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

Physiologic Concepts That May Revise the Interpretation and Implications of HbA1C in Clinical Medicine: An American Perspective

HbA1c, a routinely used integrated measure of glycemic control, is traditionally thought to be equivalent to mean blood glucose in hematologically normal individuals. Therefore, particularly as the methodology of measuring HbA1c has been standardized, clinical decisions dependent on mean blood glucose are often predominantly decided based on the interpretation of measured HbA1c. In this commentary, however, now that a more routine method of measuring red cell life span has been developed, we present evidence that the relationship between HbA1c and mean blood glucose is influenced by variation in red blood cell survival even in the hematologically normal. This variation has consequences for the appropriate interpretation of HbA1c in diverse clinical conditions such as the diagnosis of diabetes and management of diabetes in chronic kidney disease.

Use of an oral stable isotope label to confirm variation in red blood cell mean age that influences HbA1c interpretation

HbA1c is commonly used to monitor glycemic control. However, there is growing evidence that the relationship between HbA1c and mean blood glucose (MBG) is influenced by variation in red blood cell (RBC) lifespan in hematologically normal individuals. Correction of HbA1c for mean RBC age (MRBC ) requires a noninvasive, accurate, and affordable method to measure RBC survival. In this study, we evaluated whether a stable isotope approach would satisfy these requirements. RBC lifespan and MRBC were determined in a group of nine hematologically normal diabetic and nondiabetic subjects using oral (15) N-glycine to label heme in an age cohort of RBC. The MRBC was 58.7 ± 9.1 (2SD) days and RBC lifespan was 106 ± 21 (2SD) days. This degree of variation (±15-20%) is consistent with previous studies using other techniques. In a subset of seven subjects, MRBC determined with the biotin label technique were available from approximately five years prior, and strongly correlated with the stable isotope values (R(2) = 0.79). This study suggests that the MRBC is stable over time but varies substantially among individuals, and supports the importance of its variation in HbA1c interpretation. The characteristics of the stable isotope method support its suitability for studies to directly evaluate the impact of variation in MRBC on the interpretation of HbA1c.

Efficacy of carbohydrate counting in type 1 diabetes: a systematic review and meta-analysis

BACKGROUND

Although carbohydrate counting is the recommended dietary strategy for achieving glycaemic control in people with type 1 diabetes, the advice is based on narrative review and grading of the available evidence. We aimed to assess by systematic review and meta-analysis the efficacy of carbohydrate counting on glycaemic control in adults and children with type 1 diabetes.

METHODS

We screened and assessed randomised controlled trials of interventions longer than 3 months that compared carbohydrate counting with general or alternate dietary advice in adults and children with type 1 diabetes. Change in glycated haemoglobin (HbA1c) concentration was the primary outcome. The results of clinically and statistically homogenous studies were pooled and meta-analysed using the random-effects model to provide estimates of the efficacy of carbohydrate counting.

FINDINGS

We identified seven eligible trials, of 311 potentially relevant studies, comprising 599 adults and 104 children with type 1 diabetes. Study quality score averaged 7·6 out of 13. Overall there was no significant improvement in HbA1c concentration with carbohydrate counting versus the control or usual care (-0·35% [-3·9 mmol/mol], 95% CI -0·75 to 0·06; p=0·096). We identified significant heterogeneity between studies, which was potentially related to differences in study design. In the five studies in adults with a parallel design, there was a 0·64% point (7·0 mmol/mol) reduction in HbA1c with carbohydrate counting versus control (95% CI -0·91 to -0·37; p<0·0001).

INTERPRETATION

There is some evidence to support the recommendation of carbohydrate counting over alternate advice or usual care in adults with type 1 diabetes. Additional studies are needed to support promotion of carbohydrate counting over other methods of matching insulin dose to food intake.

FUNDING

None.

Detection of proteins by hyphenated techniques with endogenous metal tags and metal chemical labelling

The absolute and relative quantitation of proteins plays a fundamental role in modern proteomics, as it is the key to understand still unresolved biological questions in medical and pharmaceutical applications.

Applications of reversible covalent chemistry in analytical sample preparation

Reversible covalent chemistry (RCC) adds another dimension to commonly used sample preparation techniques like solid-phase extraction (SPE), solid-phase microextraction (SPME), molecular imprinted polymers (MIPs) or immuno-affinity cleanup (IAC): chemical selectivity. By selecting analytes according to their covalent reactivity, sample complexity can be reduced significantly, resulting in enhanced analytical performance for low-abundance target analytes. This review gives a comprehensive overview of the applications of RCC in analytical sample preparation. The major reactions covered include reversible boronic ester formation, thiol-disulfide exchange and reversible hydrazone formation, targeting analyte groups like diols (sugars, glycoproteins and glycopeptides, catechols), thiols (cysteinyl-proteins and cysteinyl-peptides) and carbonyls (carbonylated proteins, mycotoxins). Their applications range from low abundance proteomics to reversible protein/peptide labelling to antibody chromatography to quantitative and qualitative food analysis. In discussing the potential of RCC, a special focus is on the conditions and restrictions of the utilized reaction chemistry.

HbA1c in pregnancy

During pregnancy, the glucose levels vary according to the hormonal changes and the metabolic needs necessary to maintain fetal nutrition but strict glycemic control is essential to minimize the maternal and fetal morbidity and mortality of pregnancies complicated by diabetes. Although considered the "gold standard" for diagnosis, measurement of glucose in the blood is subject to several limitations, many of which are not widely appreciated. Measurement of A1c for diagnosis is appealing as with one number, a total, integrated view of glycemia over time is derived though it has some inherent limitations. Thus, supplementation with HbA1c, as is common outside pregnancy, seems appropriate. Before pregnancy, the target for metabolic control in women with diabetes is HbA1c values near the normal range. However, the upper normal range of HbA1c during normal pregnancy is only sparsely investigated with different methods though recently a number of papers have been published regarding the determination of reference ranges for HbA1c in pregnancy. These changes may have clinical implications for the assessment and management of glycemic control in diabetic pregnancy and calls for establishment of separate reference limits of HbA1c levels in different trimesters as compared to general population.

