No correlation between glycemic control and an increase in erythrocyte aldose reductase activity in type I and type II diabetic patients.

Aldose reductase, the first enzyme of the polyol pathway, has been related to the pathogenesis of diabetic complications. The regulation of the enzyme in diabetes patients, however, has not yet been clarified. We recently reported that the activity of aldose reductase was increased in erythrocytes of insulin-dependent diabetes mellitus patients but short-term hyperglycemia did not affect the enzyme activity. It is still unclear, however, whether or not the increase in the enzyme activity is caused by long-term hyperglycemia and thus would be seen equally in both type I (insulin-dependent diabetes mellitus) and type 2 (non-insulin-dependent diabetes mellitus) individuals. To further clarify these issues we measured erythrocyte aldose reductase activity in 46 type I patients and 30 type II patients who had variable glucose control and in 16 nondiabetic subjects. We compared the enzyme activity with plasma glucose levels and hemoglobin A1c levels. The results show that erythrocyte aldose reductase activity is increased in both type I and type II patients as compared with nondiabetic subjects (7.1 +/- 0.3 U/L and 6.8 +/- 0.4 U/L erythrocytes versus 5.6 +/- 0.2 U/L erythrocytes, p less than 0.001 and p less than 0.01, respectively), but there were no significant differences between the two groups of diabetic patients. The enzyme activity varied by approximately four times among the diabetic individuals but there was no correlation between the enzyme activity and plasma glucose or hemoglobin A1c levels. We conclude that the increased activity of erythrocyte aldose reductase seen in diabetes is not related to hyperglycemia.     

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)     

Erythrocytic sorbitol contents in diabetic patients correlate with blood aldose reductase protein contents and plasma glucose levels, and are normalized by the potent aldose reductase inhibitor fidarestat (SNK-860)

The accumulation of sorbitol by the activated polyol pathway is considered to be a major cause of diabetic neuropathy. Because the erythrocytic sorbitol contents reportedly reflects that in nerves, erythrocytic sorbitol measurement would be useful for confirming the effect of an aldose reductase inhibitor (ARI). In this study, we examined erythrocytic sorbitol contents in healthy subjects and diabetic patients under fasting and postprandial conditions. Then, the contributions of blood aldose reductase (AR) contents and plasma glucose levels to the accumulated erythrocytic sorbitol contents were also analyzed. Erythrocytic sorbitol contents in the healthy subjects were 11.7 and 12.5–12.6 nmol/g Hb in fasting and postprandial status, respectively. In contrast, the erythrocytic sorbitol contents in diabetic patients were apparently higher (approximately 2.5-fold), but fidarestat treatment restored the elevated erythrocytic sorbitol contents to normal. In the diabetic patients, erythrocytic sorbitol contents were highly correlated with blood AR contents multiplied by the plasma glucose levels, whereas in the normal and fidarestat-treated diabetic patients no such correlation was observed. Taken together, these results suggest both the blood AR contents and the plasma glucose levels are factors determining erythrocytic sorbitol contents in diabetic patients. Notably, the potent ARI fidarestat was shown to normalize elevated erythrocytic sorbitol contents.     

Activation of erythrocyte aldose reductase in man in response to glycaemic challenge.

Flux via the polyol pathway, which comprises the enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH), has been implicated in the debilitating complications of diabetes. Previous studies in this laboratory have indicated that erythrocyte AR activities are increased (by 72%) in insulin-dependent diabetic patients. To investigate the mechanism underlying this activation, the response of AR activity to oral glucose challenge was investigated in eight overnight-fasted human volunteers. Glucose consumption led to a transient activation (by 76%: P less than 0.01) of erythrocyte AR, which paralleled the rise and subsequent fall in blood glucose concentrations. It is concluded that erythrocyte AR activity is acutely modulated in response to hyperglycaemia by an as yet unknown mechanism.     

High levels of erythrocyte aldose reductase and diabetic retinopathy in NIDDM patients

Summary Erythrocyte aldose reductase was determined in 90 NIDDM patients by a two-site ELISA using recombinant human aldose reductase. The level of aldose reductase did not correlate with age, duration of diabetes, fasting blood glucose and HbA_1cof the patients. Among 38 patients with diabetes for more than 10 years, aldose reductase in those with retinopathy (including non-proliferative and proliferative) was significantly higher than in those without, while no difference in the means of the average HbA_1c, maximum and minimum blood pressure levels was observed between the two groups. The results indicate that the level of aldose reductase in the erythrocyte of diabetic patients is associated with the presence of retinopathy.Abbreviations NIDDM Non-insulin-dependent diabetes mellitus - IDDM insulin-dependent diabetes mellitus     

