory for clinical applications.

The efficacy of aldose reductase inhibitors in the management of diabetic complications. Comparison with intensive insulin treatment and pancreatic transplantation

Recently, aldose reductase inhibitors (ARIs) have been registered in several countries for the improvement of glycaemic control. However, their efficacy is still controversial. ARIs inhibit the enhanced flux of glucose through the polyol pathway. As such, they can never be more effective than normoglycaemia, and so their potential benefits and limitations should be considered relative to the effects of prolonged euglycaemia. The clinical effects of ARIs can be put into perspective by assessing the effects of improved glycaemic control attained in randomised trials of intensive insulin treatment [such as the Diabetes Control and Complications Trial (DCCT)] and after pancreatic transplantation. Although direct comparison of these 3 interventions is hampered by differences in patient populations, duration and methods of follow-up and in the potency of ARIs, the effects of these 3 metabolic interventions and their course in time appear remarkably similar. For neuropathy, all 3 interventions induce an increase in average motor nerve conduction velocity of approximately 1 m/sec during the first months of treatment. At the same time, improvement of painful symptoms may occur. These changes probably largely represent a metabolic amelioration of the condition of the nerves. Around the second year of treatment with all 3 forms of metabolic improvement, an acceleration of nerve conduction of a similar magnitude occurs, with signs of structural nerve regeneration and some sensory recuperation. Experience with ARIs in nephropathy is still limited, but similar improvements in glomerular filtration rate and, less consistently, in urinary albumin excretion were found during short term normoglycaemia produced by all 3 forms of treatment. Comparison of a small number of studies, however, shows differences between intensive insulin regimens, pancreatic transplantation and ARIs in effects on retinopathy. Retinopathy often temporarily deteriorates in the early phases of improved glycaemic control, but this is not noted with ARIs. New microaneurysm formation was slightly reduced in a single long term study with the ARI sorbinil, but the preventive effects on the overall levels of retinopathy seemed less strong than in normoglycaemia trials of similar duration. However, the pharmacodynamic effects on inhibiting the polyol pathway differ among ARIs, and the half-life of the inhibiting effect of sorbinil may have been too short for a complete reduction of polyol pathway activity. The trials of prolonged intensive insulin therapy and pancreatic transplantation have demonstrated that very strict metabolic control must be maintained continuously for many years before a significant reduction of complications can be demonstrated.(ABSTRACT TRUNCATED AT 400 WORDS)

Red blood cell sorbitol in diabetic children

When red blood cells (RBC) were incubated with various concentrations of glucose, the RBC sorbitol level increased in a concentration-dependent manner. The elevated RBC sorbitol level was not reduced by further incubation in a glucose-free medium. In both diabetic and non-diabetic children, an increase of RBC sorbitol levels occurred in the oral glucose tolerance test and the return to baseline was delayed in diabetics compared with non-diabetics. In the majority of diabetics (87%), RBC sorbitol levels exceeded the upper limit of the normal range, which was arbitrarily determined as the mean + 2 s.d., in healthy non-diabetic children and adults. A good correlation was observed between RBC sorbitol levels and plasma glucose levels (r = 0.644). In both diabetics and non-diabetics, no correlation was observed between RBC sorbitol levels and age, and in diabetics the RBC sorbitol level was not related to the duration of disease. A good correlation was observed between RBC sorbitol levels, and hemoglobin Alc (HbAlc) or fructosamine levels in diabetic children.

The effect of aldose reductase inhibition on erythrocyte polyols and galactitol accumulation in diabetic patients