ATP-dependent mechanism protects spectrin against glycation in human erythrocytes

Human erythrocytes are continuously exposed to glucose, which reacts with the amino terminus of the β-chain of hemoglobin (Hb) to form glycated Hb, HbA1c, levels of which increase with the age of the circulating cell. In contrast to extensive insights into glycation of hemoglobin, little is known about glycation of erythrocyte membrane proteins. In the present study, we explored the conditions under which glucose and ribose can glycate spectrin, both on the intact membrane and in solution and the functional consequences of spectrin glycation. Although purified spectrin could be readily glycated, membrane-associated spectrin could be glycated only after ATP depletion and consequent translocation of phosphatidylserine (PS) from the inner to the outer lipid monolayer. Glycation of membrane-associated spectrin led to a marked decrease in membrane deformability. We further observed that only PS-binding spectrin repeats are glycated. We infer that the absence of glycation in situ is the consequence of the interaction of the target lysine and arginine residues with PS and thus is inaccessible for glycation. The reduced membrane deformability after glycation in the absence of ATP is likely the result of the inability of the glycated spectrin repeats to undergo the obligatory unfolding as a consequence of interhelix cross-links. We thus postulate that through the use of an ATP-driven phospholipid translocase (flippase), erythrocytes have evolved a protective mechanism against spectrin glycation and thus maintain their optimal membrane function during their long circulatory life span.

The effect of oral antidiabetic agents on A1C levels: a systematic review and meta-analysis

OBJECTIVE

Previous reviews of the effect of oral antidiabetic (OAD) agents on A1C levels summarized studies with varying designs and methodological approaches. Using predetermined methodological criteria, we evaluated the effect of OAD agents on A1C levels.

RESEARCH DESIGN AND METHODS

The Excerpta Medica (EMBASE), the Medical Literature Analysis and Retrieval System Online (MEDLINE), and the Cochrane Central Register of Controlled Trials databases were searched from 1980 through May 2008. Reference lists from systematic reviews, meta-analyses, and clinical practice guidelines were also reviewed. Two evaluators independently selected and reviewed eligible studies.

RESULTS

A total of 61 trials reporting 103 comparisons met the selection criteria, which included 26,367 study participants, 15,760 randomized to an intervention drug(s), and 10,607 randomized to placebo. Most OAD agents lowered A1C levels by 0.5-1.25%, whereas thiazolidinediones and sulfonylureas lowered A1C levels by approximately 1.0-1.25%. By meta-regression, a 1% higher baseline A1C level predicted a 0.5 (95% CI 0.1-0.9) greater reduction in A1C levels after 6 months of OAD agent therapy. No clear effect of diabetes duration on the change in A1C with therapy was noted.

CONCLUSIONS

The benefit of initiating an OAD agent is most apparent within the first 4 to 6 months, with A1C levels unlikely to fall more than 1.5% on average. Pretreated A1C levels have a modest effect on the fall of A1C levels in response to treatment.

Proteolytic degradation by cathepsin D of glycated hemoglobin from diabetes patients gives rise to hemorphin-7 peptides

Previous studies showed a significantly reduced level of hemorphins in the serum of diabetes patients. In order to elucidate the biochemical mechanisms responsible for this anomaly, the influence of hemoglobin glycation on hemorphin generation was studied. The glycation of hemoglobin occurs in the blood of diabetes patients and this could modify its enzymatic digestion and the resulting proteolytic products. Several samples of hemoglobin were obtained from the blood of type 1 diabetes patients (n=8) and normal healthy control subjects (n=2). The glycated hemoglobin samples were classified on the basis of their HbA1c values expressed as a percentage of total hemoglobin. Four solutions of glycated hemoglobin characterized by HbA1c values of 6%, 9.1%, 10.7% and 12.1% were treated with cathepsin D and the hemorphins obtained following the proteolysis were compared to controls. It was found that hemorphins were produced whatever the level of glycation of hemoglobin and also that the degree of glycation had no effect on the quantity of hemorphins released. Thus the alteration of hemoglobin does not seem to be the essential reason for the decrease in hemorphin concentrations in the sera of diabetic patients.

Ribosylation rapidly induces alpha-synuclein to form highly cytotoxic molten globules of advanced glycation end products

Background

Alpha synuclein (α-Syn) is the main component of Lewy bodies which are associated with several neurodegenerative diseases such as Parkinson's disease. While the glycation with D-glucose that results in α-Syn misfold and aggregation has been studied, the effects of glycation with D-ribose on α-Syn have not been investigated.

Methodology/Principal Findings

Here, we show that ribosylation induces α-Syn misfolding and generates advanced glycation end products (AGEs) which form protein molten globules with high cytotoxcity. Results from native- and SDS-PAGE showed that D-ribose reacted rapidly with α-Syn, leading to dimerization and polymerization. Trypsin digestion and sequencing analysis revealed that during ribosylation the lysinyl residues (K₅₈, K₆₀, K₈₀, K₉₆, K₉₇ and K₁₀₂) in the C-terminal region reacted more quickly with D-ribose than those of the N-terminal region. Using Western blotting, AGEs resulting from the glycation of α-Syn were observed within 24 h in the presence of D-ribose, but were not observed in the presence of D-glucose. Changes in fluorescence at 410 nm demonstrated again that AGEs were formed during early ribosylation. Changes in the secondary structure of ribosylated α-Syn were not clearly detected by CD spectrometry in studies on protein conformation. However, intrinsic fluorescence at 310 nm decreased markedly in the presence of D-ribose. Observations with atomic force microscopy showed that the surface morphology of glycated α-Syn looked like globular aggregates. thioflavin T (ThT) fluorescence increased during α-Syn incubation regardless of ribosylation. As incubation time increased, ribosylation of α-Syn resulted in a blue-shift (∼100 nm) in the fluorescence of ANS. The light scattering intensity of ribosylated α-Syn was not markedly different from native α-Syn, suggesting that ribosylated α-Syn is present as molten protein globules. Ribosylated products had a high cytotoxicity to SH-SY5Y cells, leading to LDH release and increase in the levels of reactive oxygen species (ROS).

Conclusions/Significance

α-Syn is rapidly glycated in the presence of D-ribose generating molten globule-like aggregations which cause cell oxidative stress and result in high cytotoxicity.

D-Ribosylated Tau forms globular aggregates with high cytotoxicity

Although the glycation of Tau that is involved in paired helical filament formation in Alzheimer's disease has been widely studied, little attention has been paid to the role of D-ribose in the glycation of Tau. Here, we show that Tau is rapidly glycated in the presence of D-ribose, resulting in oligomerization and polymerization. Glycated derivatives appeared after 24 h incubation. Western blotting indicated the formation of advanced glycation end-products (AGEs) during initial stages of glycation. Thioflavin T-positive (ThT-positive) aggregations that appeared from day 4 indicated the globular-like features. Atomic force microscopy revealed that the surface morphology of ribosylated Tau40 was globular-like. Kinetic studies suggested that D-ribosylated Tau is slowly oligomerized and rapidly polymerized with ThT-positive features. Moreover, D-ribosylated Tau aggregates were highly toxic to SHSY5Y cells and resulted in both apoptosis and necrosis. This work has demonstrated that D-ribose reacted with Tau protein rapidly, producing ThT-positive aggregations which had high cytotoxicity.