Inhibition of aldose reductase in human erythrocytes by vitamin C

Ascorbic acid, or vitamin C, has been reported to lower erythrocyte sorbitol concentrations, and present studies were performed to determine the mechanism of this effect. Incubation of erythrocytes with increasing concentrations of glucose (5-40 mM) progressively increased erythrocyte sorbitol contents, reflecting increased flux through aldose reductase. At extracellular concentrations of 90 microM, both ascorbic acid and its oxidized form, dehydroascorbate, decreased intracellular sorbitol by 25 and 45%, respectively. This inhibition was not dependent on the extracellular glucose concentration, or on erythrocyte contents of free NADPH or GSH. To test for a direct effect of ascorbate on aldose reductase, erythrocyte hemolysates were prepared and supplemented with 100 microM NADPH. Hemolysates reduced glucose to sorbitol in a dose-dependent manner that was inhibited with a Ki of 120 microM by the aldose reductase inhibitor tetramethylene glutaric acid. Above 100 microM, ascorbic acid also lowered hemolysate sorbitol generation by about 30%. Studies with ascorbic acid derivatives showed that the reducing capacity of ascorbic acid was not required for inhibition of sorbitol production from glucose in erythrocyte hemolysates. These results show that high, but physiologic, concentrations of ascorbic acid can directly inhibit erythrocyte aldose reductase, and provide a rationale for the use of oral vitamin C supplements in diabetes.     

Erythrocyte sorbitol levels in patients with thyrotoxicosis

Hyperglycemia and impaired glucose tolerance are frequently observed in patients with hyperthyroidism. However, little is known about whether altered polyol metabolism in hyperthyroidism is present or not. To examine changes in polyol metabolism in hyperthyroidism, we investigated changes in erythrocyte sorbitol, glyceraldehyde reductase (GAR) and sorbitol dehydrogenase (SDH) activities during hyperthyroid and euthyroid states in patients with thyrotoxicosis. Mean levels of erythrocyte sorbitol and GAR were 32.0 +/- 1.6nM/g.Hb and 147.1 +/- 0.3mU/g.Hb, respectively. In thyrotoxic patients in a hyperthyroid state, these values were significantly higher than those in euthyroid controls. Mean level of erythrocyte SDH in thyrotoxic patients was weak but was significantly increased in comparison with that of euthyroid controls. However, mean levels of erythrocyte sorbitol and GAR were remarkably reduced to 23.6 +/- 1.4nM/g.Hb and 125.3 +/- 4.6mU/g.Hb in thyrotoxic patients in a euthyroid state after treatment with anti-thyroid drugs or by subtotal thyroidectomy. Mean level of SDH, on the other hand, was increased after the treatment. In addition, positive correlations were observed between the level of erythrocyte sorbitol or GAR, and the level of free thyroxine(FT4) or free triiodothyronine(FT3). A negative correlation was observed between the level of erythrocyte SDH and the level of FT4 or FT3. These results suggest that the level of erythrocyte sorbitol may be increased through direct acceleration of erythrocyte GAR activity by increased thyroid hormone levels in patients with thyrotoxicosis.     

Effects of two new aldose reductase inhibitors, AL-1567 and AL-1576, in diabetic rats

Two new potent aldose reductase inhibitors, AL-1567 (DL-spiro(2-fluoro-9H-fluoren-9,4'-imidazolidine)-2',5'-dione) and AL-1576 (spiro-(2,7-difluoro-9H-fluoren-9,4'-imidazolidine)2',5'-dione), have been characterized with respect to in vitro activity toward rat lens and human placental aldose reductase and in vivo activity in uncontrolled, severely diabetic rats dosed acutely with the compounds. The IC50 values for inhibition of rat lens aldose reductase are 2.7 X 10(-8) mol/L for AL-1567 and 8.5 X 10(-9) mol/L for AL-1576; very similar IC50 values were measured for each compound with the human placental enzyme. When the compounds were administered orally once per day to 3-week diabetic rats for a period of eight days, the ED50 values for normalization of lens sorbitol levels were 0.60 mg/kg for AL-1567 and 0.05 mg/kg for AL-1576, and for normalization of sciatic nerve sorbitol levels; 0.22 mg/kg for AL-1567 and 0.04 mg/kg for AL-1576. Compared with published data on other aldose reductase inhibitors evaluated in very similar diabetic rat models, both compounds have unusually high activity in lens, and AL-1576 appears to be the most active such compound in both lens and sciatic nerve reported thus far. The evidence linking increased sorbitol pathway activity to diabetic complications, such as cataract and neuropathy in animal models, suggests that aldose reductase inhibitors will be useful therapeutic agents in human diabetics.     

