Hemoglobin carbamylation in uremia

Heterogeneity of hemoglobin A1d: assessment and partial characterization of two new minor hemoglobins, A1d3a and A1d3b, increased in uremic and diabetic patients, respectively

We have separated and quantified two new minor hemoglobins named HbA1d3a and HbA1d3b. The level of HbA1d3a was significantly higher in uremic than in non-uremic patients (3.00 +/- 0.50% vs. 1.28 +/- 0.26% of total hemoglobin). It correlated well with carbamylated hemoglobin (r = 0.80, n = 81, p < 0.002) and with plasma urea concentration (r = 0.78, n = 81, p < 0.002). These data and the electrospray ionization mass spectrometric analysis provide strong evidence that HbA1d3a is an alpha-chain modified by carbamylation. The HbA1d3b level an diabetic patients was found to be 1.6-fold that in non-diabetic subjects (3.00 +/- 0.49 vs. 1.90 +/- 0.33). This was attributed to HbA1d3 modified by glycation. Indeed HbA1d3b correlated significantly with HbA1c (r = 0.71, p < 0.002) and with serum glucose level (r = 0.62, p < 0.002). These two new minor hemoglobins may serve as complements for the objective assessment of averaged long-term uremia and glycemia in uremic and diabetic patients.

Carbamylated hemoglobin: a potential marker for the adequacy of hemodialysis therapy in end-stage renal failure

Urea can dissociate in vivo to form isocyanic acid which can react with hemoglobin to form carbamylated hemoglobin. Previous work has shown that formation of carbamylated hemoglobin depends upon both the severity and the duration of renal failure. To determine whether carbamylated hemoglobin can be used as an assessment of the adequacy of hemodialysis treatment, we prospectively studied 55 stable patients who regularly attended our hospital dialysis program. Carbamylated hemoglobin was greater in those patients with a Kt/V of < or = 1.1 compared to those with a Kt/V of > 1.1 (120 +/- 8 micrograms VH/gHb versus 99 +/- 7, P < 0.01), and there was a negative correlation with Kt/V (r = -0.37, P = 0.007). There were positive correlations between carbamylated hemoglobin and the time-averaged urea concentration (r = 0.4, P = 0.004), and a negative correlation with the urea reduction ratio (r = -0.37, P = 0.01). Carbamylated hemoglobin may therefore be a useful marker of the degree of uremia, just as glycosylated hemoglobin is used in the assessment of patients with diabetes mellitus.

Specificity of hemoglobin A1c measurement by cation exchange liquid chromatography. Evaluation of a Mono S column method

A method was developed for evaluating the specificity of the ion exchange chromatographic HbA1c method by Pharmacia which uses a Mono S column. To investigate hemoglobin fractions potentially interfering in HbA1c analysis, the chromatographic resolution was enhanced and a peak integration program was used which enables the quantitation of overlapping and even shoulder-like peaks. Hemoglobin was incubated with glucose in vitro and the chromatograms were analyzed before and after incubation. Five minor peaks were detected, close to hemoglobin A1c, which could not be conclusively identified as hemoglobins previously described. Four of these peaks, the stable ones, were included in the fraction of the chromatogram to be measured as HbA1c by the routine method. In practice, these peaks comprise 20%-35% of the routine HbA1c result in diabetic patients and 30%-45% in non-diabetic subjects. Three of these minor peaks were non-dependent on glucose. The chromatography and the peak integration method described can also be used to study hemoglobin adducts other than glycohemoglobin.

Carbamylation of erythrocyte membrane proteins: an in vitro and in vivo study

OBJECTIVES

To establish the degree of erythrocyte membrane protein carbamylation in uremic and nonuremic patients, and to characterize the in vitro binding of cyanate to the individual proteins of the cytoskeletal matrix.

DESIGN AND METHODS

For in vivo studies, erythrocyte ghosts were digested with proteinase K and the released peptides colorimetrically assayed for carbamylation, using the diacetyl monoxime reagent, and quantitated using homocitrulline. For in vitro studies, erythrocyte ghosts were incubated with [14C] cyanate, and the membrane proteins separated by SDS-PAGE. Cyanate incorporation was quantitated by liquid scintillation counting and imaging densitometry of the excised bands.

RESULTS

Erythrocytes from uremic patients were found to have a greater level of carbamylation than those from nonuremic patients (47.09 +/- 7.80 and 25.89 +/- 6.92 nmol homocitrulline/mg proteolyzed protein released, respectively). In vitro incorporation of [14C] cyanate into membrane protein followed the sequence: spectrin > ankyrin > Band 4.1 > Band 3 > actin > Band 7.

CONCLUSIONS

The increased level of erythrocyte membrane protein carbamylation in uremic compared to nonuremic patients may lead to membrane destabilization and contribute to the decreased erythrocyte survival time observed in uremia.

Direct and indirect cost of urea excretion

Urea, the major end product of protein metabolism in mammals, is the most abundant solute in the urine. Urea excretion is thought to result from filtration curtailed by some passive reabsorbtion along the nephron. This reabsorption is markedly enhanced by vasopressin and slow urinary flow rate (V), the fraction of filtered urea excreted in the urine (FEurea) falling from approximately 60% at high V to only approximately 20% at low V. In concentrated urine, normal urea excretion can be maintained only if urea filtration is elevated. This can be achieved by increasing plasma urea concentration (Purea) and/or GFR. We have shown that both parameters do increase when normal rats are submitted to chronic alterations in the water intake/vasopressin axis within the normal range of physiologic regulation. This situation is very similar to that observed after alterations in protein intake. In both cases more urea needs to be filtered, either because more of it has to be excreted, or because the efficiency of its excretion is reduced. A common mechanism is proposed to explain the rise in GFR observed in the two situations. In summary, our studies demonstrate that the antidiuretic effects of vasopressin are responsible for a significant elevation of GFR. This GFR adaptation limits the rise in Purea, a favorable effect because urea is not as harmless as usually thought. However, this hyperfiltration might have deleterious consequences in diseased kidneys.