Immunochemical detection of glycated lens crystallins and their circulating autoantibodies in human serum during aging

PURPOSE

The aim of this investigation was to exploit lens-specific glycated crystallins as an immunogen to detect human glycated crystallins and their circulating autoantibodies in human serum during aging in relation to the development of cataract.

METHODS

Polyclonal antibodies were produced against human total lens proteins (40-80 years) in rabbits. The specificity of the antibodies produced were determined by antibody capture assay using purified human lens crystallins (high molecular weight fraction [HMW]+alpha, HMW+alpha-glycated, beta, beta-glycated, gamma, and gamma-glycated) as antigens. The cross-reactivity of these lens specific antibodies against rat beta-, beta-glycated, gamma-, and gamma-glycated lens crystallins was also analyzed. A non-competitive enzyme linked immunosorbent assay (ELISA) methodology was developed for the detection of circulating lens crystallins in human sera using HMW+alpha, HMW+alpha-glycated, beta-, and beta-glycated crystallins from humans and gamma- and gamma-glycated crystallins from rats as immobilized antigens. Circulating autoantibodies were also detected in human sera by antibody capture assay. The methodology was validated by evaluating 60 human serum samples collected from cataract patients and 30 human serum samples from apparently normal subjects belonging to the same age group.

RESULTS

The polyclonal antibodies raised against human total lens proteins showed 90% and 65% cross-reactivity with rat gamma- and beta-crystallins, respectively, by ELISA. Further, these polyclonal antibodies were capable of detecting both native and in vitro synthesized glycated crystallins. Their IC50 values were observed to be (i) human total lens proteins (55 ng), (ii) human HMW+alpha (16.45 ng), (iii) human HMW+alpha-glycated (273 ng), (iv) human beta- (37.82 ng), (v) human beta-glycated (260 ng), (vi) rat gamma- (105.34 ng), and (vii) rat gamma-glycated (313 ng). The immunochemical analysis of human serum indicated a significant change (p<0.001) in the levels of circulating beta-glycated and gamma-glycated crystallins in the age group of 40-80 years with respect to their control groups. However, there was no statistically significant change in the levels of HMW+alpha-glycated crystallins in the age group of 40-80 years as compared to their age-matched controls. Notably, the levels of serum gamma-glycated crystallins were found to be threefold higher than that of HMW+alpha-glycated and beta-glycated crystallins in the age group of 70-80 years. Circulating autoantibodies to HMW+alpha-glycated, beta-glycated, and gamma-glycated crystallins were detected in the serum of both apparently normal and cataract patients in the age group of 40-80 years by antibody capture assay. The levels of these autoantibodies were significantly higher at every time point compared to their respective controls. Autoantibodies to gamma-glycated crystallins were found to be twofold and 3.2 fold higher as compared to the levels of autoantibodies to beta-glycated and HMW+alpha-glycated crystallins, respectively. Western blot and immunohistochemical analysis substantiated the observations made in non-competitive ELISA.

CONCLUSIONS

During the course of aging, leakage of lens crystallins (HMW+alpha, HMW+alpha-glycated, beta, beta-glycated, gamma, and gamma-glycated) elicit an immune response resulting in the formation of autoantibodies in cataract patients (40-80 years) as compared to age matched controls. This is the first experimental report where polyclonal antibodies raised against lens-specific glycated crystallins were capable of detecting the early leakage of glycated crystallins in human subjects. This immunochemical approach has implications in the early detection of senile cataract.

Enhanced glycation of hemoglobin and plasma proteins is associated with increased lipid peroxide levels in non-diabetic hypertensive subjects

BACKGROUND

Accumulating evidences indicate that lipid peroxidation and protein glycation play a vital role in the pathogenesis of cardiovascular disease. The purpose of the present study was to evaluate the levels of lipid peroxides and glycated proteins in non-diabetic hypertensive patients and to assess the possible nexus between them, among these subjects.

METHODS

Thirty hypertensive patients and 25 normotensive subjects were enrolled in the present study. Lipid peroxides, glycated hemoglobin, and fructosamine levels were estimated in both groups.

RESULTS

Lipid peroxides, glycated hemoglobin, and fructosamine levels were significantly increased in hypertensive subjects in comparison with normotensive subjects. When partial correlation analysis was performed, malondialdehyde was significantly associated with glycated hemoglobin and fructosamine.

CONCLUSIONS

An increased glycation of proteins was found in non-diabetic hypertensive subjects. These data also support the premise that lipid peroxidation per se plays a role in glycation of hemoglobin and plasma proteins.

Evidence for the role of lipid peroxides on glycation of hemoglobin and plasma proteins in non-diabetic asthma patients

BACKGROUND

Collective evidences reveal that malondialdehyde (MDA), reduced glutathione (GSH) and ascorbic acid can modulate protein glycation. We investigated the concentrations of MDA, GSH, ascorbic acid and protein glycation in asthma patients to delineate the possible association among these parameters.

METHODS

Blood was collected from 18 asthma patients and 16 age and sex matched control subjects. Glycated hemoglobin (HbA1C), GSH, MDA, vitamin C, fructosamine and glucose were assessed in both groups. The effect of H2O2 on glycation of hemoglobin was studied by incubating normal healthy erythrocytes with either 5 or 50 mmol/l glucose concentration.

RESULTS

Plasma of asthma patients revealed significantly higher concentrations of lipid peroxides and fructosamine concentrations than the matched controls. Glycated hemoglobin concentrations were also found to be significantly increased. Ascorbic acid and GSH concentrations were decreased significantly in the test group when compared with the healthy control group. When the effects of fasting glucose, GSH and ascorbic acid on the concentrations of HbA1C and fructosamine were refuted by partial correlation analysis, MDA was found to be a significant determinant of HbA1c and fructosamine in patients with asthma. The in vitro model with human erythrocytes showed an enhancement of protein glycation by H2O2.

CONCLUSION

An increased glycation of proteins was found in asthma patients. These data also support the premise that lipid peroxides per se do have a role to play in glycation of hemoglobin and plasma proteins.

