Glycated albumin, not hemoglobin A1c, predicts cardiovascular hospitalization and length of stay in diabetic patients on dialysis

BACKGROUND

The utility of glycated hemoglobin (HbA1c) and glycated albumin (GA) in diabetic dialysis patients remains unknown. GA was previously associated with all-cause hospitalization and patient survival. Relationships between GA, HbA1c, and casual plasma glucose (PG) with cause-specific cardiovascular (CV) disease, infectious disease (ID), and vascular access- (VA) related hospitalization rates and length of stay (LOS) were assessed.

METHODS

444 prevalent diabetic dialysis patients had monthly PG, quarterly GA, and all HbA1c values recorded for 2.33 years; hospitalizations within 17 and 30 days of testing were evaluated. Best-fit, time-dependent Cox models were constructed in unadjusted, case-mix-adjusted (age, sex, race, BMI, diabetes duration, dialysis vintage), and case-mix- plus lab-adjusted (hemoglobin, albumin, phosphorus) models.

RESULTS

Mean ± SD diabetes duration was 18.5 ± 10.8 years and dialysis vintage 2.9 ± 2.6 years. In fully adjusted models, CV hospitalization rates were associated with increasing GA (HR 1.32; 95% CI 1.11-1.57; p = 0.002 at 17 days; HR 1.21; p = 0.02 at 30 days) and PG (HR 1.10; 95% CI 1.02-1.17; p = 0.01 at 17 days; HR 1.07; p = 0.03 at 30 days), not HbA1c (HR 1.24; 95% CI 0.89-1.73; p = 0.21 at 17 days; HR 1.26; p = 0.10 at 30 days). LOS for CV admissions was positively associated with GA (HR 1.18; 95% CI 1.01-1.39; p = 0.03), not PG (HR 1.04; 95% CI 0.99-1.10; p = 0.15) or HbA1c (HR 1.03; 95% CI 0.92-1.15; p = 0.21). Admissions due to ID and VA complications (and LOS) did not correlate with these assays.

CONCLUSIONS

Improved glycemic control based on GA and PG predicted CV-related hospitalizations; GA also predicted CV hospitalization LOS. HbA1c did not predict cause-specific hospitalizations in dialysis populations.

A novel method to quantify in vivo transferrin glycation: applications in diabetes mellitus

BACKGROUND

In vitro glycation of transferrin leads to increased oxidative stress by impairing iron-binding antioxidant capacity. The aim of this study is to develop a method to evaluate in vivo transferrin glycation in diabetes.

METHODS

We adapted the nitroblue tetrazolium assay to measure in micro-well plates the fructosamine content of transferrin isolated from serum by immunocomplexation.

RESULTS

Introduction of the immunocomplexation step did not affect the analytical performance of the fructosamine measurement and analytical variability was lower than 7%. The diabetic group (n=107) had significantly higher transferrin glycation (1.39+/-1.12 versus 0.79+/-1.09 micromol fructosamine/g transferrin in the non-diabetic group, n=91, p<0.0005) and this was most pronounced in type 1 diabetes (1.95+/-1.02 versus 1.06+/-1.04 micromol fructosamine/g transferrin in type 2, p<0.0005). Transferrin glycation was associated with parameters of glycaemic control but did not correlate with serum iron or total iron-binding capacity. Total iron-binding capacity was lower in type 1 diabetes (63+/-9 versus 69+/-12 micromol/l in type 2, p<0.05) and was mainly determined by transferrin concentration.

CONCLUSIONS

These results indicate that the adapted nitroblue tetrazolium assay combined with immunocomplexation of serum transferrin is suitable to detect differences in in vivo transferrin glycation between non-diabetic, type 1 and type 2 diabetic subjects.