Erythrocyte sorbitol level has previously been used as a measure of the efficacy of aldose reductase inhibitors, but its value is limited by fluctuations related to variations in blood glucose concentration. The aim of the study was to compare sorbitol content with the ability to accumulate galactitol during ex vivo incubation with galactose, of erythrocytes taken from diabetic patients following administration of a single 600 mg dose of the aldose reductase inhibitor, ponalrestat. Twelve patients were studied in a placebo-controlled crossover trial. Blood glucose levels were not statistically different during the placebo and ponalrestat treatment periods except at 1 h after the dose was taken (10.6 +/- 6.7 vs 7.7 +/- 4.6 mmol l-1 (+/- SD), p less than 0.05). Ponalrestat reduced erythrocyte sorbitol concentrations compared with placebo at 3, 5 and 7 h (0.82 +/- 0.36, 0.69 +/- 0.23, and 0.83 +/- 0.35 mg l-1 vs 1.79 +/- 0.67, 1.68 +/- 0.65, and 1.57 +/- 0.59 mg l-1 respectively, p less than 0.005) and 24 h post-dose (1.57 + 0.45 vs 2.01 + 0.73 mg l-1, p less than 0.05). Ponalrestat also reduced erythrocyte galactitol accumulation at 3, 5 and 24 h post-dose from 5.53 +/- 2.41, 5.43 +/- 1.89, and 5.42 +/- 1.96 mg l-1 2-h-1 to 1.47 +/- 0.30, 1.76 +/- 0.41, and 4.12 +/- 0.72 mg l-1 2-h-1 respectively, p less than 0.01. Galactitol accumulation rate appeared to be a less variable parameter than erythrocyte sorbitol and was not influenced by fluctuations in blood glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldose reductase inhibition in diabetic neuropathy: clinical and neurophysiological studies of one year's treatment with sorbinil

In a double-blind placebo-controlled trial the effect of Sorbinil (250 mg daily) on diabetic neuropathy was examined. After a 2-month run-in placebo period (with three major assessments) 21 patients were randomized to Sorbinil and 10 to placebo, and all were studied for a further 12 months with neurophysiological measurements at 3-month intervals of nerve conduction velocity in multiple nerves, autonomic function tests, vibration thresholds as well as clinical examination and an extensive self-assessment of symptoms. Two subjects on Sorbinil treatment developed a hypersensitivity reaction and were withdrawn. Metabolic control and severity of neuropathy was not significantly different between groups. There were no changes in symptoms as judged by self-assessment scores. No patient entered the trial with neuropathic ulcers but ulceration developed in 4 patients during Sorbinil treatment and in 1 of the placebo group. No beneficial effect of Sorbinil was demonstrated on either the clinical manifestation or on the neurophysiological measurements made in these neuropathic diabetic patients over 12 months of treatment.

Peripheral nerve function in relation to quality of metabolic control in diabetes

It has been proposed that increased polyol pathway activity resulting from hyperglycaemia induces neuropathy in diabetic patients. Recently, it has been demonstrated that erythrocyte sorbitol content is a reflection of polyol pathway activity in nerve. In the present study the relationship of erythrocyte sorbitol and glycosylated haemoglobin (HbA1) levels to peripheral nerve functions were investigated in 62 diabetic patients. Peroneal and median motor nerve conduction velocities, H-M intervals of the Hoffmann reflex, thermal discrimination thresholds and vibratory perception thresholds, either singly or in combination, were not significantly correlated with erythrocyte sorbitol or HbA1 levels. These findings show that a single measurement of erythrocyte sorbitol or HbA1 content cannot predict impairment of nerve function in diabetic subjects.

Comparison of aldose reductase inhibitors in vitro. Effects of enzyme purification and substrate type

Aldose reductase (EC 1.1.1.21) was purified approximately 5000-fold from bovine lens by ammonium sulphate fractionation and chromatography on DEAE-Sephacel and Matrex OA. Inhibition of this enzyme was found to depend upon the assay substrate. Tested against the purest form of enzyme, the inhibitor Sorbinil gave IC50 values of approximately 100 microM with the model substrate 4-nitrobenzaldehyde (4NB) and 0.4-1.4 microM with the physiological substrate glucose. A similar effect of substrate was found for the inhibitor Statil (IC50 450-750 nM with 4NB, 26-71 nM with glucose substrate). The implications of these results towards the assessment of aldose reductase inhibitors in vitro are discussed.

Red cell sorbitol levels during hyper and hypoglycaemic clamp studies in insulin-dependent diabetic patients

The red cell sorbitol concentration has been suggested as a measure of polyol pathway activity. Red cell sorbitol levels were higher in 53 patients having insulin-dependent diabetes mellitus (IDDM) than in 16 control subjects. Six patients having IDDM underwent hyperglycaemic 'clamp' studies; the red cell sorbitol level returned to the normal range when the blood glucose was clamped at 5 mmol/l for 1 h and rapidly increased when it was clamped at 15 and 25 mmol/l for a further hour at each level. Seven patients with IDDM were rendered hypoglycaemic; red cell sorbitol levels rapidly fell to a level less than, but not significantly different from normal. The results of these studies suggest that in IDDM red cell sorbitol levels are a reflection of prevailing blood glucose concentration and do not indicate long-term sorbitol accumulation in other tissues.