HbA1c standardisation destination--global IFCC Standardisation. How, why, where and when--a tortuous pathway from kit manufacturers, via inter-laboratory lyophilized and whole blood comparisons to designated national comparison schemes

Glycohaemoglobins were first used in routine clinical laboratories for diabetes monitoring around 1977 and at the time all methods had either no calibrators, or used material with assayed values derived from individual manufacturers' assays. Over the next five to fifteen years, lyophilised and whole blood sample exchanges were shown to improve inter-laboratory variability markedly. The use of a precise HPLC method as the "standard method" in the Diabetes Control and Complications Trial (DCCT) led to significant further improvement. National standardisation schemes in the mid to late 1990s in the USA, Japan and Sweden further improved the quality and accuracy of HbA1c assays in clinical use. The work of the IFCC Working Group on Standardisation of HbA1c in establishing true International Reference Methods for HbA1c and the successful preparation of pure HbA1c calibration material should lead to further improvements in inter-method and inter-laboratory variability, essential to the long-term monitoring of patients with diabetes.

Possible link between glycated hemoglobin and lipid peroxidation in hyperthyroidism

BACKGROUND

Glycated hemoglobin (HbA1C) levels are enhanced by elevated glucose concentrations. Glycation of hemoglobin is also modulated by lipid peroxides, ascorbic acid and reduced glutathione (GSH). We determined the strength of the relationships among these variables in a group of hyperthyroid patients.

METHODS

Twenty-two untreated hyperthyroid patients and 17 healthy controls were recruited for the study. Whole blood GSH, HbA1C, plasma lipid peroxides, ascorbic acid and fasting glucose were analyzed in both the groups. Direct and partial correlation analysis was performed to explore the possible relationships between these variables.

RESULTS

In hyperthyroid patients, HbA1C and lipid peroxides levels were found to be significantly increased than the controls. Ascorbic acid and GSH were decreased significantly in the test group when compared with the healthy control group. With partial correlation analysis, fasting glucose and lipid peroxides were found to have a significant positive correlation with HbA1C. Ascorbic acid and GSH showed no significant association with HbA1C levels.

CONCLUSION

These data suggest that HbA1C levels are closely associated with fasting glucose and lipid peroxides in hyperthyroid patients. Therefore, serum lipid peroxides level should be kept in mind while interpreting HbA1C as a long-term glycemic index in hyperthyroid cases.

Advanced glycation end products in uremia

The term "advanced glycation end products" (AGEs) stands for a heterogeneous group of amino acid derivatives that are formed via glycation processes between peptide-bound lysine or arginine derivatives and carbonyl compounds, processes originally known from food systems as "Maillard reactions." AGEs accumulate in plasma and tissues with advancing age, diabetes, and particular renal failure. In vivo and in vitro studies indicate that AGEs represent an important class of uremic toxins. This review focuses on the chemistry behind the formation of AGEs, possible mechanisms underlying the accumulation of AGEs in uremia, clinical and therapeutic implications, and possible nutritional consequences.

Determination of haemoglobin A(1c) by liquid chromatography using a new cation-exchange column

The use of a new cation-exchange column, ProPac SCX-10, for the determination of haemoglobin A(1c) (HbA(1c)) by high-performance liquid chromatography is described. After optimization of the analytical method for the separation of the various isoforms of haemoglobin with the ProPac SCX-10 column, the method was applied to the determination of HbA(1c) in blood from 59 volunteers. Three of the 59 had previously been diagnosed as diabetics. Interference studies for carbamylation, acetylation and pre-HbA(1c) were carried out via "in-vitro" experiments. No interference due to carbamylation was observed at the urea values normally found in uremic patients undergoing dialysis. No interference from pre-HbA(1c) was detected either. The method is able to separate haemoglobin A (alpha(2)beta(2)), haemoglobin S (haemoglobin from sickle cell anaemia patients) and haemoglobin A(2) (alpha(2)delta(2)) without interference. The method of Hampel was applied to detect outliers. A value of 3.29+/-0.44% (2sigma) for HbA(1c) was obtained in the analysis of 56 blood samples from non-diabetics. This average value is lower than that reported by most of the methods currently used in routine analyses.

Evidence for the existence of two distinct functions for the inducible NO synthase in the rat kidney: effect of aminoguanidine in rats with 5/6 ablation

The functional role of the NO synthase (NOS) isoforms in the normal or diseased kidney is uncertain. This study examined the renal expression of the endothelial (eNOS), neuronal (nNOS), and inducible (iNOS) isoforms by both immunohistochemistry and Western blot analyses in sham-operated rats (S) and in rats subjected to 5/6 nephrectomy (Nx). Primary antibodies from two different sources were used to detect iNOS. Additional S and Nx rats were chronically treated with aminoguanidine (AG), a selective iNOS inhibitor. All three isoforms were clearly expressed in S kidney. Their renal abundance, evaluated by Western blot analysis, fell in Nx rats. With the use of anti-iNOS antibodies from two distinct sources, the immunohistochemical analysis showed the presence of what appeared to be two distinct iNOS fractions: a "tubular" fraction, present in S and with decreased intensity in Nx; and an "interstitial" fraction, observed only in inflamed areas of Nx rats. AG treatment greatly attenuated renal injury in Nx rats by a direct antiinflammatory effect, likely related to iNOS inhibition, rather than to amelioration of renal hemodynamics or to reduced protein glycation. These observations suggest that: (1) the functional role of the renal iNOS isoform may vary dramatically under different physiologic conditions; (2) caution should be taken in the interpretation of immunohistochemical iNOS data, because antibodies from different sources may detect different iNOS fractions; and (3) AG treatment may become useful in the treatment of human progressive nephropathies, even those not associated with diabetes or aging.

Approved IFCC reference method for the measurement of HbA1c in human blood

HbA1C is the stable glucose adduct to the N-terminal group of the beta-chain of HbA0. The measurement of HbA1c in human blood is most important for the long-term control of the glycaemic state in diabetic patients. Because there was no internationally agreed reference method the IFCC Working Group on HbA1c Standardization developed a reference method which is here described. In a first step haemoglobin is cleaved into peptides by the enzyme endoproteinase Glu-C, and in a second step the glycated and non-glycated N-terminal hexapeptides of the beta-chain obtained are separated and quantified by HPLC and electrospray ionisation mass spectrometry or in a two-dimensional approach using HPLC and capillary electrophoresis with UV-detection. Both principles give identical results. HbA1c is measured as ratio between the glycated and non-glycated hexapeptides. Calibrators consisting of mixtures of highly purified HbA1c and HbA0 are used. The analytical performance of the reference method has been evaluated by an international network of reference laboratories comprising laboratories from Europe, Japan and the USA. The intercomparison studies of the network showed excellent results with intra-laboratory CVs of 0.5 to 2% and inter-laboratory CVs of 1.4 to 2.3%. Possible interferences have been carefully investigated. Due to the higher specificity of the reference method the results are lower than those generated with most of the present commercial methods which currently are calibrated with unspecific designated comparison methods. The new reference method has been approved by the member societies of the International Federation of Clinical Chemistry and Laboratory Medicine and will be the basis for the future uniform standardization of HbA1c routine assays worldwide.

