The measurement of glycosylated hemoglobin in man and animals by aminophenylboronic acid affinity chromatography

Diabetes mellitus alters the mechanical properties of the native tendon in an experimental rat model

The purpose of this study was to determine the effect of the diabetic phenotype on the mechanical properties of the native patellar tendon and its enthesis. Diabetes was induced via intraperitoneal injection of streptozotocin in Lewis rats. Control (n = 18) and diabetic animals (n = 20) were killed at 12 and 19 days for analysis. Statistical comparisons were performed using Student's t-tests and a two-tailed Fisher test with significance set at p < 0.05. Pre- and post-injection intraperitoneal glucose tolerance tests demonstrated significant impairment of glycemic control in the diabetic compared to control animals (p = 0.001). Mean serum hemoglobin A1c levels at 19 days was 10.6 ± 2.7% and 6.0 ± 1.0% for the diabetic and control groups, respectively (p = 0.0001). Fifteen of sixteen diabetic animals demonstrated intrasubstance failure of the patellar tendon, while only 7 of 14 control specimens failed within the tendon substance. The Young's modulus of the diabetic tendon was significantly lower than control specimens by 19 days post-induction (161 ± 10 N m(-2) compared to 200 ± 46 N m(-2) , respectively) (p = 0.02). The metabolic condition of poorly controlled diabetes negatively affects the mechanical properties of the native patellar tendon. These altered structural properties may predispose diabetic patients to a greater risk of tendinopathy and/or traumatic rupture.

Diabetes mellitus impairs tendon-bone healing after rotator cuff repair

INTRODUCTION

Studies have demonstrated a significant decrease in skeletal mass, bone mineral density, and impaired fracture healing in the diabetic population. However, the effect of sustained hyperglycemia on tendon-to-bone healing is unknown.

MATERIALS AND METHODS

Forty-eight male, Lewis rats underwent unilateral detachment of the supraspinatus tendon followed by immediate anatomic repair with transosseous fixation. In the experimental group (n = 24), diabetes was induced preoperatively via intraperitoneal injection of streptozotocin (STZ, 65 mg/kg) and confirmed with both pre- and post-STZ injection intraperitoneal glucose tolerance tests (IPGTT). Animals were sacrificed at 1 and 2 weeks postoperatively for biomechanical, histomorphometric, and immunohistochemical analysis. Serum hemoglobin A1c (HbA1c) levels were measured at 2 weeks postoperatively. Statistical comparisons were performed using Student t tests with significance set at P < .05.

RESULTS

IPGTT analysis demonstrated a significant impairment of glycemic control in the diabetic compared to control animals (P < .05). Mean HbA1c level at 2 weeks postoperatively was 10.6 ± 2.7% and 6.0 ± 1.0% for the diabetic and control groups, respectively (P < .05). Diabetic animals demonstrated significantly less fibrocartilage and organized collagen, and increased AGE deposition at the tendon-bone interface (P < .05). The healing enthesis of diabetic animals demonstrated a significantly reduced ultimate load-to-failure (4.79 ± 1.33 N vs 1.60 ± 1.67 N and 13.63 ± 2.33 N vs 6.0 ± 3.24 N for control versus diabetic animals at 1 and 2 weeks, respectively) and stiffness compared to control animals (P < .05).

DISCUSSION

Sustained hyperglycemia impairs tendon-bone healing after rotator cuff repair in this rodent model. These findings have significant clinical implications for the expected outcomes of soft tissue repair or reconstructive procedures in diabetic patients with poor glycemic control.