Factors determining hemoglobin carbamylation in renal failure

Carbamylated hemoglobin (carhb) is formed by the reaction of hemoglobin with cyanate, a product of in vivo urea dissociation. It is found in high levels in patients with renal failure and may be useful in their clinical evaluation. Accordingly, we measured carhb by HPLC after acid hydrolysis in 73 patients with renal failure and 11 controls. Mean carhb levels (expressed as micrograms valine hydantoin/g Hb), were highest in chronic renal failure (CRF, 146 +/- 13), intermediate in end-stage renal disease on hemodialysis (ESRD, 106 +/- 7), and lowest in acute renal failure (ARF, 80 +/- 12) when compared to normal subjects (27 +/- 2). In all patients carhb was significantly correlated with BUN but not with creatinine, bicarbonate, or phosphate. For any level of BUN above 80 mg/dl, carhb was substantially higher in CRF than in ARF. Predialysis BUN and urea reduction ratio (URR) were significant predictors of carhb in ESRD. To investigate the effect of time of exposure and BUN level on the rate of carbamylation of hemoglobin, blood from normal subjects and dialysis patients was incubated in vitro with urea equivalent to BUN levels of 50, 100, 150, and 200 mg/dl and assayed for carhb at 0, 5, 9, and 14 days. Carhb increased linearly over the first nine days of urea exposure and leveled off thereafter. The rate of carbamylation increased as BUN increased and was significantly higher in hemoglobin from dialysis patients than from normal subjects. These results show that the higher the level of carhb at baseline, the higher the rate of carbamylation upon exposure to increasing urea concentrations. We conclude that carhb formation is dependent on urea concentration and length of exposure to urea. The rate of carhb formation for a given urea concentration is greater in hemoglobin already carbamylated, and this may explain why carhb is higher in CRF than in ARF at BUN levels greater than 80 mg/dl. Carhb may thus be a useful index of the duration and degree of exposure to high blood urea levels in patients with renal failure, and may potentially serve as an index of the adequacy of dialysis.

Carbamylation of proteins and atherogenesis in renal failure

The incidence of atherosclerosis is greatly increased in patients with chronic renal failure, and this increased risk is only partly explained by conventional risk factors. Carbamylation of proteins occurs in renal failure as a result of reactions between urea-derived cyanate and protein amino groups. A mechanism is proposed whereby oxidation of LDL cholesterol within the arterial wall may be enhanced as a result of carbamylation. This may be accentuated as a result of inhibition of antioxidant enzymes by carbamylation. The combination of these processes may lead to increased atherogenesis.

In vitro formation of advanced glycation end products in peritoneal dialysis fluid

Glycation of proteins in the peritoneum might occur due to the extremely high glucose concentrations (75 to 214 mmol/liter) in the dialysate of patients on continuous ambulatory peritoneal dialysis (CAPD) and may be involved in the etiology of ultrafiltration failure. Formation of both early (glycated albumin) and late (advanced glycation end products; AGE, measured as protein-derived fluorescence intensity, FI) Maillard reaction products was studied in vitro in dialysis fluids obtained from seven patients on CAPD and in phosphate buffered saline (PBS) controls paired for glucose and albumin concentrations. Percentage glycated albumin (median, range) increased (P < 0.02) from baseline after 10 and 21 days in both dialysate and PBS but did not differ (P > 0.05) between the two media at any time point (day 0, 3.6, 3.1-4.5 vs. 4.1, 3.0-4.6; day 10, 19.4, 7.9-54.8 vs. 19.1, 8.7-50.1; day 21, 29.0, 12.0-75.6 vs. 30.0, 11.7-69.8). Glycated albumin formation was linearly related to the glucose concentration (r > 0.98, P < 0.001) in both dialysate and PBS at 10 and 21 days. FI (U/g/liter albumin, median, range) increased (P < 0.02) from baseline after 10 and 21 days in dialysate but only after 21 days in PBS; this increase was significantly greater (P < 0.02) in dialysate than in PBS after 10 and 21 days (day 0, 41, 36-46 vs. 42, 37-46; day 10, 99, 88-161 vs. 51, 34-68; day 21, 113, 102-239 vs. 68, 54-91). After 21 days, FI was significantly related to glucose concentration (r = 0.90, P < 0.01) and to % glycated albumin (r = 0.92, P < 0.01) in PBS but not in dialysate (P > 0.05). AGE formation, but not glycation, decreased as a function of the dialysate dwell time and was inhibited by aminoguanidine. Our results demonstrate that formation of AGE products occurs in dialysis fluid and that factors in dialysate can modulate this process.