Human fructosamine-3-kinase: purification, sequencing, substrate specificity, and evidence of activity in vivo

Nonenzymatic glycation appears to be an important factor in the pathogenesis of diabetic complications. Key early intermediates in this process are fructosamines, such as protein-bound fructoselysines. In this report, we describe the purification and characterization of a mammalian fructosamine-3-kinase (FN3K), which phosphorylates fructoselysine (FL) residues on glycated proteins, to FL-3-phosphate (FL3P). This phosphorylation destabilizes the FL adduct and leads to its spontaneous decomposition, thereby reversing the nonenzymatic glycation process at an early stage. FN3K was purified to homogeneity from human erythrocytes and sequenced by means of electrospray tandem mass spectrometry. The protein thus identified is a 35-kDa monomer that appears to be expressed in all mammalian tissues. It has no significant homology to other known proteins and appears to be encoded by genomic sequences located on human chromosomes 1 and 17. The lability of FL3P, the high affinity of FN3K for FL, and the wide distribution of FN3K suggest that the function of this enzyme is deglycation of nonenzymatically glycated proteins. Because the condensation of glucose and lysine residues is an ubiquitous and unavoidable process in homeothermic organisms, a deglycation system mediated by FN3K may be an important factor in protecting cells from the deleterious effects of nonenzymatic glycation. Our sequence data of FN3K are in excellent agreement with a recent report on this enzyme by Delpierre et al. (Diabetes 49:1627-1634, 2000).

Characterization of glycated hemoglobin in diabetic patients: usefulness of electrospray mass spectrometry in monitoring the extent and distribution of glycation

A combination of chromatographic and mass spectrometric techniques was used to evaluate the extent and distribution of glycation within the glycated hemoglobin (GHb) molecule. Studies on quantification of hemoglobin (Hb) glycation by electrospray ionization mass spectrometry (ES-MS) of intact globins employed specimens from 10 diabetic individuals and five normal controls. Detailed structural analysis of the phenylboronate affinity chromatography/ion-exchange (IE) HPLC-separated sub-populations of GHb was performed on a specimen carrying 13.7% GHb. An efficient protocol for mapping glycation sites within alpha and beta globins was developed, e.g., Glu-C/Asp-N proteolytic digestion followed by LC-ES-MS. Relative site occupancy within discrete components of GHb was evaluated. A correlation between the degree of glycation measured at Hb level (by affinity chromatography) and at globin level (measured by ES-MS) was carried out. The above studies led us to conclude that during the process of phenylboronate chromatography GHb dimers, rather than tetramers, are bound to the affinity resin so a fraction of glycated dimers rather than tetramers is measured. This finding implies that a process of glycation affects a much higher number of native Hb tetramers than was previously contemplated. No glycation sites appear to be missed by phenylboronate affinity chromatography. We have found no evidence of the presence of multiple glycations within a single globin chain. While glycation of both globins within a dimer cannot be excluded, it is unlikely to be a significant phenomenon. According to ES-MS data, an equivalent of about one globin per alphabeta dimer of the affinity chromatography-isolated GHb carried glycation.

The narrow therapeutic window of glycated hemoglobin and assay variability

Glycated hemoglobin is measured by a variety of assays, each of which has a unique normal level. Our purpose is to show that among the different assays available in the United States, using the same patient's blood sample, assay results may vary widely and may more or less easily achieve a glycated hemoglobin value within the normal range. The following assays were compared using the same patient's blood sample for each pair of assays: glycohemoglobin affinity assay (GHB Reader; Isolab, Akron, OH) versus gel electrophoresis assay (n = 76); Isolab versus ion capture assay (IMX; Abbott Laboratories, Irving, TX) (n = 57); monoclonal antibody assay (DCA2000; Bayer Diagnostics, Pittsburgh, PA) versus IMX (n = 100); and high-performance liquid chromatography (HPLC) assay (Bio-Rad Variant A1c; Bio-Rad Laboratories, Richmond, CA) versus IMX assay (n = 55). Our analyses indicate that a relative ranking can be established for the ease of achieving a normal glycated hemoglobin level. The ranking indicates that the most stringent or difficult assays for achieving a normal level are the Isolab and DCA2000 assays. The intermediate assays are the IMX and Bio-Rad Variant, and the easiest method for achieving a normal value is the gel electrophoresis assay. Our results indicate that various glycated hemoglobin assays vary widely and are associated with more or less difficulty for an individual patient to achieve a glycated hemoglobin level within the normal range. These results are especially significant with respect to (1) the clinically narrow therapeutic window of glycated hemoglobin values in type 1 diabetes to avoid rapidly advancing severe hypoglycemia rates and chronic microvascular complication rates, and (2) the glycated hemoglobin threshold for rapidly advancing macrovascular disease in both type 1 and type 2 patients.

Measuring glycated proteins: clinical and methodological aspects

The recognition that nonenzymatic glycation, wherein free sugar condenses with certain reactive protein amino groups, is increased in hyperglycemic states has led to improved means for assessing diabetic control through the measurement of concentrations of glycated proteins in the circulation. This article reviews the chemistry, methods for measurement, and clinical relevance of circulating glycated proteins in the management of diabetes.

Preparation of a candidate primary reference material for the international standardisation of HbA1c determinations

We prepared a candidate primary reference material for the forthcoming international standardisation of beta-N-terminal glycated hemoglobin A measurements. It consists of well-defined mixtures of purified beta-N-terminal glycated hemoglobin A and non-glycated hemoglobin A. First, beta-N-terminal glycated hemoglobin A and non-glycated hemoglobin A were isolated, purified to homogeneity, and characterised. The techniques used were cation exchange and affinity chromatography for the purification, and high performance liquid chromatography, capillary isoelectric focusing, electrospray ionisation mass spectrometry, and peptide mapping for the characterisation. Hemoglobins from blood of healthy, non-diabetic volunteers were obtained with a purity of > 99.5% for non-glycated hemoglobin A and of > 98.5% for beta-N-terminal glycated hemoglobin A. However, results from peptide mapping indicate that the beta-N-terminal glycated hemoglobin A preparations still contain some non-beta-N-terminal glycated hemoglobins, co-eluting with beta-N-terminal glycated hemoglobin A. The exact content of beta-N-terminal glycated hemoglobin A in these preparations could be determined by a procedure consisting of standard addition, enzymatic cleavage and quantification of the resulting beta-N-terminal peptides to be in the range from 95-97.5%. Since the beta-N-terminal glycated hemoglobin A and non-glycated hemoglobin A content could be exactly determined in the materials prepared, mixtures of both components could be successfully used to calibrate the candidate reference methods.

