Long-term assessment of glucose control by haemoglobin-AGE measurement

3-Deoxyglucosone: metabolism, analysis, biological activity, and clinical implication

3-Deoxyglucosone (3-DG) is synthesized via the Maillard reaction and the polyol pathway, and is detoxified to 3-deoxyfructose and 2-keto-3-deoxygluconic acid. 3-DG rapidly reacts with protein amino groups to form advanced glycation end products (AGEs) such as imidazolone, pyrraline, N'-(carboxymethyl)lysine and pentosidine, among which imidazolone is the AGE most specific for 3-DG. As demonstrated by using gas chromatography-mass spectrometry or high-performance liquid chromatography, plasma 3-DG levels are markedly increased in diabetes and uremia. Although the plasma 3-DG levels had been controversial, it was clearly demonstrated that its plasma level depends on the deproteinization method by which either free or total 3-DG, presumably bound to proteins, is measured. In diabetes, hyperglycemia enhances the synthesis of 3-DG via the Maillard reaction and the polyol pathway, and thereby leads to its high plasma and erythrocyte levels. In uremia, however, the decreased catabolism of 3-DG, which may be due to the loss of 3-DG reductase activity in the end-stage kidneys, may lead to high plasma 3-DG level. The elevated 3-DG levels in plasma and erythrocytes may promote the formation of AGEs such as imidazolone, as demonstrated by immunohistochemistry and immunochemistry using an anti-imidazolone antibody. Although AGE-modified proteins prepared in vitro exhibit a variety of biological activities, known AGE structures have not yet been demonstrated to show any biological activities. Because 3-DG is potent in the formation of AGEs and has some biological activities, such as cellular toxicity, it may be more important in the development of diabetic and uremic complications than the known AGE structures. By demonstrating that treatment with an aldose reductase inhibitor reduces the erythrocyte levels of 3-DG and AGEs, such as imidazolone, light is shed on the mystery of how aldose reductase inhibitors may prove beneficial in diabetic complications. These evidences suggest that 3-DG plays a principal role in the development of diabetic and uremic complications.

The role of oxidative stress and NF-kappaB activation in late diabetic complications

A common endpoint of hyperglycemia dependent cellular changes is the generation of reactive oxygen intermediates (ROIs) and the presence of elevated oxidative stress. Therefore, oxidative stress is supposed to play an important role in the development of late diabetic complications. Formation of advanced glycation end products (AGE's) due to elevated nonenzymatic glycation of proteins, lipids and nucleic acids is accompanied by oxidative, radical-generating reactions and thus represents a major source for oxygen free radicals under hyperglycemic conditions. Once formed, AGE's can influence cellular function by binding to several binding sites including the receptor for AGE's, RAGE. Binding of AGE's (and other ligands) to RAGE results in generation of intracellular oxidative stress and subsequent activation of the redox-sensitive transcription factor NF-kappaB in vitro and in vivo. Consistently, activation of NF-kappaB in diabetic patients correlates with the quality of glycemic control and can be reduced by treatment with the antioxidant alpha-lipoic acid. The development of techniques allowing for a tissue culture independent measurement of NF-kappaB activation in patients with diabetes mellitus gives insights into the molecular mechanisms linking diabetes mellitus and hyperglycemia with formation of advanced glycated endproducts and generation of oxidative stress finally resulting in oxidative stress mediated cellular activation.

Comparison of advanced glycation endproducts on haemoglobin (Hb-AGE) and haemoglobin A1c for the assessment of diabetic control

Glycation process in vivo results in two different products: early and advanced glycation endproducts (AGEs). The mechanism of early product formation has been well described, with HbA1c as the best-studied example. The finding that advanced glycation endproducts are also formed on haemoglobin suggests that HbA1c is a precursor for Hb-AGE formation. HbA1c has been well established as an important indicator for glycaemia monitoring, but the diagnostic role of Hb-AGE has not yet been clarified. A question is whether HbA1c and Hb-AGE are competitive or complementary parameters. In our study, Hb-AGE was quantified by the competitive ELISA technique using polyclonal anti-AGE-RNase antibodies to detect AGE immunoreactivities of proteins precipitated in red cell hemolysate. Results are expressed as AGE units/mg Hb. Hb-AGE was analysed in three groups of patients divided according to HbA1c values as follows: group I (n = 25) HbA1c < 7%, Hb-AGE = 6.93 (5.7-7.3) U/mg; group II (n = 25) HbA1c = 7-10%, Hb-AGE = 8.62 (7.7-10.2) U/mg; and group III (n = 25) HbA1c > 10%, Hb-AGE = 12.47 (10.8-13.9) U/mg (median (interquartile range)). A close relation between the amounts of red cell HbA1c and Hb-AGE was observed in all diabetic subjects (n = 75) r = 0.77, P < 0.001. Patients with HbA1c level > 8% were considered to be in poor glycaemic control and those with HbA1c < 8% in good control. In the well-controlled subgroup (n = 33), HbA1c and Hb-AGE were less tightly correlated (r = 0.37, P <0.001). However, in those patients with a higher level of HbA1c = 12.55 (8.9-13.3)% (n = 42), the related Hb-AGE was 11.5 (10.3-12.8) U/mg Hb, yielding a more significant correlation (r = 0.51, P < 0.001). The content of Hb-AGE did not correlate with age (r = 0.09), diabetes duration (r = 0.05) or severity of retinopathy and/or nephropathy. The observed difference may reflect a different kinetic rate of HbA1c production and subsequently the rate of Hb-AGE formation. The discrepancy in the correlation between HbA1c and Hb-AGE suggests that they are complementary rather than opposed parameters. The amount of haemoglobin-linked AGEs does not correlate with the presence or absence of retinopathy and/or nephropathy. It seems that Hb-AGE represents only the metabolic status, equally in the subjects with and without diabetic microangiopathy.

Levels of fructosyllysine in collagen are low compared with the loss of lysine in diabetic rats: involvement of oxidation in increased cross-linking

1. It is thought that early glycation products on lysine residues of collagen are re-arranged to form collagen cross-links (advanced glycation end-products), which are increased in hyperglycaemia. We have previously reported that 4 week vitamin E supplementation in diabetic rats protected against the development of increased collagen cross-linking. In the present study we analysed early glycation products as fructosyllysine and amino acid content of tail tendon collagen in rats. 2. The amount of lysine residues in early collagen glycation products in diabetic rats was estimated to be less than 0.2 mol/mol type I collagen, whereas in the diabetic rats the loss of lysine residues was estimated to be 2.5 mol/mol. However, vitamin E supplementation protected against the loss of lysine residues without affecting early glycation products. 3. Thus, it is demonstrated that the amount of early collagen glycation products is quite small compared with the loss of lysine residues in diabetic rats and we suggest that the oxidative modification of lysine residues may be involved in the increased cross-linking observed in hyperglycaemia.

Diabetic vascular complications

1. Diabetic vascular complications can be arbitrarily divided into micro- and macrovascular complications, the major microvascular complications being nephropathy, retinopathy and neuropathy. Macrovascular complications are due to accelerated atherosclerosis and include ischaemic heart disease, cerebrovascular disease and peripheral vascular disease. 2. It is postulated that metabolic and haemodynamic factors interact leading to the development of diabetic vascular complications. 3. Advanced glycation appears to be an important pathway in the pathogenesis of diabetic complications with evidence that the inhibitor of this process, aminoguanidine, attenuates the development of a range of diabetic vascular complications. 4. The results of experimental studies have led to large-scale clinical trials of various therapeutic agents that act to interfere with the metabolic and haemodynamic pathways implicated in the progression of diabetic complications.