Decreased clearance of uraemic and mildly carbamylated low-density lipoprotein

Low-density lipoprotein (LDL) was in vitro carbamylated with potassium cyanate and the clearance was studied in man. A minor carbamylation of LDL decreased the clearance of LDL by 41% (94% of amino groups free) and by 18% (90% of amino groups free). When LDL was extensively carbamylated its clearance was substantially accelerated. Moreover, the clearance of LDL isolated from 14 haemodialysis patients (uremic-LDL) was studied in rabbits. Uraemic-LDL, injected into rabbits simultaneously with the LDL of a healthy control subject, was cleared more slowly than the control-LDL (difference in fractional catabolic rate -6.5%, P = 0.02). We also examined the lipid peroxidation of the carbamylated LDL by measuring the amount of thiobarbituric-acid reactive substances (TBARS) and formation of conjugated dienes during exposure of carbamylated LDL to 5 microM Cu2+. The carbamylated and native LDL had similar lipid peroxidation and propensity for oxidation. In summary, both the uraemic-LDL and minimally carbamylated LDL had a decreased clearance in vivo, which may contribute to the accelerated atherosclerosis in uraemic patients.

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure. In patients with chronic renal failure, cardiovascular disease accounts for a significant proportion of all deaths. Several factors contribute to the "accelerated" atherosclerosis in this population, including hyperlipidemia, the pathogenesis of which is multifactorial. We investigated low-density lipoprotein (LDL) metabolism in a remnant model of chronic renal failure in the guinea pig. After one and two-thirds nephrectomy, creatinine clearance decreased to one-sixth normal. Plasma cholesterol and triglyceride (TG) levels increased with induction of renal failure. Analysis of lipoprotein composition disclosed significant TG enrichment of both uremic very-low-density lipoprotein (VLDL) and uremic LDL compared with control lipoproteins. Plasma clearance of homologous LDL was evaluated in turnover studies in control and uremic guinea pigs. To discriminate between differences in catabolism related to the uremic lipoprotein particle and to the uremic host milieu, a crossover protocol was used comparing the fractional catabolic rate (FCR) after simultaneous injection into control and uremic animals of 125I-control LDL and 131I-uremic LDL. The FCR of native LDL was slower in uremic animals than in controls. In addition, FCR of uremic LDL was significantly less than that of control LDL in both groups. Degradation studies in cultured fibroblasts indicated substantially reduced degradation of uremic LDL compared with control LDL. These results suggest dual abnormalities of LDL catabolism in renal failure that are not only related to alterations in clearance mechanisms in the uremic environment, but also suggest important functional differences in the LDL particle itself isolated from uremic animals.

Glycated haemoglobin: an assessment of high capacity liquid chromatographic and immunoassay methods

We have assessed five high-throughput systems for the measurement of glycated haemoglobin and have reviewed published evaluations of individual analysers. All systems offered better precision than a widely used electroendosmosis method. The low pressure chromatography and immunoassay systems demonstrated greater between-batch imprecision than the high performance liquid chromatography analysers, the latter achieving the proposed analytical goal of between-batch coefficients of variation less than 5%. Agreement between all systems measuring HbA1 was good but there was variability amongst observed HbA1c values. The systems were also assessed for their quality of chromatographic separation, simplicity of operation, flexibility, cost and potential for interference by other haemoglobins.

Carbamylation-induced alterations in low-density lipoprotein metabolism

Low-density lipoprotein was derived from carbamyl (carbamyl-LDL) by incubating LDL in potassium cyanate (KCNO). The proportion of free amino groups in the carbamyl-LDL was negatively correlated (r = -0.95) with the time of incubation in potassium cyanate (ranged from 5 to 360 min). The carbamylation did not change the chemical composition or the flotation characteristics of the LDL particles. However, the electrophoretic mobility of carbamyl-LDL was distinctly increased with the extent of carbamylation. The carbamyl-LDL had substantially decreased binding to the LDL apoB/E receptors of the bovine adrenocortical membranes when compared to the control-LDL. The reduced binding was already observed when only 9% of the free amino groups were derived from carbamyl. A minor carbamylation of LDL (less than 20% of the free amino groups) decreased the in vivo clearance of LDL from rabbit plasma. However, when more than 20% of the free amino groups were derived the carbamyl-LDL had accelerated clearance compared to the control-LDL. LDL isolated from uremic patients was cleared in rabbits at a slower rate than LDL isolated from a control subject. Providing that carbamylation of LDL could also occur in vivo resulting in similar alterations of the LDL binding to the LDL B/E receptors, as observed in the present study, the uremia-related accelerated atherosclerosis could have one additional mechanistic explanation.

Carbamylated haemoglobin in normal, diabetic and uraemic patients

Carbamylated haemoglobin arises from the non-enzymic modification of haemoglobin monomers by isocyanate derived from the spontaneous dissociation of urea. We measured carbamylated haemoglobin by high performance liquid chromatography in healthy subjects, non-uraemic hospital patients, diabetics, and different groups of uraemic patients. Carbamylated haemoglobin levels were found to be raised in uraemic subjects, but were independent of age, sex, glycaemic state and haemodialysis procedure. There was no significant difference in carbamylated haemoglobin levels between two groups of patients having different modes of dialysis treatment, probably indicating a similar degree of uraemic exposure in these patients.