Neutrophil aldose reductase activity and its association with established diabetic microvascular complications.

Direct investigation of the polyol pathway is rarely possible in studies of human diabetes. A spectrophotometric assay has been developed for the measurement of aldose reductase and sorbitol dehydrogenase activity in the neutrophil. Neutrophil aldose reductase activity was increased in patients with Type 1 diabetes with complications (median 40 (interquartile range 28-48) u, where 1 unit of enzyme activity = nmol NADPH min-1 10(8)-cells-1) compared with those without complications (20 (16-36) u, p less than 0.01) and normal control subjects (20 (8-36) u, p less than 0.01). In Type 2 diabetes, patients with complications also had higher aldose reductase activity (40 (28-52) u) than those without complications (24 (16-36) u, p less than 0.01). There were no differences between patients without complications and normal control subjects. Sorbitol dehydrogenase activity was decreased in diabetic patients (p less than 0.02) but not significantly different between diabetic patients with and without complications.     

In vitro retinal and erythrocyte polyol pathway regulation by hormones and an aldose reductase inhibitor.

The effects of a high-glucose medium, insulin, and an aldose reductase inhibitor (ONO-2235) on sorbitol accumulation were compared in the human erythrocyte and the rabbit retina, while the effects of epinephrine on in vitro sorbitol accumulation were investigated in the human and rabbit retina. In both erythrocytes and the retina, linear increments of sorbitol accumulation were observed in a dose-dependent manner with 5 to 50 mM glucose. These increments were markedly inhibited by 100 microM ONO-2235 but not by insulin (400 microU/ml). In the presence of 5 mM glucose, a dose-dependent increase of the sorbitol content of the rabbit retina was seen following epinephrine stimulation (0.4-4.0 microM and this was markedly reduced by 100 microM ONO-2235. Moreover, both 50 mM glucose and 4.0 microM epinephrine increased the sorbitol content of the retina from a diabetic patient, and the glucose-induced increment in sorbitol was significantly reduced by 100 microM ONO-2235. Our data suggested that aldose reductase inhibitors might be useful for the treatment of diabetic retinopathy, since the polyol pathway appears to be an important factor in its pathogenesis, and that catecholamines might have some role in the activation of the retinal polyol pathway.     

Correlation between erythrocyte aldose reductase activity and the width of skeletal-muscle capillary basement membrane in insulin-dependent diabetes mellitus

Thickening of capillary basement membrane has been demonstrated in diabetic subjects, and it is considered to be the characteristic pathological lesion of diabetic microvascular disease. There are studies reporting the effects of inhibitors of aldose reductase, the first enzyme of the polyol pathway, on the thickening of the capillary basement membrane. These observations indicate a significant role of the polyol pathway in the development of microvascular disease. However, it is unknown whether or not there is any correlation between the thickness of the capillary basement membrane and the activity of aldose reductase in diabetic patients. To clarify this issue, we measured the width of skeletal-muscle basement membrane and erythrocyte aldose reductase activity in 27 insulin-dependent diabetic and 8 nondiabetic individuals. The results showed that both the aldose reductase activity and the width of capillary basement membrane were increased in diabetic patients as compared to nondiabetic individuals (6.89 ± 0.38 versus 5.15 ± 0.60 mL/mU erythrocytes, p < 0.05 and 2257 ± 166 versus 1136 ± 69 Å, p < 0.0001, respectively) (means ± SE), but marked variability was observed in both the enzyme activity and the basement membrane thickness among the diabetic patients. There was a significant correlation between the capillary basement membrane thickness and the activity of erythrocyte aldose reductase (r = 0.51, p < 0.01) in diabetic patients. Our data suggest that the polyol pathway plays an important role in thickening of capillary basement membrane in diabetic individuals, and the variability in aldose reductase activity seen among diabetic patients may result in the varying susceptibility to the development of diabetic microvascular disease.     