Fructosamine assay using albumin extracted from serum

Albumin extracted from serum by a simple technique using trichloroacetic acid and ethanol has been applied to a fructosamine assay using nitroblue tetrazolium. A fructosamine assay using extracted albumin sample was carried out without interference from low molecular weight substances with reducing activities and other proteins with varying concentrations, half-lives and reducing activities. 1-Deoxy-1-p-toluidino-D-fructose (DTF) was selected as a calibrator for the assay because it is a stable Amadori reaction product with a constant glycation rate. Albumin fructosamine value was calculated using the calibration curve of DTF. The corrected albumin fructosamine value was expressed as the amount of albumin fructosamine per gram of extracted albumin taking into consideration the variation in albumin concentrations in sera from patients. The corrected albumin fructosamine values correlated more closely with the fasting blood glucose levels (r=0.735) than the serum fructosamine values corrected for albumin concentrations (r=0.514) (p<0.05).

Erythrocyte membrane glycation and NA(+)-K(+) levels in NIDDM

Although there are many investigations on protein glycation in diabetic patients, many detailed studies are needed on this subject. In this study, the correlation between red cell membrane and serum protein glycation was investigated in NIDDM. The relation of membrane glycation to intracellular Na(+) and K(+) levels was also considered. Forty patients with NIDDM and 22 healthy subjects were included in the study. The membrane proteins were isolated, and total protein (TP(m)) and fructosamine (FA(m)) levels were determined. Serum glucose, fructosamine (FA(s)) and total protein (TP(s)) levels were also measured. HbA(1C), red blood cell (RBC) and reticulocyte (RET) counts in whole blood were made in all samples. NA(+) and K(+) levels of both serum and RBC were determined. The patient group had lower levels of K(+)(RBC) (P<.001) and Na(+)(s) (P<.05) and RBC count (P<.05), and higher levels of FA(m) (P<.001), Na(+)(RBC) (P<.01), K(+)(s) (P<.01), glucose (P<.001) and HbA(1C) (P<.001) than those of controls. The ratios of FA(s)/TP(s) (P<.001) and FA(m)/TP(m) (P<.001) were higher in patients than in control. As a result, HbA(1C) levels and the ratio of FA(m)/TP(m) were high in NIDDM patients (P<.001) and these patients have slight negative correlations in FA(m)/TP(m) and FA(s)/TP(s) (P<.05). On the other hand, that there is no correlation between RBC membrane protein glycation and RBC Na(+) and K(+) levels may be caused by the fact that the membrane protein glycation is lower than that of other soluble proteins and that the membrane proteins are functional with respect to Na(+)-K(+) transport.

Serum immunoglobulin concentrations in diabetic patients

The relationship between glycated haemoglobin (an index of long-term diabetic control), fructosamine (an index of intermediate-term diabetic control), and serum IgA, IgG, and IgM was studied in 110 diabetic patients (41 Type 1 and 69 Type 2) and compared with 111 healthy non-diabetic subjects. Significant increases in serum IgA (by 82.7%, p < 0.001) and IgG (by 35.2%, p < 0.001) concentrations were observed whereas the concentration of IgM was significantly decreased (by 46.7%, p < 0.001) in diabetic patients compared with non-diabetic subjects. Using Spearman's rank correlations, IgA correlated with fructosamine (r = 0.77, p < 0.001), HbA1 (r = 0.76, p < 0.001), and albumin (r = -0.58, p < 0.001) for the entire population sample but only fructosamine (r = 0.19, p < 0.05) and HbA1 (r = 0.28, p < 0.001) correlated with IgA in diabetic patients, respectively. It is concluded that abnormal levels of IgA, IgG, and IgM are very common in diabetic patients in whom serum IgA concentrations are influenced by the degree of glycaemic control. Whether changes in IgA and other immunoglobulins are implicated in the pathogenesis of diabetic complications (such as susceptibility to infection) deserve further study.