Identification and relative quantification of specific glycation sites in human serum albumin

Glycation (or non-enzymatic glycosylation) is a common non-enzymatic covalent modification of human proteins. Glucose, the highest concentrated monosaccharide in blood, can reversibly react with amino groups of proteins to form Schiff bases that can rearrange to form relatively stable Amadori products. These can be further oxidized to advanced glycation end products (AGEs). Here, we analyzed the glycation patterns of human serum albumin (HSA) in plasma samples obtained from five patients with type 2 diabetes mellitus. Therefore, glycated peptides from a tryptic digest of plasma were enriched with m-aminophenylboronic acid (mAPBA) affinity chromatography. The glycated peptides were then further separated in the second dimension by RP-HPLC coupled on-line to an electrospray ionization (ESI) tandem mass spectrometer (MS/MS). Altogether, 18 Amadori peptides, encompassing 40% of the HSA sequence, were identified. The majority of the peptides were detected and relatively quantified in all five samples with a high reproducibility among the replicas. Eleven Lys-residues were glycated at similar quantities in all samples, with glycation site Lys(549) (K(Am(Glc))QTALVELVK) being the most abundant. In conclusion, the established mAPBA/nanoRP-HPLC-ESI-MS/MS approach could reproducibly identify and quantify glycation sites in plasma samples, potentially useful in diagnosis and therapeutic control.

Analysis of amadori peptides enriched by boronic acid affinity chromatography

Glycation of peptides and proteins by D-glucose is a universal, nonenzymatic reaction with important implications for the pathogenesis and diagnosis of many diseases, including diabetes mellitus. Whereas some modification sites have been identified in serum albumin and hemoglobin, a general approach to map glycation sites for nonabundant proteins present in complex mixtures, such as serum, is still missing. Here, we describe a universal enrichment procedure for glycated peptides using boronic acid affinity chromatography in the first dimension followed by reversed-phase chromatography, coupled either online to electrospray ionization mass spectrometry (ESI-MS) or offline to matrix-assisted laser desorption/ionization (MALDI) MS. This two-dimensional approach was optimized for high recoveries and low cross reactivities. For bovine serum albumin, a total of 31 Amadori peptides were identified in a tryptic digest corresponding to 26 different glycation sites.

Changes in glycated haemoglobin levels in diabetic rats measured with an automatic affinity HPLC

1. The level of glycated haemoglobin (GHb) in diabetic rats was measured using a newly developed automatic high-performance liquid chromatography (HPLC) with a boronate affinity column that requires only 2.5 min per sample for analysis. 2. Levels of GHb were 2.7% in normal 7-week-old Sprague-Dawley rats. These levels increased gradually following the abrupt induction of hyperglycaemia by intravenous injection of streptozotocin (STZ), reaching a maximal level of 10.1% after 6 weeks. 3. Glycosylated haemoglobin (HbA1) levels measured by cation-exchange chromatography were also increased by STZ treatment, although HbA1 values were lower than GHb measured by affinity column HPLC. 4. In Wistar fatty rats, GHb levels declined gradually over 5 weeks following the administration of pioglitazone (0.75 or 2.25 mg/kg per day) as a food admixture, which reduced plasma glucose (PG) levels to normal levels within 1 week. Glycated haemoglobin levels after 5 weeks treatment with pioglitazone correlated better with the area under the curve for PG over the entire 5 week treatment period than with the PG level at the end of treatment. 5. In addition, GHb determined by affinity column HPLC correlated well with HbA1 measured by cation-exchange chromatography, although the GHb value was higher than the HbA1 value. 6. Glycated haemoglobin levels in db/db and KKAy mice were higher than those in control normoglycaemic animals and were also higher than HbA1 values measured by the cation-exchange method, although the two values did show good correlation. 7. These results indicate that the newly developed affinity column HPLC system is useful for evaluating total GHb levels in rats as an index of antidiabetic treatment.