Candidate reference methods for hemoglobin A1c based on peptide mapping

A reference method that specifically measures hemoglobin (Hb) A1c is an essential part of the reference system for the international standardization of Hb A1c/glycohemoglobin. We have developed a new method for quantification, based on the specific N-terminal residue of the hemoglobin β-chains. Enzymatic cleavage of the intact hemoglobin molecule with endoproteinase Glu-C has been optimized to obtain the β-N-terminal hexapeptides of Hb A1c and Hb A0. These peptides have been separated by reversed-phase HPLC and quantitated by electrospray ionization-mass spectrometry (method A) or by capillary electrophoresis (method B). With these peptides and hyphenated separation techniques, it has been possible to overcome the insufficient resolution of currently used protein separation systems for Hb A1c.

Potential of electrospray mass spectrometry for quantifying glycohemoglobin

An electrospray ionization–mass spectrometric procedure has been developed for determining glycohemoglobin. Whole-blood samples from 78 diabetic and 50 nondiabetic subjects (glycation range 3–15%, as determined by electrospray mass spectrometry) were diluted 500-fold in an acidic denaturing solvent and introduced directly into a mass spectrometer. The resulting mass spectra were then processed to estimate the percentage of glycohemoglobin present in the sample. Total analysis time, including plotting the spectra and computing the percentage of glycation, was ∼3 min. The imprecision (CV) of the method was &lt;5.1% for inter- and intrabatch analyses for total glycohemoglobin in the range 3.6–14%. Comparison of the mass spectrometric results with those from established affinity chromatographic procedures showed good overall agreement. The relative glycation of the α- and β-chains was determined directly and was shown to be constant (0.64:1) over the glycation range measured. Only single glucose attachment to both the α- and β-chains was observed.

Investigation of the mechanism underlying the variability of glycated haemoglobin in non-diabetic subjects not related to glycaemia

The Islington Diabetes Survey identified two groups of non-diabetic individuals, low and high glycators, who remained consistently classified 4.4 +/- 0.2 years after the original study. To investigate the mechanism for this grouping, 12 original subjects, 5 with low and 7 with high levels of glycated haemoglobin relative to their 2 h blood glucose, were studied. Glycated albumin and fructosamine measurements gave comparable classifications, with three individuals being misclassified for each measurement; in addition glycated albumin was positively correlated with mean blood-glucose concentration (r = 0.53; P < 0.05). Fasting plasma glucose concentration was greater than the intra-erythrocyte concentration (P < 0.05), but their ratio was reduced in low compared to high glycators (0.77 +/- 0.12 and 0.94 +/- 0.13, P < 0.0001). No differences between groups were found for plasma insulin, urea or non-esterified fatty acids; plasma or intra-erythrocyte inorganic phosphate or vitamin C; nor plasma, erythrocyte or urinary total amino acids. Erythrocyte 2,3-diphosphoglycerate, a catalyst of glycation, was elevated in high compared to low glycators (5.61 +/- 0.26 and 4.81 +/- 0.24 mmol/l, P < 0.001). Mean centile glycated haemoglobin was positively correlated with intra-erythrocyte pH (r = 0.55; P < 0.05) and negatively with plasma total amino acids (r = -0.57, P < 0.05). These data indicate that the intra-erythrocyte environment of high glycators favours glycation of haemoglobin. This could have important consequences for diabetic patients in terms of monitoring their glycaemic control and in the progression of those complications related to non-enzymic glycation of intracellular proteins.

Crystallin composition of human cataractous lens may be modulated by protein glycation

BACKGROUND

This study was designed to establish whether increased glycation of human crystallins could be related to an increased susceptibility to aggregation and insolubilization. The study was focused particularly on the glycation levels and composition of low-molecular-weight (LMW) peptides present in human cataractous lenses.

METHODS

Lens crystallins from the water-soluble fraction were separated on a preparative scale by gel filtration. Each crystallin was purified and its glycation level evaluated as furosine content. The peptides were further purified by reverse-phase chromatography. The amino acid composition of each of these peptides was also determined by RP-HPLC using PITC pre-column derivatization.

RESULTS

The high-molecular-weight (HMW), alpha L-crystallin and LMW crystallins from diabetic patients present high furosine content. LMW peptides were shown to constitute a heterogeneous population of three major peptides with a lysine content similar to that observed for native crystallins. These peptides were shown to present glycation levels ten times higher than those observed for the crystallins. Glycated proteins from insoluble fraction were found to be mostly urea soluble and were present at higher concentration in diabetic cataracts.

CONCLUSIONS

LMW peptides are suggested to play a major role in protein aggregation and insolubilization, probably via a mechanism involving protein glycation. This process seems to be particularly relevant to diabetic cataract development.

Novel magnetic microspheres on the basis of poly(vinyl alcohol) as affinity medium for quantitative detection of glycated haemoglobin

A water-in-oil suspension cross-linking technique using poly(vinyl alcohol) as polymer phase to prepare novel magnetic microbeads is described. By dispersing a conventional Fe3O4 pigment in the polymer phase and subsequently suspending the mixture in a vegetable oil phase with a defined viscosity, spherical magnetic microspheres are obtained. Bead sizes ranging from 1 to 50 microns and above can be obtained by exploiting well defined preparation parameters such as polymer concentration and oil and polymer viscosity. The performance of the magnetic matrices for the separation and quantification of glycated haemoglobin was tested using a m-aminophenylboronic acid matrix. The feasibility of this detection method for blood sugar diagnosis is discussed using a commercial column test kit for comparison.

Measurement of non-enzymic glycosylation with a radiochemical assay

1. A radiochemical assay has been developed to measure non-enzymic glycosylation (glycation), which is free from interference from enzymic glycosylation and which can be used in batch analysis. 2. Glycated residues were isolated by affinity chromatography on mini-columns, after reduction with Na[3H]BH4 and hydrolysis. 3. The specific activity of the labelled borohydride was measured by a novel procedure. 4. The method was used to measure glycation of albumin, immunoglobulin G and fibrinogen isolated from diabetic and non-diabetic subjects.