Application of the time-resolved immunofluorometric assay to the study of C3 complement component glycation in vitro and in vivo

A time-resolved immunofluorometric assay (TR-IFMA) was used for the measurement of glycated C3. The very high sensitivity of this technique allowed the direct measurement of glycated and non-glycated proteins (especially C3) in chromatography eluates. C3 glycation in vitro after incubation with 20 mmol/l glucose was always less than 3.5% by day 5. As determined with the TR-IFMA, the means +/- standard deviations of glycated C3 were 0.20% +/- 0.04 for non-diabetic subjects and 0.88% +/- 0.06 for insulin-dependent diabetic patients. The low percentages of glycated C3 in both our in vitro and in vivo studies show that this protein is subject to only moderate rates of glycation.

Immunologic detection and measurement of glycated apolipoprotein B with site specific monoclonal antibodies

Nonenzymatic glycation of apolipoprotein B (apo B) is a post-secretory modification of low density lipoprotein (LDL) that affects its atherogenic potential and is implicated in the accelerated atherosclerosis associated with diabetes. To facilitate assessment of apo B glycation, we produced hybridomas secreting monoclonal antibodies specific for glycated apo B. SP 2/0 myeloma cells were fused with spleen cells from BALB/c mice immunized with purified apo B glycated non-reductively in vitro. Specificity of monoclonal antibodies secreted by the cloned cell line designated ES12 was demonstrated by immunoblotting and by direct ELISA, wherein the antibodies reacted with glycated epitopes residing in LDL but not in other plasma proteins, and did not react with nonglycated apo B or nonglycated LDL. Immunoblotting of human plasma with ES12 monoclonal antibody yielded an approx. 180,000 molecular weight component showing co-identity with apo B, indicating site specificity for glycated epitopes residing in apo B of the LDL complex and absence of reactivity with other nonenzymatically glycated plasma proteins. This reactivity of ES12 with the physiologic form of glycated apo B that occurs in vivo differs from properties of other antibodies raised against glycated lipoproteins, which recognized glycated residues only after reductive conversion to glucitol-lysine and which do not discriminate between different glycated proteins. In a competitive ELISA, mean concentration of glycated LDL, measured as apo B equivalents, in eight separate plasma samples was 19.7 +/- 1.9 micrograms/ml, representing 3.5 +/- 0.3% of total apo B. The ES12 monoclonal antibody allows specific determination of plasma glycated LDL concentrations, which may have diagnostic and pathogenetic importance.

Polyclonal immunoglobulin M: location of glycation sites

A four step purification procedure for polyclonal human serum IgM was elaborated, including ultracentrifugation, ammonium sulfate and polyethyleneglycol precipitations and diffusion-exclusion gel chromatography. IgM was glycated in vitro both in the presence of [14C]glucose and with unlabeled glucose. Influence of incubation time up to 10 days and of glucose concentration between 10 and 60 mmol/l were studied. With 10 mmol/l glucose, a molar ratio glucose/IgM of 5.7 was attained in 10 days. Increase of glucose concentration up to 60 mmol/l led to a molar ratio of 16.0. Both basal and in vitro glycation were evaluated by 3H-labeling by gel filtration and Concanavalin A-Sepharose chromatographies. Glycation occurs mainly on the heavy chains (> 85%), particularly on the Fd region.

Blood glycated haemoglobin, serum fructosamine, serum glycated albumin and serum glycated total protein as measures of glycaemia in diabetes mellitus

Blood glycated haemoglobin (HbAlc), serum fructosamine (FA), serum glycated albumin (GA), and serum glycated total protein (GTP) were determined in 61 subjects (19 pregnant women with gestational diabetes, 24 pregnant women with insulin-dependent diabetes mellitus [IDDM] and 18 nonpregnant subjects with IDDM). FA, GA, and GTP correlated with HbAlc similarly (r = 0.791, 0.816, and 0.794, respectively, p < 0.001). In a subgroup of 22 subjects data on blood glucose home monitoring was recorded and used for calculating mean blood glucose as an index of average glycaemia preceding sampling of the glycation products. Mean blood glucose levels preceding sampling of HbAlc by 2 months and FA, GA, or GTP by three weeks correlated significantly with HbAlc (r = 0.668, p < 0.001) and GA (r = 0.441, p < 0.05) whereas no significant correlation was found between mean blood glucose and FA (r = 0.003) or GTP (r = 0.252). In conclusion, such methods which measure specifically the non-enzymatic glycation of a single species of protein (i.e. FPLC for HbAlc and affinity chromatography for GA) are to be preferred for assessing glycaemia.

