Study of aldose reductase inhibition in intact lenses by 13C nuclear magnetic resonance spectroscopy

Imaging of 13C-labeled glucose and sorbitol in bovine lens by 1H-detected 13C nuclear magnetic resonance spectroscopy

Bovine lenses were incubated in a solution containing [1-(13)C]glucose (50 mM) for 1, 2 and 4 days. Spectroscopic images of [1-(13)C]glucose and [1-(13)C]sorbitol were constructed using (1)H-detected gradient-enhanced heteronuclear multiple-quantum coherence (GE-HMQC) in a 2.0-tesla magnetic field. Accumulations of [1-(13)C]glucose and [1-(13)C]sorbitol were mainly observed at the periphery of the lens. Their distributions corresponded to the cortex. (1)H-detected (13)C nuclear magnetic resonance (NMR) spectroscopic imaging by GE-HMQC successfully demonstrated the distribution of [1-(13)C]glucose and [1-(13)C]sorbitol at the periphery of bovine lenses.

The glucose/insulin system and vitamin C: implications in insulin-dependent diabetes mellitus

The cellular uptake of vitamin C (ascorbic acid, ASC) is promoted by insulin and inhibited by hyperglycemia. If a rise in plasma ASC is uncoupled from insulin replacement in insulin-dependent diabetes mellitus (IDDM) then the degree of hyperglycemia could account for "tissue scurvy" in IDDM. Leukocyte ASC is lower in IDDMs compared with nondiabetics when vitamin C consumption is adequate and our data suggest that this is a variable component of the pathophysiology of IDDM. The complications of diabetes mellitus are believed to result from either the intracellular accumulation of sorbitol or the nonenzymatic glycoxidation of proteins or both. With respect to the abnormal cellular accumulation of sorbitol, vitamin C supplementation has been shown to be effective in several studies of adults with diabetes; the situation regarding the prevention of protein glycoxidations by supplementation is presently unclear. The roles of ASC as an aldose reductase inhibitor and a water soluble antioxidant in body fluids are potentially very important as adjuncts to tight glycemic control in the management of diabetes. Tissue saturation and maximal physiologic function in IDDM may require supplemental vitamin C intake.

Micronutrients as nutriceutical interventions in diabetes mellitus

Hyperglycemia portends chronic complications in insulin-dependent diabetes mellitus (IDDM) and substantial benefits are associated with "tight" glycemic control. Other interventions should either enhance glycemic control per se or add benefit to an established degree of glycemic control. Several micronutrients enhance insulin action and others offer promise in countering the untoward consequences of hyperglycemia. Supplements of micronutrients including the vitamins niacin (as niacinamide), C and E and the minerals zinc, chromium and vanadium have been studied. For the purpose of this review, the term "nutriceutic" refers to supplementation on the order of 2 to 10 times the RDA for which a benefit is linked to a mechanism of action. Benefits associated with "nutriceutic" supplementation are reported in small trials for vitamins C and E and these supplements are safe and affordable from food or tablet sources. A dietary strategy adding 200-600 mg of vitamin C and 100 IU of vitamin E to a healthy dietary pattern is worthy of consideration as an intervention for individuals with IDDM.

Inhibition of aldose reductase: 13C NMR studies in isolated peripheral nerve

We report 13C NMR measurements of the flux through aldose reductase in isolated rat sciatic nerve, and its inhibition by an aldose reductase inhibitor of the sulphonylnitromethane class. [1-13C] galactose was used as substrate, and the rate of production of [1-13C] dulcitol was measured. Quantitation required the use both of internal extracellular, and external, standards. The mean net forward flux (+/- SD) was 20 +/- 11 nmol/(mL nerve water)/min (n = 10). In the presence of the inhibitor, flux was reduced significantly (p < 0.001) to 13% of control. Since dulcitol is symmetrical, an estimate of the backward flux, to [6-13C] galactose, is also possible; under our conditions, this was negligible.