Aldose reductase in the etiology of diabetic complications: 2. Nephropathy

The progression of diabetic nephropathy can be arrested by an improvement in diabetic control. High glucose concentrations increase the flux through the aldose reductase pathway, and it has been proposed that this may contribute to renal damage. Aldose reductase is present in both the glomerulus and the renal tubule. Biochemical changes associated with increased sorbitol production have been demonstrated in animal models, including myo-inositol depletion, reduced Na⁺-K⁺ ATPase activity, and activation of the pentose phosphate and glucuronate-xylose pathways. Selective inhibition of aldose reductase reverses these biochemical changes and prevents some of the structural and functional abnormalities in diabetic rats. The potential beneficial effects of aldose reductase inhibitors on diabetic kidney disease in man are at present being investigated.     

Erythrocyte sorbitol level as a predictor of the efficacy of epalrestat treatment for diabetic peripheral polyneuropathy

The relationship between the effect of aldose reductase inhibitors (ARIs) on the activation of the polyol pathway and on diabetic neuropathy has not been fully established. To address this issue, we investigated the effect of epalrestat (150 mg/day), an ARI, on erythrocyte sorbitol levels as an index of polyol activation and on nerve function test results in 43 patients with diabetic peripheral polyneuropathy. After 6 months of epalrestat administration, erythrocyte sorbitol levels did not decrease in patients as a whole. However, a decrease in erythrocyte sorbitol levels during epalrestat administration was significantly correlated with baseline erythrocyte sorbitol levels (ρ=−.47, P<.01): The higher the level at baseline, the greater the decrease after epalrestat treatment. Moreover, the mean sorbitol level during epalrestat treatment was associated with the beneficial effect of epalrestat on vibration sensitivity as measured with a C-128 tuning fork (ρ=−.66, P<.01) and/or a pallesthesiometer TM-31A (ρ=.53, P<.05). On the other hand, erythrocyte sorbitol levels did not reflect the prognosis of nerve conduction velocity. These findings at least partly suggest a causal relationship between polyol activation and the development of diabetic neuropathy. Aldose reductase inhibitor treatment may be clinically useful in the control of polyol activation, especially in patients with excessive accumulation of sorbitol.     

Alteration of urinary sorbitol excretion in WBN-kob diabetic rats - treatment with an aldose reductase inhibitor

An accelerated polyol pathway in diabetes contributes to the development of diabetic complications. To elucidate diabetic nephropathy involving also renal tubular damage, we measured urinary sorbitol concentration concomitantly with urinary N-acetyl-D-glucosaminidase (NAG) excretion in WBN-kob diabetic rats.Twenty-four-hour urinary sorbitol concentrations increased in the diabetic rats in parallel with whole blood sorbitol concentrations. An increase in 24-h urinary NAG excretion coincided with the elevated urinary sorbitol levels in the diabetic rats. The administration of epalrestat, an aldose reductase inhibitor, reduced the increased whole blood and urinary sorbitol concentrations and urinary NAG excretion concomitantly with renal aldose reductase inhibition in the diabetic rats.These results indicate that diabetic nephropathy involves distorted cell function of renal tubules, and that treatment with epalrestat may prevent at least the progress of the nephropathy.     

Localization of aldose reductase and sorbitol dehydrogenase in the nervous system of normal and diabetic rats

Summary Intraneuronal accumulations of sorbitol and fructose have been postulated to predispose the nervous system to the cerebral edema associated with the treatment of diabetic ketoacidosis. In the present study, the enzymes of the pathway for the production of sorbitol and fructose, aldose reductase and sorbitol dehydrogenase, were localized histochemically in brain, spinal cord and sciatic nerve. Enzyme activity was limited to the choroidal epithelium, ependymal cells, and pia mater in normal, 2- and 10-week streptozotocin diabetic and vehicle-treated rats. Sorbitol dehydrogenase activity was located in blood vessels and perineurium of the sciatic nerve in these groups of rats. Comparison of diabetic and vehicle groups did not demonstrate any alteration in the activity of either enzyme in the central nervous system. However, there was a decrease in sorbitol dehydrogenase activity in the blood vessels in the sciatic nerve in 50% of the 10-week diabetic rats.     

Redox state-dependent and sorbitol accumulation-independent diabetic albuminuria in mice with transgene-derived human aldose reductase and sorbitol dehydrogenase deficiency