Influence of serum proteins on fructosamine concentration in multiple myeloma

Serum fructosamine levels in 36 subjects with various types of multiple myeloma and in 64 normal controls were evaluated by means of a Nitroblue tetrazolium colorimetric assay. Only the IgA myeloma group showed significantly raised serum fructosamine values (P less than 0.001). In the IgG myeloma group, which showed a higher mean serum protein concentration, serum fructosamine levels were not significantly different from controls. The study shows that elevated IgA levels do influence serum fructosamine and this effect should be taken into due consideration in order to avoid possible misinterpretations in evaluating this widely used index of glucose metabolism.

Abnormal serum immunoglobulin concentrations in patients with diabetes mellitus

Since the recently reported relationship between serum fructosamine and IgA concentrations appears to throw doubt on the clinical utility of fructosamine as a measure of hyperglycemic status if IgA concentration is not taken into account, we studied serum immunoglobulin concentrations in 169 diabetics and their relationship with various clinical and analytical parameters. Over 41% of the patients studied had abnormal serum IgA concentrations. Serum IgA concentration was negatively correlated with serum albumin, and among IDDM patients was positively correlated with age (so that the prevalence of abnormal IgA was 57.7% among IDDM patients aged over 30 years). Among NIDDM patients, abnormal IgA concentrations were especially prevalent among those being treated with oral hypoglycemics. Abnormal IgA was also more frequently found in both IDDM and NIDDM patients, who had been under treatment for 10 years or more. Abnormal IgG concentrations were found in 11.8% of the diabetics, and the mean IgM concentration found in the patients was 41.6% lower than in the normoglycemic group. We conclude that abnormal serum IgA concentrations are very common in diabetic patients and that further research should be carried out to verify whether the determination of serum immunoglobulins, IgA in particular, is of clinical use for monitoring diabetes or evaluating its secondary effects.

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.

Serum fructosamine/haemoglobin A1 ratio predicts the future changes in haemoglobin A1 in type 2 (non-insulin dependent) diabetic patients

The aim of the study was to define clinical interpretation of the parallel measurements of serum fructosamine and HbA1 in diabetic patients. We studied 14 type 2 diabetic patients over a 16-wk period. The cross-sectional analysis showed no correlation between serum fructosamine and HbA1 concentrations during the period of changing glycaemic control. The correlations, however, became significant (P less than 0.05) at 12 (r = 0.60) and 16 (r = 0.87) weeks, i.e. after glycaemia had stabilised. Longitudinal analysis of individual patients' data over the 16-wk period showed a significant correlation between serum fructosamine and HbA1 (r = 0.55 to r = 0.94) which was present in 8 out of 14 patients. The changes in fructosamine concentration preceded those observed in HbA1. The ratio of fructosamine/HbA1 predicted the changes in HbA1 over the following month (r = 0.54, P less than 0.001). Thus, we demonstrated that the parallel measurement of fructosamine and HbA1 provides information on future trends in HbA1 concentration in diabetic patients.

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.