Inhibition of aldose reductase from human retina

Aldose reductase was prepared from a pool of 21 male and 16 female human retinas by ammonium sulphate fractionation (40-75% saturation) and chromatography on DEAE-Sephacel and Matrex-OA. The overall purification was 132-fold with 50% recovery of enzyme activity. The concentrations of the aldose reductase inhibitors Sorbinil, Statil and M79175 required to give 50% inhibition (IC50 value) of enzyme activity with the model substrate 4-nitrobenzaldehyde (4NB) were 3.4 microM, 2.3 microM and 0.22 microM respectively. This indicated that M79175 was the most effective inhibitor tested of aldose reductase with 4NB in vitro. These inhibitors were more effective when tested against aldose reductase activity with glucose, the substrate which might play a role in the pathogenesis of diabetic complications. Sorbinil gave an IC50 (glucose) of 0.40 microM; M79175 and Statil were more effective. At an inhibitor concentration of 0.1 microM the %-inhibitions observed were: Sorbinil 20% M79175 55%, Statil 76%. Thus Statil was the most potent compound tested against human retinal enzyme using the more physiological substrate in vitro. This report provides the first direct evidence that human retinal aldose reductase is susceptible to inhibition by compounds designed for chemotherapy of diabetic complications, and indicates that the concentrations of inhibitor required for a substantial block of activity in vitro are lower than those attained in plasma in man.

Inhibition of hexonate dehydrogenase and aldose reductase from bovine retina by sorbinil, statil, M79175 and valproate

Aldose reductase inhibitors (A.R.I.s), developed as potentially therapeutic agents for the treatment of complications of long-term diabetes, were found to be potent inhibitors of aldose reductase (ALR2) partially purified from bovine retina (IC50 values: Statil 0.89 microM, Sorbinil 2 microM, M79175 greater than 1 microM). These compounds varied, however, in their ability to inhibit hexonate dehydrogenase (ALR1), a closely related enzyme isolated from the same source (IC50 values: Statil greater than 1 microM, Sorbinil 3.9 microM, M79175 0.18 microM). Statil and Sorbinil were active against ALR2 at very low concentrations (approx. 5% inhibition at 100 pM), but did not inhibit ALR1 at less than or equal to 10 nM. In contrast, M79175 (structurally very similar to Sorbinil) and M7HEQ (a flavonoid) were preferential inhibitors of ALR1. Valproate, a compound of value in the treatment of epilepsies, was a poor inhibitor of ALR2 (18% at 1 mM). Furthermore, valproate was found to be a relatively poor inhibitor of ALR1, particularly in comparison with M79175.

Applicability of red blood cell sorbitol measurements to monitor the clinical activity of sorbinil

Increased flux of glucose through the polyol pathway with resultant resultant accumulation of tissue sorbitol is implicated in the pathogenesis of the chronic complications of diabetes. Sorbinil is a potent inhibitor of aldose reductase (the first enzyme in the polyol pathway) and has been shown to normalize sorbitol levels in relevant tissues (eg, nerve, kidney, lens) of experimentally-induced diabetic animals. The purpose of this study was to identify a relatively noninvasive method of monitoring the intrinsic (or biochemical) efficacy of sorbinil in diabetic man. Specifically, the objective was to identify a readily accessible tissue that would be reflective of polyol pathway activity and the activity of sorbinil in clinically relevant but less accessible tissues. Based on several previous studies, which demonstrated that sorbitol accumulation in human red blood cells (RBCs) was a function of ambient glucose concentrations, either in vitro or in vivo, our investigations were conducted to determine if RBC sorbitol accumulation would correlate with sorbitol accumulation in lens and nerve tissue of diabetic rats; the effect of sorbinil in reducing sorbitol levels in lens and nerve tissue of diabetic rats would be reflected by changes in RBC sorbitol; and sorbinil would reduce RBC sorbitol in diabetic man.