Correlation of glycosylated hemoglobin measured by affinity vs ion exchange chromatography with mean blood sugar in pediatric IDDM patients

Affinity chromatography provides a more specific estimate of glycosylated hemoglobin (GlyHb) than does ion exchange chromatography (HbA1). However, whether GlyHb correlates closer than HbA1 with mean blood glucose has not been established. GlyHb and HbA1 were measured in pediatric IDDM patients attending a clinic (n = 285 visits) over a one year period and correlated with the mean of a patient's blood glucose measurements from records of home blood glucose monitoring. Mean GlyHb was higher than mean HbA1 (10.8% vs 9.6%) as was its standard deviation (2.2% vs 1.5%). While both GlyHb (r = 0.75) and HbA1 (r = 0.65) were strongly correlated with estimates of mean blood glucose, the correlation with GlyHb was significantly stronger than with HbA1 for the entire spectrum of metabolic control (P = 0.03), as well as for a segregated group of 'poorly controlled' patients with mean blood glucose greater than 150 mg/dl (P = 0.04). The results suggest that GlyHb is more accurate than HbA1 for estimating metabolic control and that GlyHb shows greater discriminating power than HbA1, especially at high concentrations of blood sugar. The mean blood glucose can be estimated from the equation: mean blood glucose (mg/dl) = (11.3 x GlyHb) + 32.

Comparison of reliability of plasma fructosamine and glycosylated hemoglobin assays for assessing glycemic control in diabetic patients on hemodialysis

To search for a reliable marker of medium-term integrated blood glucose level in diabetics on maintenance hemodialysis (HD), plasma fructosamine and glycosylated hemoglobin (Hb) levels were determined every week and blood glucose levels were determined four times every day over 3 weeks. The mean values of fructosamine (mol/L per 40 g of albumin) and of glycosylated Hb of other the study period correlated (r = .746, P less than .001) for combined materials of diabetic and nondiabetic subjects. However, plasma fructosamine values at the end of the study period did not correlate with the overall mean blood glucose values during the preceding 8 to 21 days (r = .372, NS). In contrast, glycosylated Hb values correlated closely with the same mean blood glucose values (r = .703, P less than .001). Fructosamine values significantly decreased during a HD, irrespective of the increases in albumin and total protein. In conclusion, glycosylated Hb was a reliable marker of long-term integrated blood glucose even in diabetics on HD. However, fructosamine was not a reliable marker of medium-term integrated blood glucose in these patients.

Carbamoylation of hemoglobin in uremic patients determined by antibody specific for homocitrulline (carbamoylated epsilon-N-lysine)

Polyclonal antisera to epsilon-amino-carbamoyl-lysine (homocitrulline) on guinea pig carbamoylated low density lipoprotein were used as probes to identify homocitrulline. These antisera detect homocitrulline in the immunogen and on other carbamoylated blood proteins including those from different species. In vivo carbamoylation of intracellular hemoglobin obtained from uremic patients and carbamoyl-proteins, carbamoylated in vitro, located on erythrocyte and lymphocyte cell surface membranes were detected. Solid phase, direct and competitive ELISAs for homocitrulline were used to analyze carbamoylated blood proteins. In uremic patients carbamoylation of protein by endogenous urea-derived cyanate occurs on the epsilon-amino group of lysine residues. Identification of carbamoylated proteins from uremic patients using an immunochemical probe of site specific antibody provides a way to monitor post-translational modification of proteins due to uremia and may give further insight for understanding the pathophysiology of end stage renal disease.

Unexplained variability of glycated haemoglobin in non-diabetic subjects not related to glycaemia

We have studied levels of glycated haemoglobin in a sample of 223 people aged over 40 years without known diabetes mellitus screened in a community study. Each had a glucose tolerance test and glycated haemoglobin measured by four methods - agar gel electrophoresis with and without removal of Schiff base, affinity chromatography and isoelectric focusing. The correlation coefficients between 2 h blood glucose and levels of glycated haemoglobin were between 0.43 and 0.64. This poor correlation was not explained on the basis of assay or biological variability of either 2 h blood glucose or glycated haemoglobin. Multiple regression analysis showed that other assays of glycated haemoglobin contributed to the variance of any single glycated haemoglobin value by 0.1%-52.9% (median 12.8%) compared to the variance of 18.6%-41.4% (median 30.8%) explained by 2 h blood glucose alone, suggesting that in a non-diabetic population, the degree of glucose intolerance may explain only one third of the variance of glycated haemoglobin levels, but other factors operate to produce consistent changes in levels of glycated haemoglobin. Investigation of 42 subjects with consistently high (20 subjects) or low (22 subjects) levels of glycated haemoglobin relative to their 2 h blood glucose level showed no difference in age, gender, body mass index, haemoglobin levels or smoking, although 50% of low glycators had impaired glucose tolerance. Neither ambient blood-glucose levels, as estimated on two five-point blood-glucose profiles, nor dietary intake of carbohydrate, starch, sugars, fibre or alcohol, explained the difference between high and low glycators. The determinants of the consistent interindividual differences in levels of glycated haemoglobin in non-diabetic subjects remain to be determined.

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.

Serum fructosamine in uraemia, myeloma and acute inflammatory disorders--relationship to serum glucose and albumin levels

There has been considerable interest in the serum fructosamine assay as a measure of glycated serum proteins. We have measured serum fructosamine in three groups of patients--those with uraemia; those with multiple myeloma; and those with acute inflammatory conditions--none of whom were known to have diabetes. Serum fructosamine was significantly higher in the uraemic group than in the other two, and also than in a control group. When allowance was made for prevailing serum albumin levels fructosamine was shown to be increased in the acute inflammatory group also. There was a significant correlation between random plasma glucose and serum fructosamine only when fructosamine was adjusted for prevailing albumin levels. In control and uraemic subjects there was a significant positive correlation between serum fructosamine and albumin levels, whereas in the myeloma group there was a negative correlation with serum protein. These data would suggest the need to take into account serum albumin levels and protein composition if serum fructosamine is accurately to reflect short-term integrated glycaemia.