Ion-exchange microchromatography and thiobarbituric acid colorimetry for the measurement of canine glycated hemoglobins

Nonenzymatic glycation of hemoglobin is a slow, continuous, and irreversible process which takes place during the whole lifespan of the erythrocyte. When hemolytic diseases are ruled out, the levels of glycated hemoglobins reflect the time-averaged serum glucose concentration for the preceding weeks. Canine hemoglobin also binds physiologically to intraerythrocytic glucose to form a glycated fraction which provides information on the animal's long-term glycemic status. This study describes an overall evaluation of ion-exchange microchromatography and thiobarbituric acid (TBA) colorimetry for the measurement of canine glycated hemoglobins. The intra- and inter-assay coefficients of variation (CVs) found were less than 5% in normal and diabetic canine samples, and both assays proved linear over the analytical range tested, which was wide enough to include the expected clinical values. Under our laboratory's conditions, the reference range for HbA(1) was 5.82 +/- 0.62% and for HbA(1)c was 2.35 -/+ 0.47%. Sample stability was lower using the ion-exchange procedure, with increases in HbA(1) observed after 4 days in whole blood and hemolysates stored at room temperature, after 12 days in whole blood stored at 4 degrees C, and after 7 days in hemolysates stored at 4 degrees C and -20 degrees C. In the case of TBA colorimetry, whole blood was stable for at least 21 days at room temperature and at 4 degrees C, and hemolysates were stable for 18 days at room temperature, at least 21 days at 4 degrees C, and up to 3 months at -20 degrees C.

Glycosylated haemoglobin assay of canine blood samples

A method for assaying canine glycosylated haemoglobin was evaluated. The method is a turbidimetric inhibition immunoassay and the final reaction is bichromatically measured using a multichannel automatic analyser. Within-run coefficients of variation (2.07 to 4.46 per cent) were permissible, but between-run coefficients of variation (2.10 to 8.25 per cent) were slightly more elevated. The detection limit of this assay is 0.052 per cent. A sample dilution of 10 microliters of sample and 500 microliters of haemolysing reagent is recommended for routine analysis of canine blood samples. A normal reference interval of 1.39 +/- 0.70 per cent was obtained from the glycosylated haemoglobin analysis in 82 healthy dogs and no statistically significant differences in relation to age or gender were observed. Some changes in glycosylated haemoglobin concentrations were noted throughout the ovarian cycle, although differences between dogs were evident. Since this assay specifically measures the glycosylated haemoglobin content in canine blood samples, it could be very useful for monitoring diabetic dogs.

Effect of curcumin on the advanced glycation and cross-linking of collagen in diabetic rats

A close association between increased oxidative stress and hyperglycemia has been postulated to contribute significantly to the accelerated accumulation of advanced glycation end products (AGEs) and the cross-linking of collagen in diabetes mellitus. In the present work, we report the influence of curcumin, an efficient antioxidant, on the level of AGEs and the cross-linking of collagen in diabetic rats. Diabetic rats were given curcumin (200 mg/kg body wt) orally for a duration of 8 weeks. The antioxidant status in serum and the level of AGEs, cross-linking and browning of collagen in tail tendons and skin were investigated. The oxidative stress observed in diabetic rats was reduced significantly by curcumin administration. Nonenzymic antioxidants such as vitamin C, vitamin E, and glutathione were maintained at near normal values in curcumin-treated diabetic animals. Similarly, the accumulation of lipid peroxidation products in diabetic serum was reduced significantly by curcumin. Accelerated accumulation of AGE-collagen in diabetic animals, as detected by ELISA, was prevented by curcumin. Extensive cross-linking of collagen in the tail tendon and skin of diabetic animals was also prevented to a greater extent by curcumin treatment. A correlation between the level of AGEs and collagen cross-linking was noted, suggesting the involvement of advanced glycation in cross-linking. It was also noted that the preventive effect of curcumin on the advanced glycation and cross-linking of collagen was more pronounced than its therapeutic effect. However, the Maillard reaction fluorescence in both tail and skin collagen remained unaltered by curcumin. This study confirms the significance of free radicals in the accumulation of AGEs and cross-linking of collagen in diabetes. It supports curcumin administration for the prevention of AGE-induced complications of diabetes mellitus.