Discrepancy in HbA1c measurements performed at different local laboratories and at a selected central reference laboratory

As participants in a general practice intervention study, 66 patients had their HbA1c measured both at a local and at a selected central reference laboratory. A discrepancy in the results was observed, as 97% of the results measured locally were lower than the centrally determined results. Bias (as calculated from mean value of measured HbA1c) between local laboratories and the central laboratory was measured to -1.47% HbA1c. A bias of this magnitude gave "problems" both to the general practitioners, patients and laboratories. To reduce the "problems" a bias of 0.5% HbA1c is estimated to be acceptable. But, to avoid these "problems" totally, a bias of 0.25% HbA1c is estimated to be the highest allowed bias. For HbA1c, a control system for both control of method standardisation and for specificity is described.

The use of glipizide combined with intensive insulin treatment for the induction of remissions in new onset adult type I diabetes

To determine if glipizide could enhance remission induction in new onset type 1 diabetes compared to intensive insulin treatment alone, 27 patients with type 1 diabetes were intensively treated in an open randomized trial with subcutaneous injections for one month. The insulin was randomly either discontinued (Group A) or the insulin discontinued and glipizide begun (Group B) Three patients in Group A (22%) and 7 in Group B (54%, p < .05) underwent insulin-free remissions for 10.3 +/- 4.4 and 8.7 +/- 2.6 months, respectively (p = NS). Mean blood glucose levels during insulin treatment were lower in patients entering remissions (94 +/- 3 mg/dl versus 102 +/- 5 mg/dl, p < 0.05). C-peptide levels were performed 0, 4, 8, and 24 weeks after insulin treatment. When all patients were examined, mean stimulated C-peptide levels at 4 weeks (0.58 +/- 0.09 pm/ml) were increased compared to time 0 (0.32 +/- 0.05 pm/ml, p < 0.02). Patients not entering remission had higher 4-week stimulated values (0.67 +/- 0.12 pm/ml) compared to time 0 values (0.29 +/- 0.06 pm/ml, p < .01), whereas remission patients' mean C-peptide levels remained similar at 0, 4, 8 and 24 weeks. These data indicate that a) insulin treatment plus glipizide induces higher rates of remission compared to intensive insulin treatment alone, b) the intensity of initial metabolic control may be an important determinant for remission induction, and c) endogenous insulin secretion is not associated with remission induction, suggesting that glipizide alters insulin sensitivity or is immunomodulatory in the context of new onset type 1 diabetes.

Direct evidence for the alterations in protein structure and conformation upon in vitro nonenzymatic glycosylation

The formation of nonenzymatic glycosylation products appears to be a link between chronic hyperglycaemia and long-term diabetic complications. However, little is known concerning the glycation-induced modifications in the structure and conformation of proteins, which possibly underlie their altered functional characteristics. This study conveys a direct evidence for and compares the glucose-induced modifications in the conformation of three proteins with various half-lives: bovine serum albumin, human haemoglobin and bovine tendon collagen. These proteins incubated in vitro with glucose in various media containing optionally EDTA and Fe2+ ions contained up to 4-10 times as much attached glucose as did their relevant controls, and the extent of glycation was the highest in the samples incubated under air or in the absence of EDTA. The fluorescence and ESR data indicate that the Trp in albumin molecule, given albumin glycation-induced structural modifications, became more exposed to water surrounding solution whereas the Trp residues of haemoglobin remained shielded from water; also collagen fluorescence derived from the supposedly newly formed covalent crosslinks is vastly increased, and particularly when collagen was glycated under air or in the presence of Fe2+ ions. Possible mechanisms underlying the increased mobility of selected protein domains and glycation-mediated alterations in protein conformation are considered and discussed.

Nifedipine versus fosinopril in uninephrectomized diabetic rats

Antihypertensive agents have been shown to exert inequivalent effects on glomerular injury in experimental renal disease models. To compare the consequences of dissimilar antihypertensive regimens on the development of diabetic glomerulopathy, studies were performed in three groups of uninephrectomized moderately hyperglycemic diabetic rats. One group (DM) received no therapy except insulin. The remaining groups received insulin and either the angiotensin I converting enzyme inhibitor, fosinopril (FOS), or the calcium channel blocker, nifedipine (NIF). Both drugs lowered blood pressure comparably. At four to eight weeks, DM rats exhibited elevation of the single nephron glomerular filtration rate (SNGFR), due to elevations of the glomerular capillary plasma flow rate (QA) and the glomerular capillary hydraulic pressure (PGC). Neither NIF nor FOS affected values for SNGFR or QA. However, while FOS lowered PGC and increased Kf, NIF did not affect these parameters. In longer term (8 month) studies, DM rats exhibited progressive albuminuria and glomerular sclerosis. FOS markedly limited development of albuminuria and glomerular injury, but NIF was ineffective in limiting either parameter of glomerular injury. Thus, in contrast to the beneficial effects of converting enzyme inhibitors, chronic calcium channel blockade with nifedipine fails to limit PGC or glomerular injury in diabetic rats. These findings lend further support to the concept that different classes of antihypertensive agents are not equally effective in protecting against diabetic glomerulopathy.

Increased permeability across the blood-nerve barrier of albumin glycated in vitro and in vivo from patients with diabetic polyneuropathy

The blood-nerve transfer of human plasma albumin glycated with D-glucose was investigated by measuring the permeability coefficient-surface area product (PS) of the blood-nerve barrier to radioiodinated albumin in normal adult rat sciatic nerve. Human albumin (ALB) from normal individuals, freshly isolated by CM-Affi-Gel Blue affinity chromatography, was glycated in vitro for 1, 3, 10, 19, and 30 weeks. Glycated ALB (gALB) was separated from the nonglycated form by boronate-affinity chromatography. The efficiency of this separation was assessed by chromatography of ALB glycated with [14C]glucose and by rechromatography of isolated ALB and gALB after radioiodination. The gALB was also shown to have a higher molecular weight and be completely separated from ALB after SDS/pore gradient electrophoresis in a Tris borate/EDTA buffer. After 1 week of glycation, the gALB PS was 2.2-fold greater than the ALB PS (0.724 +/- 0.063 x 10(-6) vs. 0.328 +/- 0.053 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001) and it increased with the time of glycation reaching a maximum value of 16.2-fold greater at 30 weeks (4.656 +/- 1.117 x 10(-6) vs. 0.288 +/- 0.042 x 10(-6) ml.g-1.s-1; mean +/- SD; P less than 0.0001). No change was observed in the residual endoneurial plasma volume. In addition, the PS of gALB isolated from patients with diabetic polyneuropathy was significantly increased (P less than 0.0001) compared to the PS for ALB isolated from the same patients. It is hypothesized that the increased permeability of gALB and presumably other glycated serum components across the blood-nerve barrier, as well as the observed quantitative increase in ALB, IgG, and IgM in sural nerve biopsies from patients with diabetic polyneuropathy contribute to the development of diabetic polyneuropathy over a prolonged period of time by mechanisms that might involve osmotic changes in the nerve microenvironment, direct toxic effects of glycated macromolecules on cells within the endoneurium, or nerve damage by classical immunological mechanisms due to trapping of glycated immunoglobulins within nerve.