Carbamylated haemoglobin in chronic renal failure

Carbamylated haemoglobin was measured by quantifying the release of isopropyl hydantoin by the acid hydrolysis of globin using gas liquid chromatography. Carbamylated haemoglobin was evaluated in 167 subjects including patients with a wide spectrum of renal disease. Grossly elevated concentrations of isopropyl hydantoin (mean values greater than 80 ng IPH/mg globin) were found in chronic renal failure, dialysis and transplant patients with renal failure compared to healthy subjects (15-39 ng IPH/mg globin). The elevated carbamylated haemoglobin correlated with renal function. Carbamylated haemoglobin may act as an indicator of uraemic status and has a potential clinical usefulness and may also have a pathophysiological significance.

Rapid and inexpensive microdetermination of serum fructosamine results in diabetics, uraemics, diabetics with uraemia and healthy subjects

This paper describes a simple, reliable and inexpensive method for rapid determination of serum fructosamine. The assay is based on commercially available reagents and utilizes equipment accessible in most laboratories (i.e. an automated ELISA-reader interfaced with an IBM computer). In contrast to HbA1c determination, the fructosamine method presented can be used in diabetes complicated by uraemia. In the clinically relevant measuring range, fructosamine is uninfluenced by serum albumin concentration in diabetics with or without uraemia. Eighty microlitres of non-haemolysed capillary serum suffices for a duplicate determination. One year's storage of normal serum induced no change in serum fructosamine estimates. s-Fructosamine in 18 healthy subjects was 2.1 +/- 0.3 mmol/l (mean +/- SD) in venous blood and 2.2 +/- 0.4 mmol/l in capillary blood. In diabetics 3.4 +/- 0.6 mmol/l and 3.3 +/- 0.6 mmol/l (n = 38) were found. The method is well-suited for routine use in the diabetic out-patient clinic.

Glycated haemoglobins

The association between elevated levels of glycated haemoglobins and diabetes mellitus has been known for twenty years [92]. Since then the determination of glycated haemoglobins has become a valuable tool for the objective assessment of long-term glycaemia in diabetic patients. The marked clinical interest in reliable measurements of glycated haemoglobins has stimulated the development and perfection of the necessary methodology. Limitations of the techniques have led to investigation of the underlying causes. Some of them led to the recognition of processes that were not known to occur in vivo before, such as glycation at sites other than the amino terminus of the beta-chains, modification of haemoglobin by reactants other than glucose or the existence of labile haemoglobin adducts. With ideal methodology these features would have gone unnoticed. Furthermore, the determination of glycated haemoglobin in large populations of diabetic patients has lead to the discovery of new, clinically silent mutant haemoglobins. Today, the routine determination of glycated haemoglobins in diabetic patients probably represents the broadest screening for mutant haemoglobins. The experience with glycated haemoglobins shows that overcoming difficulties in their determination, and progress in biomedical research, are closely intertwined.

Fructosamine assay: an index of medium-term metabolic control parameters in diabetic disease

The fructosamine levels and some metabolic control parameters were evaluated in 120 diabetics and in 60 controls. Higher levels of fructosamine were found in diabetics compared to normals (3.22 +/- 0.06 vs. 1.82 +/- 0.05 mmol/l; P less than 0.001). Positive correlations were found between fructosamine and metabolic control parameters in diabetic subjects. Moreover, non-insulin-dependent diabetic subjects had better correlations. The study of fructosamine variation in relation to plasma glucose change showed that fructosamine levels were modified after at least 2 weeks of good metabolic control. In conclusion, the fructosamine assay could be a useful method for medium-term metabolic control evaluation.

Glycated serum protein determination: comparison between thiobarbituric acid and fructosamine assays

Glycated serum proteins were evaluated with fructosamine (GSP-fructosamine) and thiobarbituric acid (GSP) assays in 60 normal and 60 diabetic subjects. Higher levels of GSP-fructosamine and GSP were found in diabetics in comparison to controls. GSP-fructosamine levels were positively related to GSP values and the other metabolic control parameters. Moreover, the fructosamine assay is sensitive, reproducible, rapid and easy to perform. Thus, fructosamine assay could be a useful method for glycated serum protein evaluation in diabetic patients.

Impact of metabolic control in progression of clinical diabetic nephropathy

Renal clearance of 51Cr-EDTA as a measure of glomerular filtration rate was followed prospectively for 21 months in 18 Type 1 (insulin-dependent) patients with juvenile-onset diabetes and nephropathy. Hypertension was treated aggressively, attaining a mean blood pressure of 154/88 mmHg in the supine and 126/82 mmHg in the standing position. The mean glycosylated haemoglobin value (HbA1c) during the observation period was found to correlate well with the mean of random blood glucose values (r = 0.72). It also correlated to the rate of glomerular filtration rate decline over time, whether the latter was calculated as slope coefficient for all available data (r = -0.52, p less than 0.05) or based on the first and last observations only (r = -0.57, p less than 0.05). In a multiple linear stepwise regression analysis also including mean arterial blood pressure, the correlation between glomerular filtration rate decline and HbA1c was significant at p less than 0.01; this explained one-third of the progression, while mean arterial pressure could not be shown to contribute. It is concluded that hyperglycaemia, contrary to the general belief, is a risk factor for the progression of clinical diabetic nephropathy with reduced glomerular filtration rate.