Aims/hypothesis We investigated the role played by sorbitol accumulation in the kidney in the development of diabetic albuminuria.Methods We created mice (hAR-Tg:SDH null) with transgene-derived human aldose reductase and sorbitol dehydrogenase (SDH) deficiency, and analysed (i) the contribution of accumulated sorbitol to urinary albumin excretion rate, and (ii) the effect of the aldose reductase inhibitor, epalrestat, on the diabetic redox state, including decreased renal reduced glutathione concentrations or increased lactate to pyruvate ratios in the diabetic kidney.Results Compared to littermates, non-diabetic transgenic mice had a 2.6-fold increase in aldose reductase mRNA. In a diabetic group, aldose reductase mRNA in hAR-Tg mice was 2.7-fold higher than in littermates. In the diabetic and non-diabetic groups, hAR-Tg:SDH null mice had the highest sorbitol content among all four genetic types including hAR-Tg:SDH null, SDH null, hAR-Tg and littermates. The urinary albumin excretion rate in non-diabetic groups was similar in the four genetic types of mouse. In diabetic groups it was greater than in non-diabetic groups, but did not correlate with the sorbitol content among the four genetic types of mouse. When aldose reductase inhibitor and streptozotocin were given simultaneously at 6 weeks of age, epalrestat prevented diabetic increases in urinary albumin excretion rate and completely prevented diabetic decreases in reduced glutathione concentrations and diabetic increases in lactate to pyruvate ratios, even in the presence of transgenic aldose reductase.Conclusions/interpretation The degree of diabetic albuminuria in genetically modified mice is dependent on the redox state and independent of polyol accumulation; aldose reductase inhibitor can prevent diabetic albuminuria by normalising diabetic redox changes.Abbreviations AR Aldose reductase - SDH sorbitol dehydrogenase - UAE urinary albumin excretion rate - GSH reduced glutathione - hAR-Tg human aldose reductase-transgenic mouse - L/P lactate/pyruvate     

(AC)(n) polymorphism of aldose reductase gene and diabetic microvascular complications in type 2 diabetes mellitus.

Recent studies suggest that the gene encoding aldose reductase, the enzyme that converts glucose to sorbitol, may confer susceptibility to microvascular disease. The aim of this study therefore, was to investigate the relationship between the aldose reductase gene and type 2 diabetic microvascular complications such as diabetic nephropathy and retinopathy. DNA from 127 Korean patients with type 2 diabetes was typed for an (AC)(n) dinucleotide repeat polymorphic marker at the 5'-end of the aldose reductase gene using polymerase chain reaction. No significant difference in the frequency of the putative risk allele Z-2 was found in patients of nephropathy and retinopathy groups compared with the uncomplicated group (32.2, 34.1 vs. 25.1%, respectively, P>0.05). Similarly, no difference was found in the frequency of the putative protective allele Z+2 among any of the study groups. In conclusion, the results of the study in Korean type 2 diabetic patients do not support the hypothesis that polymorphism at the 5' end of the aldose reductase gene contributes to the susceptibility to diabetic microvascular complications.     

Caloric restriction counteracts age-related changes in the activities of sorbitol metabolizing enzymes from mouse liver

The influence of caloric restriction (CR) on hepatic sorbitol-metabolizing enzyme activities was investigated in young and old mice. Aldose reductase and sorbitol dehydrogenase activities were significantly lower in old CR mice than in old controls. Young CR mice showed decreased aldose reductase activity and a trend towards decreased sorbitol dehydrogenase when compared to controls. Metabolites of the pathway, namely sorbitol, glucose and fructose were decreased by CR in young and old mice. Pyruvate levels were decreased by CR in both young and old mice, while lactate decreased only in old CR. Malate levels increased in old CR but remained unchanged in young CR, when compared with controls. Accordingly, the lactate/pyruvate and malate/pyruvate ratios in young and old CR mice were increased, indicating increased NADH/NAD and NADPH/NADP redox couples, respectively. The results indicate that decreased glucose levels under CR conditions lead to decreased sorbitol pathway enzyme activities and metabolite levels, and could contribute to the beneficial effects of long-term CR through decreased sorbitol levels and NADPH sparing.     

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

Summary Erythrocytes from diabetic patients before and after treatment with the aldose reductase inhibitor, sorbinil, were analyzed by a capillary gas chromatographic method for sorbitol and myo-inositol. The mean erythrocyte sorbitol level in the diabetic patients was significantly higher than in the control subjects (13.1±0.9 and 5.2±0.3 nmol/ml erythrocytes, respectively, mean±SEM, p< 0.001). The mean erythrocyte myo-inositol level in diabetic patients was not different from that in control subjects (43.2±2.9 and 40.5±1.9nmol/ ml erythrocytes, respectively). Sorbinil treatment reduced the elevated sorbitol levels in the diabetic patients to normal or slightly below normal, but did not affect the erythrocyte myo-inositol concentration. It is concluded that the erythrocyte is not a suitable model to monitor a possible effect of sorbinil on myo-inositol concentration in less accessible tissues.