Glycated haemoglobin in various pathological conditions: investigations based on a new, fully automated method

A new automated affinity chromatographic method for determining glycated haemoglobin (GHb) in dogs and cats was tested. The method appeared to be practical, quick and accurate. The reference range, calculated on the basis of 50 healthy dogs and 43 healthy cats, lay between 2.4 and 3.4 per cent in dogs and 2.0 and 2.9 per cent in cats. Concentrations were not influenced by age or gender. GHb levels obtained for 21 dogs and 18 cats with newly diagnosed diabetes mellitus were significantly higher than those of the control animals, ranging from 4.5 to 8.6 per cent (median 6.1) in dogs and from 2.7 to 6.0 per cent (median 3.8 per cent) in cats. The GHb levels in 31 normoglycaemic dogs with anaemia ranged from 2.3 to 4.3 per cent (median 3.3 per cent), and those of 22 normoglycaemic cats with anaemia from 2.6 to 3.9 per cent (median 3.2 per cent); both sets of levels were significantly elevated compared to control group values. GHb concentrations in animals with polycythaemia, azotaemia or liver disease showed no significant deviations from the control group; in individual cases they were slightly elevated compared to the reference range. The automated measuring method employed can be used to determine GHb in dogs and cats. Anaemic animals should generally be excluded from the GHb determination.

A rapid- and short-acting hypoglycemic agent KAD-1229 improves post-prandial hyperglycemia and diabetic complications in streptozotocin-induced non-insulin-dependent diabetes mellitus rats

We investigated therapeutic effects of a rapid- and short-acting non-sulfonylurea hypoglycemic agent, calcium (2S)-2-benzyl-3-(cis-hexahydro-2-isoindolinylcarbonyl)propionate dihydrate (KAD-1229), on streptozotocin (STZ)-induced non-insulin-dependent diabetes mellitus (NIDDM) rats. The effects exerted by KAD-1229 on the post-prandial plasma glucose rise in STZ-induced mild NIDDM (mNIDDM) rats were different from those of sulfonylureas. When KAD-1229 with liquid meal (10 kcal/kg) was given to the mNIDDM rats, the plasma glucose migration was similar to that of normal healthy rats. On the contrary, glibenclamide had little or no effect on the plasma glucose rise 0.5-1 hr after oral administration, and its effect was only evident 2-5 hr after dosing. Tolbutamide showed similar hypoglycemia to that induced by glibenclamide at 2-5 hr with insufficient efficacy at 0.5 hr. Gliclazide sufficiently suppressed the level of post-prandial plasma glucose. However, its complete inhibition of post-prandial plasma glucose was associated with the extra-hypoglycemia 1-5 hr after oral administration. We also tested the efficacy of KAD-1229 in more severe STZ-induced NIDDM (sNIDDM) rats to elucidate the effects of the drug on the long-term glycemic controls and diabetic complications. When the sNIDDM rats were treated with 10 mg/kg KAD-1229 twice a day for about 17 weeks, increases in fasting plasma glucose and hemoglobin A1c were inhibited. Furthermore, treatment with KAD-1229 suppressed the development of microalbuminuria and cortical cataract. We conclude that the rapid- and short-acting insulinotropic agent KAD-1229 is able to improve the deterioration in the glycemic controls and inhibit the development of diabetic complications in STZ-induced NIDDM rats.

Protective effect of vitamin E supplementation on increased thermal stability of collagen in diabetic rats

Products similar to non-enzymatic glycation end products are known to arise from the interactions between proteins and lipid peroxides in vitro. In this study, we assessed the effect of vitamin E, which possibly modifies lipid peroxide, on advanced glycation end products or similar products in vivo by measuring the fluorescence and thermal rupture time of tail tendon collagen in streptozotocin-induced diabetic rats. The diabetic rats and non-diabetic rats were fed a vitamin E supplemented diet, and a control diet starting 3 days after the streptozotocin injection. After the 4-week treatment, the serum lipid peroxide levels expressed as thiobarbituric acid reactants in the diabetic rats on control diet (15.9 +/- 2.6 mumol/l[SEM]) were significantly (p less than 0.05) higher than in the non-diabetic rats (7.9 +/- 1.3 mumol/l on control diet and 3.3 +/- 0.4 mumol/l on supplemented diet), but the levels in the diabetic rats on supplemented diet (7.9 +/- 2.3 mumol/l) were reduced to the normal levels. No significant differences were found in serum glucose and glycated haemoglobin levels within the diabetic rats on control and supplemented diets. Both the fluorescence and thermal rupture time of collagen were significantly (p less than 0.05) increased in the diabetic rats on both diets compared with those in the corresponding non-diabetic rats. Although there was no significant difference in the collagen-linked fluorescence within the diabetic rats on control and supplemented diets, the thermal rupture time was significantly (p less than 0.01) shortened with supplemented diet (10.8 +/- 0.7 min on supplemented diet vs 15.0 +/- 0.7 min on control diet).(ABSTRACT TRUNCATED AT 250 WORDS)