Androgen therapy in haemodialysis patients. II. Effects on red cell metabolism

Activities of the red cell enzymes hexokinase, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, lactic dehydrogenase and aspartate aminotransferase were measured in 17 chronic haemodialysis patients receiving androgen therapy, 15 untreated chronic haemodialysis patients and 15 normal subjects. Compared to normal subjects, untreated haemodialysis patients had similar reticulocyte counts but significantly increased levels of all five enzymes studied. This finding suggests the presence of a younger red cell population in the peripheral blood and is consistent with the shortened red cell survival known to occur in this clinical setting. Red cell enzyme activities in untreated haemodialysis patients were significantly correlated with one another and with the serum phosphate level. Moreover, in this population, red cell DPG content was directly related to hexokinase and glucose 6-phosphate dehydrogenase activities while haemoglobin-oxygen affinity (P50) was inversely related to all five enzyme activities. In contrast, in androgen-treated haemodialysis patients, despite higher reticulocyte counts, red cell enzyme activities were the same or lower than those in the untreated haemodialysis group and only slightly higher than those in normal subjects, suggesting an overall older red cell population. Moreover, relationships of red cell enzymes to one another, to serum phosphate levels and to both red cell DPG content and haemoglobin-oxygen affinity were significantly different in androgen-treated subjects than in the untreated haemodialysis group. These changes are consistent with a direct effect of androgens on red cell metabolism and an improved red cell survival during androgen therapy.

Nonenzymatically glycosylated proteins

Nonenzymatic glycosylation takes place in all proteins with a free-reacting lysine or valine in the presence of glucose. The formation of glycosylated plasma albumin, hemoglobin (Hb A1c), and skin collagen provides a diagnostic index of short- to long-term time-concentration of glucose in vivo. A wide range of assay methods are available, with affinity chromatographic, isoelectric focusing, and spectrophotometric methods providing the best accuracy and versatility. Glycosylated hemoglobin assays indicate glucose pressure over the previous 2 to 3 months and are of diagnostic value in general diabetic control, while glycosylated plasma albumin determinations are preferable in acute episodes in the life of a diabetic (e.g., pregnancy, infection, stress, trauma, surgery), since they provide an overview of changing blood glucose values of the previous 2 to 4 weeks. Glycosylated collagen estimations reflect tissue aging and are relevant in healing processes. Glycosylation alters the biologic activity of proteins, and these may relate to the manifold complications concomitant on the lifelong elevation of blood and tissue glucose in the diabetic (C6a). Assays for glycosylated hemoglobin have been routinely performed in clinical chemistry laboratories for a decade, and convenient determination for other nonenzymatically glycosylated proteins is proceeding apace.

Effects of carbamoylating agents on tumor metabolism

Carbamoylation of macromolecules occurs by the displacement of hydrogen on several groups, but the most stable addition at neutral pH is on amino groups. This reaction occurs predominantly with proteins and results from the administration in vivo of inorganic cyanate or organic isocyanates. The latter act more rapidly, but also are more rapidly hydrolyzed in aqueous solution. This instability has been a factor limiting study of the pharmacological properties of organic isocyanates. However, organic isocyanates are released from some nitrosoureas of value in cancer therapy such as 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU) and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU). The carbamoylating activities of BCNU and CCNU are generally considered less significant than their alkylating activity in the action of these drugs on tumors, but carbamoylation may serve to inhibit DNA repair. There is evidence that carbamoylating agents can exert selective inhibitory effects on metabolite uptake and macromolecular synthesis in neoplastic tissues. Such selectivity is much more notable in vivo than in vitro. In the case of cyanate, the selectivity in vivo has been variously attributed to a requirement for metabolic activation, to selective effects on circulation in solid tumors, and to diminished pH in tumors. It is the distinction between such factors and the identification of critical cellular targets which provide major challenges in present studies on the effects of carbamoylating agents on tumor metabolism.

Androgen therapy in hemodialysis patients: I. Effects on red cell oxygen transport

Measures related to red cell oxygen transport were evaluated in 17 hemodialysis patients receiving androgen therapy, 15 untreated hemodialysis patients and 15 normal subjects. Hemoglobin levels were higher in androgen-treated patients than in the untreated population and were directly related to the reticulocyte index. Hill coefficients were normal, and in vivo P5O values were increased to the same degree in both dialysis groups. However, DPG and serum phosphate explained 70% and 12%, respectively, of the variance in P5O in untreated patients but only 2% and 5% in androgen-treated subjects. In contrast, sample pH explained 34% of the variance in P5O in the androgen-treated group and less than 1% in the untreated dialysis population. Despite the relative importance of pH as a determinant of P5O in patients on androgen therapy, the Bohr coefficient in this group was only about half of that in untreated dialysis subjects. Androgen-treated patients also had lower red cell ATP levels. Finally, the expected correlation of MCHC with pH was noted in untreated dialysis subjects but not in patients receiving androgens. We conclude that androgen therapy in hemodialysis patients in addition to increasing red cell production, directly alters red cell metabolism. Moreover, although the androgen regimens used did not change the net oxygen transport characteristics of hemoglobin, they decreased the responsiveness of hemoglobin-oxygen affinity to changes in pH, DPG and phosphate. Thus, red cell adaptation to changes in oxygen supply and/or demand may be limited in androgen treated patients, and the improvement in clinical performance expected from androgen-stimulated erythropoiesis may not be realized.