Impaired agglutination of IgM resulting from non-enzymatic glycation in diabetes mellitus

Non-enzymatic glycation of circulating immunoglobulin G, A and M, measured by boronic acid affinity chromatography was found to be significantly increased in diabetes mellitus. The mean value of glycated IgM was already high in the control group: 49.58% (+/- 10.3% SD) and increased up to a mean value of 61.00% (+/- 8.5%) in the diabetic group. Similarly IgG and IgA glycation was significantly higher in the diabetic group than in controls: IgG 21.6% (+/- 3.4%) vs. 14.1% (+/- 2.9%; P less than 0.01); IgA 14.7% (+/- 4.9%) vs. 7.7% (+/- 1.3%; P less than 0.01). To investigate how glycation would alter IgM function, serum proportions from diabetic patients with blood group O were separated into glycated and non-glycated fractions by affinity chromatography and, adjusted to the same concentrations, tested against group A1 erythrocytes. Agglutination, which is mainly an IgM-mediated reaction, was significantly lower in the glycated than in the non-glycated fraction of IgM. The correlation between glycation of IgM and the reduction of agglutination titres in the glycated fraction was significant (r = 0.88, P less than 0.001). We conclude that impaired IgM function may be caused by non-enzymatic glycation in diabetes mellitus with possible consequences for host resistance in the early phase of infection.

The effect of hypoalbuminaemia, hyperbilirubinaemia and renal failure on serum fructosamine concentration in non-diabetic individuals

We have investigated the effects of hypoalbuminaemia, hyperbilirubinaemia and renal failure on serum fructosamine concentration in 39 non-diabetic patients. All patients were hypoalbuminaemic (median serum albumin 25 g/l, range 12-34 g/l). Group 1 (n = 19) were patients with hypoalbuminaemia alone, group 2 (n = 7) with hypoalbuminaemia and impaired renal function (median serum creatinine 226 mumol/l, range 154-461 mumol/l) and group 3 (n = 13) were subjects with hypoalbuminaemia and hyperbilirubinaemia (median serum bilirubin 34 mumol/l, range 19-83 mumol/l). Serum fructosamine was significantly lower in all three groups compared to age-matched normoalbuminaemic controls, but there was no significant difference in fructosamine concentrations between the groups. There was a correlation between fructosamine concentration and serum albumin. (r = 0.82, p less than 0.001) in all three groups combined. Serum fructosamine did correlate with serum bilirubin in patients with normal renal function (r = 0.0, p less than 0.001). In patients with abnormal renal function there was no correlation between serum fructosamine and either urea (r = 0.22, ns) or creatinine (r = 0.31, ns). Albumin is the major factor affecting serum fructosamine concentrations. Moderate hyperbilirubinaemia does not affect fructosamine concentration. No difference in fructosamine concentration could be demonstrated in patients with renal failure.

Production and characterization of monoclonal antibodies against human glycoalbumin

Hybridomas secreting monoclonal antibodies specific for nonenzymatically glycated albumin were produced by fusion of SP2/0 myeloma cells with spleen cells from BALB/c mice immunized with unreduced nonenzymatically glycated albumin prepared from human plasma. Wells containing hybridomas secreting antibodies against glycoalbumin were identified by binding, in an enzyme-linked immunosorbent assay, to glycoalbumin isolated from human plasma or to albumin that had been glycated in vitro. The colony designated A717, which secreted antibodies discriminating between glycated versus unglycated albumin, was cloned four times by limiting dilution and used for further study, performed with monoclonal antibody purified from mouse ascites fluid. Specificity of A717 was demonstrated by immunoblotting and by ELISA, wherein the monoclonal antibody reacted preferentially with glycated albumin but insignificantly with unglycated albumin. Immunoblotting of human plasma with A717 on nitrocellulose yielded a single band, the electrophoretic mobility of which corresponded with that of authentic glycated albumin, indicating site specificity for glycated epitopes residing in albumin but not in other nonenzymatically glycated serum proteins. A717 differs from other antibodies raised against glycated albumin and other proteins, which recognize glycated residues only after reductive conversion to glucitol-lysine and which do not discriminate between different glycated proteins. Thus, this report describes the establishment of the first hybridoma secreting monoclonal antibody raised against unreduced glycated albumin, which is the physiologic form occurring in vivo, and for the epitope when it resides in albumin but not other proteins.