A neuropathic deficit, decreased sweating, is prevented and ameliorated by euglycemia in streptozocin diabetes in rats

Decreased sweating, especially of feet and legs, occurs in human diabetic neuropathy. It might be studied in experimental diabetes to characterize it, elucidate its mechanisms, and determine whether it can be prevented or treated. The pilocarpine-induced sweat responses (SR) in the hind foot pads of groups of control and streptozocin diabetic rats, in good (GC) and in poor (PC) glycemic control and with a crossover design after 20 wk of diabetes, were evaluated with the silicone mold sweat test to determine the number of sweat droplets per group of foot pads. The SR was dose dependent and reproducible. The SR disappeared with denervation and reappeared with reinnervation; denervation hypersensitivity did not develop. In the GC group, euglycemia was achieved by regulating the caloric intake and using multiple daily injections of Ultralente insulin. The SR was not different from that of the control group for up to 136 d. In the PC group, the SR became abnormal (P less than 0.005) at 16 d and progressively worsened: 40% of baseline values at 14 wk (P less than 0.001). After restoring euglycemia in the PC group, a normal SR occurred at 12 d. These results show that one human neuropathic deficit, failure of sweating, can be prevented or ameliorated by good glycemic control.

Clinical use and time relationship of changes in affinity measurement of glycosylated albumin and glycosylated hemoglobin

Simple techniques for measurement of glycosylated hemoglobin and glycosylated albumin by affinity chromatography on m-aminophenylboronic acid agarose columns have recently been developed. This study explored the time course of changes in glycoalbumin versus those of glycohemoglobin in response to rapid changes in ambient glucose concentration. One would predict that glycoalbumin levels would change more rapidly than glycohemoglobin levels due to the shorter half-life of albumin than hemoglobin. This was found to be the case in a group of rabbits rendered diabetic with alloxan. Glycoalbumin levels plateaued 4 weeks after alloxan administration, while glycohemoglobin levels were still rising. In a group of diabetic patients in whom glucose levels were initially poorly controlled, strict diet or intensive insulin management were used to rapidly bring glucose levels under control. In this group of patients, the glycoalbumin values entered the normal range and plateaued, while glycosylated hemoglobin levels were still falling. Glycoalbumin determination by affinity chromatography is a valuable adjunct to glycosylated hemoglobin determination in evaluating near term control of blood sugar values.

Assessment of long-term glycemia in type I diabetes using multiple blood glucose values stored in a memory-containing reflectometer

The relationship between repetitive hemoglobin A1 values and daily blood glucose tests performed by 20 insulin-dependent diabetic outpatients was assessed over a six-week period using a modified reflectance meter capable of storing blood glucose determinations automatically. An average of four and a half determinations per subject per day was recorded with a range of average blood glucose values between 82 +/- 2 mg/dl and 316 +/- 5 mg/dl (mean +/- SE). The relationship between average blood glucose and hemoglobin A1 values was significant when hemoglobin A1 values at the end of the six-week period were correlated with the mean blood glucose level over that period (r = 0.55, p less than 0.02), but improved when a more remote hemoglobin A1 value obtained at 10 weeks was used (r = 0.64, p less than 0.005). Hemoglobin A1 values covering two-week intervals were extremely poor in reflecting average glycemia. The average fasting blood glucose level in these subjects was highly correlated with the overall daily blood glucose values (r = 0.89, p less than 0.0001), although the coefficients of variation of these parameters averaged 43 +/- 3 percent and 47 +/- 2 percent, respectively, and were greater than that of the hemoglobin A1 values over six weeks (10 +/- 2 percent). It is concluded that labile blood glucose control in patients with insulin-dependent diabetes is accurately reflected by the average fasting blood glucose level, although multiple determinations must be employed. Satisfactory assessment may be made by use of hemoglobin A1 value provided that the hemoglobin A1 determination follows a sufficiently long period of time, presumably related to the turnover of glycosylated hemoglobin.