Glycosylated haemoglobin in uraemia

Glycosylated haemoglobin (GHb) was measured in 71 patients with stable chronic renal failure by the thiobarbituric acid (TBA) reaction and by agar gel electrophoresis. Nineteen patients were diabetic. Of the non-diabetics, 22 were treated conservatively (including 8 children), 15 by maintenance haemodialysis, and 15 by continuous ambulatory peritoneal dialysis. GHb measured by both methods correlated with postprandial blood glucose levels. There was a significant discrepancy between the two methods only in patients with serum urea concentrations greater than 30 mmol/l, mean +/- SD, (6.8 +/- 2.6% vs 8.2 +/- 2.5% for TBA and electrophoresis, respectively). This difference, delta GHb, correlated with serum urea, serum creatinine, and serum bicarbonate, but after logistic regression of results from all 71 patients only serum urea was associated with delta GHb. Lower haemoglobin and GHb and high fetal haemoglobin concentrations in the haemodialysis group suggested increased haemolysis in these patients. Measurement of GHb by the TBA method and by agar gel electrophoresis remain useful indicators of hyperglycaemia in patients with mild, stable chronic renal failure.

Nonenzymatically glucosylated serum proteins in patients with end-stage renal disease

We report here abnormally elevated levels of nonenzymatically glucosylated whole-serum proteins in nondiabetic, as well as diabetic patients with end-stage renal disease (ESRD). Increased glucosylation of serum proteins and structural proteins has been documented in patients with diabetes mellitus, but not previously in ESRD. Increased levels of hemoglobin A1 have been reported in patients with ESRD, but appear to be due to carbamylation of hemoglobin rather than glucosylation. Using an assay that does not detect carbamylation, the present study demonstrates abnormally elevated levels of nonenzymatically glucosylated whole-serum proteins in all the chronic hemodialysis and continuous ambulatory peritoneal dialysis (CAPD) patients studied. Further, we found no change in levels of these abnormal proteins when patients were changed from hemodialysis with a dialysate that contained no glucose to one that did, or to CAPD in which large amounts of glucose are absorbed from the dialysate. The mode of dialysis does not appear to affect glucosylated protein levels in ESRD.

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.

Multiple immunoreactive forms of osteocalcin in uremic serum

Circulating osteocalcin, which normally reflects the rate of bone formation, is elevated in uremia. In 18 patients receiving maintenance hemodialysis, serum osteocalcin levels were directly related to the bone formation rate (r = 0.88, P less than 0.001), osteoblastic osteoid surface density (r = 0.65, P less than 0.01), and osteoclastic resorptive surface density (r = 0.75, P less than 0.001). Multiple regression analysis showed that osteocalcin levels remained positively correlated with osteoclastic resorption when the bone formation rate was held constant (P less than 0.01). The intimation that the coupling of bone formation and resorption could not explain the relationship between osteocalcin and resorption led us to determine whether fragments of this abundant matrix protein are released by bone resorption and retained in uremia. Sera from dialysis patients with renal osteodystrophy were fractionated by sequential gel filtration and HPLC, and assayed for immunoreactive osteocalcin. When normal serum was analyzed, a single sharp peak was found. In pooled sera from patients with high osteoclastic resorptive surfaces identified by histomorphometry, we found five additional immunoreactive peaks, while three additional peaks were detected in sera from patients with lower osteoclastic surfaces. Bio-Gel P-10 chromatography showed that these multiple peaks were of lower molecular weight than intact osteocalcin. We suggest that the liberation of bone matrix by osteoclasts contributes to the circulating osteocalcin immunoreactivity in uremia.

A comparison of the determination of glucosylated haemoglobin by isoelectric focusing and cation-exchange chromatography on minicolumns

The labile intermediate, pre A1c, formed in the glycosylation of haemoglobin A is a potential contaminant in the measurement of glycosylated haemoglobin when this is determined as the amount of HbA1c present in the sample. By isoelectric focusing on polyacrylamide gel plates this contamination could be avoided either by excision of the HbA1c leaving the neighbouring pre A1c behind on the slab, or by converting the pre A1c to HbA in glucose-free medium before electrophoresis. In cation-exchange chromatography on minicolumns (from Bio-Rad) the pre A1c was removed in the haemolysis process by borate-induced transformation to the non-interfering HbA. The chromatographic method nevertheless gave about 10% higher values than isoelectric focusing. The linearity between paired results of the electrophoretic and chromatographic methods was not perfect (p less than 0.05). Both methods measured decreasing concentrations of HbA1c equally well and with the same precision at both high and low levels (CV less than 5%). All HbF was simultaneously determined in the chromatographic method, while HbF did not interfere in the electrophoretic method. The HbA1c in whole blood samples was stable at 4 degrees C for up to 1 week. Carbon monoxide treatment made the HbA1c in haemolysates stable for at least 3 months at -70 degrees C making possible long-term control by both methods.

Amino-terminal processing of proteins: hemoglobin South Florida, a variant with retention of initiator methionine and N alpha-acetylation

The hemoglobin variant South Florida has been shown by protein sequencing and fast-atom-bombardment mass spectroscopy to have a substitution of methionine for the NH2-terminal valine of the beta-globin chain. In addition, there was complete retention of the initiator methionine on the mutant polypeptide. Approximately 20% of the protein was acetylated at the NH2 terminus of the beta chain. A search of a comprehensive data bank of protein and gene sequences revealed 84 unrelated vertebrate proteins that have not undergone cleavage of leader sequences. A highly nonrandom distribution of residues at the NH2 termini of these proteins predicts removal of the initiator methionine as well as NH2-terminal acetylation. Proteins that undergo removal commonly have serine, alanine, glycine, or valine, as the NH2-terminal residues. The first three residues favor N alpha-acetylation. Proteins that retain the initiator methionine commonly have a charged residue or methionine at the second position. Information on Hb South Florida and other hemoglobins coupled with this survey of primary sequence provides insights into the NH2-terminal processing of proteins.