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)

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Functional cysteinyl residues in human placental aldose reductase

Incubation of human placental aldose reductase (EC 1.1.1.21) with the sulfhydryl oxidizing reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM) results in a biexponential loss of catalytic activity. Inactivation by DTNB or NEM is prevented by saturating concentrations of NADPH. ATP-ribose offers partial protection against inactivation by DTNB, whereas NADP, nicotinamide mononucleotide (NMN), and the substrates glyceraldehyde and glucose offer little or no protection. The inactivation by DTNB was reversed by dithiothreitol and partially by 2-mercaptoethanol but not by KCN. When the release of 2-nitro-5-mercaptobenzoic acid was measured, 3 mol of sulfhydryl residues was found to be modified per mole of the enzyme by DTNB. Correlation of the fractional activity remaining with the extent of modification by the statistical method of C.-L. Tsou (1962, Sci. Sin. 11, 1535-1558) indicates that of the three reactive residues, one reacts at a faster rate than the other two, and that two residues are essential for the catalytic activity of the enzyme. Labeling of the total sulfhydryl by [14C]NEM and quantification of DTNB-reactive residues in the enzyme denatured by 6 M urea indicates that a total of seven sulfhydryl residues are present in the protein. The modification of the enzyme did not affect Km glyceraldehyde, but the modified enzyme had a lower Km NADPH. Kinetic analysis of the data suggests that a biexponential nature of inactivation could be due to the formation of a dissociable E:DTNB complex and the presence of a partially active enzyme species.

Indirect monitoring of carbon-13 metabolism with NMR: analysis of perfusate with a closed-loop flow system

NMR studies of living cells and of perfused organs almost invariably require a major effort in the design of either the probe or the support system. For enriched 13C studies of metabolism, it is sometimes possible to use a simpler approach, one amenable to use in narrow-bore high-resolution NMR spectrometers. Since some of the metabolic changes are reflected by changes in the chemical composition of the media, it may be necessary to provide only for NMR measurements of the perfusate. A simple apparatus is described which allows NMR measurement of the perfusate. A commercially available concentric NMR tube, modified to incorporate flow, is inserted in the flow loop and placed in the spectrometer. An example is given of the metabolism of 13C-enriched glucose to lactate by chick embryo fibroblasts grown in a hollow fiber bundle assembly.

The utilization of 13C and 31P nuclear magnetic resonance spectroscopy in the study of the sorbitol pathway and aldose reductase inhibition in intact rabbit lenses

Nuclear magnetic resonance (NMR) spectroscopy has been utilized in the study of the metabolism of intact, functioning rabbit lenses maintained in organ culture. The sorbitol pathway and aldose reductase inhibition have been studied using carbon-13 NMR spectroscopy. Incubation of lenses in high concentration [1-13C] glucose medium with and without added inhibitors allows the sorbitol pathway and glycolysis to be monitored. Various aldose reductase inhibitors have been studied and are ranked based on percentage of inhibition as follows: tolrestat greater than or equal to sorbinil greater than sulindac greater than sulindac sulfide much greater than indomethacin greater than acetylsalicylic acid greater than quercetin greater than tandearil greater than salicylic acid greater than 3,3-Tetramethyleneglutaric acid (TMG). It has been demonstrated that 13C NMR spectroscopy provides an effective method of screening potential inhibitors of aldose reductase. The aspirin substitutes ibuprofen and acetaminophen have been studied and are found to reduce sorbitol accumulation in intact rabbit lenses. The effects of myo-inositol and vitamin E on sorbitol accumulation have also been investigated. Results suggest that the various metabolic pathways within the lens are intricately connected. In a preliminary manner, the effect of diabetes on metabolism in intact lenses has been investigated using 13C NMR spectroscopy. Increased sorbitol production has been observed for diabetic lenses. 31P NMR spectroscopy has also been utilized in the study of lens metabolism and aldose reductase inhibitors. Inclusion of various inhibitors in the high concentration glucose medium results in maintenance of essentially normal phosphorus-containing metabolite levels in the lens. No clear relationship was observed between lens clarity and phosphorus metabolite levels as determined using NMR.

The study of diabetic cataractogenesis in the intact rabbit lens by deuterium NMR spectroscopy

The polyol pathway has been implicated in the process of diabetic cataractogenesis. We report the use of deuterium (2H) spectroscopy for dynamically monitoring the polyol and glycolytic pathways in the single intact rabbit lens. Using 2H labeled C-1 D-glucose, the formation of sorbitol from glucose and the metabolism of sorbitol to fructose was dynamically monitored at 5.5 mM and 35.5 mM glucose concentrations. The accumulation of sorbitol at 35.5 mM glucose concentration was prevented by the inhibition of aldose reductase using an inhibitor (Sorbinil). 2H spectra were obtained in short acquisition times because of the short T1's of deuterated metabolites. A further advantage of 2H spectroscopy is that the natural abundance resonance of water (HDO) can be used as an internal reference standard. These findings confirm previous studies and demonstrate for the first time by NMR spectroscopy activity in the polyol pathway at low glucose concentrations.