The prevention and reversibility of tissue non-enzymatic glycosylation in diabetes

The time course of non-enzymatic glycosylation (NEG) of liver, kidney, tail collagen, and haemoglobin was studied in diabetic rats. Increased NEG of liver, kidney, and collagen was detectable within 4 weeks of diabetes. The abnormal NEG of liver, kidney, and haemoglobin present after 4 weeks of untreated diabetes could be normalized by 4-8 weeks of intensive insulin therapy given by continuous subcutaneous infusion. However, the same treatment was ineffective in reversing the abnormal NEG and thermal stability of tail collagen. The differences in the development and reversibility of these tissue changes may be due to different tissue turnover rates. Insulin therapy, given from the onset of diabetes, was effective in preventing the development of collagen abnormalities. This suggests that early and vigorous treatment of diabetes is necessary to prevent collagen changes which are potentially irreversible.

Experimental chronic hypoxic neuropathy: relevance to diabetic neuropathy

The cardinal electrophysiological abnormalities in experimental diabetic (EDN) and experimental galactose (EGN) neuropathy, models in which endoneurial hypoxia has been demonstrated, are a slowing in nerve conduction velocity (NCV) and a resistance to ischemic conduction block (RICB). These electrophysiological abnormalities are also present in human diabetic neuropathy, where microangiopathy has been demonstrated to be more severe than in EDN so that endoneurial hypoxia is also likely to be present. We examined the effects of endoneurial hypoxia per se on normal nerves. In rats subjected to chronic hypoxia (10% O2) for up to 10 wk, the two electrophysiological abnormalities had developed by 4 wk and were very similar in degree to those seen in EDN and EGN. These abnormalities occurred in the absence of hyperglycemia, nerve sorbitol accumulation, or myoinositol reduction. Resting O2 consumption was reduced, the percent increase in nerve lactate under anoxic stress was increased, and nerve free sugars were normal. Hexokinase and phosphofructokinase activities were not altered substantially when studied under conditions of O2 excess. These findings indicate that hypoxia per se will cause conduction slowing and suggest that the hypoxic nerve develops RICB because of a reduced energy requirement and an increased efficiency of anaerobic glycolysis, but without major changes in the activity of its controlling glycolytic enzymes.

Comparison of three different assay procedures for the determination of HbA1 with special attention to the influence of pre-HbA1c, temperature and haemoglobin concentration

Three different methods for the measurement of glycated haemoglobin or HbA1, based on agar gel electrophoresis, ion-exchange and affinity chromatography were compared. All three showed acceptable precision (overall CV being less than 5%) and correlated well with each other (r greater than 0.945). The ion-exchange and affinity chromatography method proved to be independent of the amount of pre-HbA1c present. The electrophoresis method was independent of temperature in contrast to the other two methods, which showed a strong and comparable temperature dependency. All three methods were dependent on the haemoglobin concentration and/or protein content of the haemolysate. Both ion-exchange and electrophoresis showed significant interference by changes in haemoglobin concentrations, whereas the protein concentration significantly biased the affinity chromatography figures. Taking into account their specific merits all three methods are acceptable for routine use.