Decarbamoylating activity of ornithine transcarbamoylase

We have purified from beef liver an enzyme which decarbamoylates carbamoyl-hemoglobin and to a much lesser extent carbamoyl histones. Carbamoyl casein was a poor substrate while carbamoyl trypsin, fibrinogen and ovoalbumin were not affected. The optimal pH is 7.4. Addition of Mg++, Mn++ or Ca++ was without effect. On testing citrulline as a substrate we found high activity leading us to suspect that the activity of the decarbamoylase preparation was due to contaminating ornithine transcarbamoylase activity. Evidence for this is the similar ratio of transcarbamoylase to decarbamoylase activities of both ornithine transcarbamoylase and of the purified preparation of decarbamoylase from beef liver. Also, delta-PALO, the specific inhibitor of ornithine transcarbamoylase inhibited both preparations to the same extent. Interestingly, ornithine transcarbamoylase from bacteria also has decarbamoylase activity while aspartic transcarbamoylase does not.

Evaluation of a mini-column chromatographic procedure for the measurement of hemoglobin A1c

An ion-exchange chromatographic procedure is described which facilitates the determination of beta-chain aminoterminal modified glycated hemoglobin. The procedure includes an erythrocyte lysis reagent which eliminates the labile aldimine component (pre-A1c) and a two-stage elution step which separates HbA, 1a + b from HbA1c. This procedure also includes calibrator material which aids in correcting for temperature fluctuations during the analysis. Within-run CV's for samples with HbA1c levels between 4.0% and 13.7% were 1.4 to 3.2%. The between-run CV for an HbA1c control was 5.5%. A comparison of the present test to an ion-exchange HPLC method yielded the equation: HPLC = 0.96 (present method) -0.2% (n = 101 and r = 0.984). Two separate reference range studies yielded comparable results (n = 220/65, mean = 4.77/4.78%, S.D. = 0.68/0.55). Studies with pooled erythrocytes and various lipemic plasmas did not reveal any assay interferences. Various abnormal hemoglobins were studied for their effect on the assay.

Abnormal cell-interactive properties of low-density lipoproteins isolated from patients with chronic renal failure

Patients with renal failure on maintenance hemodialysis have accelerated rate of atherosclerosis. This, and the fact that chemically modified low-density lipoproteins (LDL) have a better capacity than native LDL to stimulate cholesteryl ester accumulation within macrophages in the vessel wall, led us to examine the possibility that some alteration in apo-LDL may take place in chronically uremic patients. We isolated LDL (d = 1.019 - 1.063 g/mL) from 18 patients with chronic renal failure and from 13 normolipidemic controls and compared the interactive properties of the different LDL preparations with cultured fibroblasts. Our results show that "uremic" LDL associates less, is degraded less, and has diminished ability to stimulate cholesteryl ester formation in fibroblasts when compared to normal LDL. LDL carbamylated in vitro showed interactive properties with fibroblasts similar to those of uremic LDL. Uremic LDL was not taken up by scavenger receptors present on rat peritoneal macrophages, similarly to normal LDL. However, the decrease in uptake by fibroblasts of uremic LDL may increase the residence time of these particles within the subendothelial region of the vessel wall, ultimately resulting in increased atherogenicity. Carbamylation of lysine residues of apoB in vivo, abnormal catabolism of LDL due to the absence of functional renal tissue, or triglyceride enrichment of LDL are among the possible explanations for the abnormal properties of uremic LDL.

B-cell response to a standardized breakfast in end-stage renal failure

Twelve uremic patients (U) on regular hemodialysis were submitted to a standardized test meal. In comparison with normal controls (C), U patients demonstrated a slight increase of HbA1 level and a definite elevation of fasting plasma C-peptide immunoreactivity. They showed glucose intolerance at 60 and 120 min. This was associated with an inappropriate insulin response as evidenced by a significantly lower insulin/glucose index at 60 min. U patients were tested again during a hemodialysis session in order to reduce the 60-min glucose intolerance. Six patients (U1) were selected because they exhibited mean fasting and 60-min glucose values similar to those of the controls. In these conditions, the insulin response at 60 min was significantly decreased in comparison to basal conditions and this could not be accounted for by a concomitant decrease of plasma alpha-aminonitrogen values. It is concluded that, in uremic patients, glucose intolerance is associated with an inappropriately low B-cell response.

Carbamylation of hemoglobin in renal failure and clinical aspects

It was revealed that in renal failure hemoglobin A1 (HbA1) was formed by the binding of substances other than glucose to Hb. It was elucidated that the formation of HbA1 increased in urea-added medium and further increased in media containing urea either stored at 37 degrees C for 1 week or heated at 100 degrees C for five minutes. The increment of HbA1 was proportional to the amount of cyanic acid produced by the treatment of the medium after the addition of urea. We found that there was the best correlation between the HbA1 level and the BUN level of 1 to 2 weeks before. Therefore, it appeared that HbA1 could be an indicator of the state of renal failure of 1 to 2 weeks earlier in patients with renal failure without impaired glucose tolerance, especially in patients on hemodialysis.