Production and characterization of monoclonal antibodies against non-A1c glycated hemoglobin

Hybridomas secreting monoclonal antibodies specific for hemoglobin nonenzymatically glycated in the non-A1c position were produced by fusion of SP 2/0 myeloma cells with spleen cells from BALB/c mice immunized with nonenzymatically glycated hemoglobin prepared from human erythrocytes. Wells containing hydridomas secreting antibodies against glycohemoglobin were identified by binding, in an enzyme-linked immunosorbent assay, to purified glycated hemoglobin. The colony designated E85, which secreted antibodies discriminating between glycated versus unglycated hemoglobin, was cloned at least four times by limiting dilution and used for further study, performed with purified monoclonal antibody. Specificity of E85 was demonstrated by immunoblotting and by ELISA, wherein the monoclonal antibody reacted with glycated hemoglobin but not with hemoglobin A1c or with unglycated hemoglobin. Immunoblotting of human plasma with E85 on nitrocellulose yielded no reactive proteins, indicating site specificity for glycated epitopes residing in hemoglobin but not in other nonenzymatically glycated proteins present in plasma. E85 differs from other antibodies raised against glycated hemoglobin and other glycated proteins, which recognize hemoglobin glycated at the N terminal valine of the beta chain (HbA1c) or which recognize glycated residues only after reductive conversion to glucitollysine and which do not discriminate between different glycated proteins. Thus, this report describes the establishment of the first hybridoma secreting monoclonal antibody raised against a physiologic (unreduced) form of non-A1c glycohemoglobin, and for the glycated epitope when it resides in glycohemoglobin but not in other proteins or in hemoglobin A1c.

Purification of glycated hemoglobin free of hemoglobin A1c and its use to produce monoclonal antibodies specific for deoxyfructosyllysine [correction of deoxyfructosyllsine] residues in glycohemoglobin

Hemoglobin nonenzymatically glycated at E-amino groups of lysine residues was purified from human erythrocyte lysates and used for immunization of BALB/c mice. Hybridomas secreting monoclonal antibodies for glycated hemoglobin were produced by fusion of mouse spleen cells with SP 2/0 myeloma cells. Immunoblotting with purified monoclonal antibody demonstrated specificity for glycated hemoglobin, with no reaction with HbAO. Glycated hemoglobin was effectively separated from other hemoglobins upon application of erythrocyte lysates to an affinity column of monoclonal antibody immobilized onto Sepharose 4B. A small fraction of purified HbA1c adsorbed to the monoclonal antibody affinity column, indicating that glycation can occur at both E-amino lysine and N-terminal valine positions in the same molecule. HbA1c did not react with the antibody after removal by immunoadsorption of molecules containing glycated lysine, confirming specificity of the antibody for deoxyfructosyl-lysine residues. The findings indicate that these monoclonal antibodies are site specific for glycated lysine amino groups in hemoglobin, and can provide rapid and efficient separation and identification of glycated hemoglobin in human erythrocyte lysates.

Evaluation of the fructosamine test in obesity: consequences for the assessment of past glycemic control in diabetes

The effect of obesity in diabetic and nondiabetic states on serum fructosamine levels, as measured by the nitro blue tetrazolium reduction method, was investigated. In 26 nondiabetic obese subjects, the mean (SD) fructosamine (1.78 +/- 0.16 mmol/L) and protein corrected fructosamine concentrations (25.7 +/- 2.5 mumol/g) were significantly lower than in nondiabetic lean control subjects (2.06 +/- 0.18 mmol/L and 30.5 +/- 2.5 mumol/g, respectively; p less than 0.01). Hemoglobin A1C, blood glucose and serum protein concentrations were normal in obese subjects. Interference from hypertriglyceridemia, hemolysis, or drugs was excluded. In diabetic subjects, fructosamine correlated with hemoglobin A1C, but the least-squares regression lines were different in 16 nonobese and in 19 obese patients, so that for the same hemoglobin A1C value, fructosamine level was 16% lower in obese compared to nonobese diabetic subjects. In vitro studies showed a significant decrease in 14C-glucose incorporation in serum proteins of obese nondiabetic subjects compared to control subjects. Similarly, the rate of formation of fructosamine in sera of obese nondiabetic subjects incubated with 12 mmol/L and 30 mmol/L glucose concentrations was slower than in sera of control subjects. In conclusion, fructosamine is underestimated in obesity, both in diabetic and nondiabetic patients, and its validity as an index of glycemic control may be impaired in obese subjects. This decrease is due to an alteration in the glycation process itself.

The principal site of glycation of human complement factor B

Accumulating amino acid sequence data have made it increasingly evident that many essential complement proteins have potentially modifiable lysine residues in putative critical functional regions. Evidence is now presented that glucose is covalently attached to lysine-266 of purified human complement Factor B as a result of glycation. Purified B was treated with NaB3H4, which reduces such bound glucose to a mixture of radiolabelled hexitols. Amino acid analysis revealed the expected radiolabelled hexitol-lysine epimers. In addition, fluorography of dried gels resolving the major high-molecular-mass h.p.l.c.-fractionated CNBr-cleavage peptides of NaB3H4-reduced B indicated that this radioactivity was specifically associated with the 15 kDa fragment derived from the N-terminal region of fragment Bb. Amino acid sequence analysis suggested that the C-terminal lysine (residue 266 of B) of the N-terminal Lys-Lys doublet of this peptide is preferentially modified. If such glycation can subsequently be shown to occur in vivo, then perhaps this modification might also be found to affect the functional activity of B and offer a potential explanation for some of the immunopathological complications of diseases exposing key plasma proteins, such as this active-site-containing proteinase of the multimeric alternative-complement-pathway C3/C5 convertases, to long-term high concentrations of glucose, such as the decreased resistance to infection and impaired chemotaxis and phagocytosis characteristic of diabetes.