Prevention of some electrophysiologic and biochemical abnormalities with oxygen supplementation in experimental diabetic neuropathy

Endoneurial hypoxia has been suggested as a mechanism of human and experimental diabetic neuropathy (EDN). We found that rats rendered diabetic for 4 months had reduced nerve blood flow (NBF) and nerve oxygen tension (PnO2). The NBF was reduced by at least 33% in EDN and 60% of the oxygen tensions in the endoneurial O2 histogram were less than 25 mm Hg (3.3 kPa) in EDN compared with only 19% in the controls. To test the hypothesis that EDN may in part be due to hypoxia, we studied the effectiveness of oxygen supplementation in preventing some electrophysiologic and biochemical abnormalities. Rats with EDN had reduced caudal nerve conduction velocity and had a resistance to ischemic conduction block. When a matched groups of rats with EDN were O2 supplemented for 4 weeks, the time to 50% block of nerve conduction and nerve conduction velocity was no longer statistically different from controls. Endoneurial free sugars (glucose, fructose, sorbitol) were markedly increased in EDN. Oxygen supplementation resulted in no change in plasma glucose; by contrast, these increased endoneurial free sugars were significantly reduced (towards normal) by 60%, 33%, and 34%, respectively. myo-Inositol, however, was further decreased by oxygen supplementation. These findings of a partial prevention of electrophysiologic and biochemical abnormalities support a role of hypoxia in the pathogenesis of EDN.

Glycosylated haemoglobin: measurement and clinical use

The discovery, biochemistry, laboratory determination, and clinical application of glycosylated haemoglobins are reviewed. Sources of error are discussed in detail. No single assay method is suitable for all purposes, and in the foreseeable future generally acceptable standards and reference ranges are unlikely to be agreed. Each laboratory must establish its own. Nevertheless, the development of glycosylated haemoglobin assays is an important advance. They offer the best available means of assessing diabetic control.

Recent advances in glycosylated hemoglobin measurements

Glycosylated hemoglobins have gained wide acceptance as an accurate index of long-term blood glucose control in diabetes mellitus. A variety of glycosylated hemoglobin assays is available. There is a high degree of correlation between results determined by these assays. The ideal laboratory method for measuring glycosylated hemoglobin in the diabetic should be accurate, precise, easily standardized, inexpensive, and rapidly performed. Unfortunately, none of the currently used methods meet all of the criteria necessary to be considered the ideal laboratory method. The most widely used methods for quantitating glycosylated hemoglobins--including ion exchange chromatography, electrophoresis, isoelectric focusing, thiobarbituric acid colorimetry, and affinity chromatography--are reviewed with respect to the important advantages and disadvantages of each method for the clinical laboratory. Techniques for quantitating glycosylated proteins other than hemoglobins, such as albumin, are also discussed.

A sensitive method for the measurement of glycosylated plasma proteins using affinity chromatography

We describe a simple, sensitive affinity technique for the routine measurement of glycosylated plasma proteins in clinical laboratories. The commercially available phenylboronic acid gel used for the chromatography has recently been marketed as a kit for this purpose (Glycogel Test Kit, Pierce Chemical Co). The manufacturers of this kit recommend loading 200 microliters neat plasma to each 1 ml gel column. This high loading is to enable the direct measurement of protein in the bound and unbound fractions at 280 nm. This loading is consistent with 10-15 mg protein being added per ml gel. Our results show that protein levels greater than 2 mg per ml gel overload the column. Therefore we used a modification of the more sensitive Bradford procedure to measure protein. The method discriminates between normals (6.29 +/- 1.87%) and diabetic patients (12.62 +/- 3.36%) and has good precision (CV 4-6%). The results obtained correlate with the colorimetric method using thiobarbituric acid (r = 0.70) and with glycosylated haemoglobin (r = 0.82).

A novel method for generating region-specific monoclonal antibodies to modified proteins. Application to the identification of human glucosylated low density lipoproteins

Modifications of plasma lipoprotein structure and function resulting from in vivo post-translational nonenzymatic glycosylation may play a role in the premature atherosclerosis of patients with diabetes mellitus. This report describes the generation and characterization of six unique murine monoclonal antibodies that bind glucosylated human plasma lipoproteins, but do not react with normal plasma lipoproteins. This was accomplished by immunizing mice with homologous glucosylated low density lipoprotein. In competitive inhibition radioimmunoassays, the dominant epitope recognized by these antibodies on glucosylated low density lipoprotein was identified as glucitollysine, the reduced hexose alcohol form of glucose conjugated to the epsilon amino group of lysine. Each of these antibodies was capable of identifying glucitollysine epitopes on all reduced glucosylated proteins studied, including high density lipoprotein, albumin, hemoglobin, and transferrin. These antibodies were also capable of identifying and quantitating glucitollysine residues on the total plasma proteins and isolated lipoproteins of normal and diabetic individuals after reduction of the proteins with NaBH4. Preliminary data suggest that diabetic total plasma proteins and isolated lipoproteins contain at least threefold more immunochemically detectable glucitollysine residues than nondiabetic plasma proteins and lipoproteins. The technique described in this report should allow production of region-specific antibodies to any immunogenic modification of a protein.

Characterization of glycosylated hemoglobins. Relevance to monitoring of diabetic control and analysis of other proteins

Boronate affinity chromatography and ion exchange chromatography were used to measure the levels of glycosylated hemoglobins in normal and diabetic hemolysates, as well as the distribution of glucose adducts on alpha-NH2-valine and epsilon-NH2-lysine residues. When analyzed by ion exchange chromatography on BioRex 70 resin, the Hb Alc peak comprised 4.4 +/- 0.6% of 15 normal hemolysates and 9.1 +/- 2.1% of 15 diabetic hemolysates. The "Hb Alc" was rechromatographed on GlycoGel B boronate affinity resin that binds vicinal hydroxyl groups of covalently linked sugars. Only 70 +/- 5% of the hemoglobin adhered to the resin. Analysis by the thiobarbituric acid colorimetric test confirmed that the affinity resin effectively separated glycosylated from nonglycosylated hemoglobin. When corrected for nonglycosylated contaminants, the mean level of Hb Alc in normal hemolysates was 2.9 +/- 0.4%, a value considerably lower than those previously reported. In addition to Hb Alc, 5.2 +/- 0.5% of the remaining hemoglobin (Hb Ao) was glycosylated. In diabetics, glycosylated Ao was increased in parallel with Hb Alc. After reduction with [3H]borohydride and acid hydrolysis, glycosylated amino acids were first purified on Affi-Gel boronate affinity resin and then analyzed by ion exchange chromatography. The glucose adducts on Hb Ao were distributed as follows: alpha-chain N-terminal valine, 14%; alpha-chain lysines, 40%; beta-chain lysines, 46%. This study has revealed several pitfalls in the analysis of nonenzymatically glycosylated proteins. Peaks isolated by ion exchange chromatography or electrophoresis are likely to be contaminated by nonglycosylated proteins. Furthermore, both the thiobarbituric acid test and [3H]borohydride reduction show variable reactivity depending upon the site of the ketoamine-linked glucose.

Separation of glycosylated haemoglobins using immobilized phenylboronic acid. Effect of ligand concentration, column operating conditions, and comparison with ion-exchange and isoelectric-focusing

Haemoglobins from diabetic and non-diabetic individuals have been separated by affinity chromatography using immobilized phenylboronate, which interacts specifically with diol-containing compounds such as glycosylated haemoglobin. The effects of ligand concentration, flow rate, column geometry, preincubation of sample, buffer composition and temperature have been investigated. Significant correlation was found between results from affinity-chromatography and ion-exchange and isoelectric-focusing methods. Isoelectric-focusing of the haemoglobin fractions obtained from affinity chromatography indicate that, in addition to haemoglobin A1c, some haemoglobin A is also bound to immobilized phenylboronic acid. Assays of haemolysates obtained from red blood cells incubated in glucose solutions suggest that unstable pre-(haemoglobin A1c) does not interfere. The assay is not affected by the presence of haemoglobin F.