The amplified detection of free and bound methoxatin (PQQ) with nitroblue tetrazolium redox reactions: insights into the PQQ-locus

Porcine kidney diamine oxidase, a PQQ-enzyme, can be directly measured by formazan production with putrescine and nitroblue tetrazolium. This cyclic reaction in air is unaffected by superoxide dimutase, suggesting a two electron transfer between substrate-reduced PQQ-locus and nitroblue tetrazolium, without intermediate formation of superoxide. With albumin-bound PQQ and detergent-exposed PQQ-loci, glycine can be oxidized by PQQ and electrons repetitively transferred through PQQ-sites to nitroblue tetrazolium, the rate of formazan production detecting picomoles of exposed PQQ-locus. Exposed PQQ-loci are also reducible with NaCNBH3. Nitroblue tetrazolium, reoxidizes the reduced PQQ-locus with formazan production. These experiments suggest that the PQQ-locus of quinoproteins contains a [ketone-ketoimine in equilibrium with ketoamine] redox center.

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.

The interaction of aminogroups with pyrroloquinoline quinone as detected by the reduction of nitroblue tetrazolium

The interaction of pyrroloquinoline quinone (PQQ) with amino groups was followed by measuring the capacity of adducts to reduce nitroblue tetrazolium (NBT). Of the natural amino acids only glycine, ornithine, and lysine interacted strongly with PQQ. The reducing activity of other less reactive amino acids, but not of lysine, was increased by ammonia, primary or secondary amines. Divalent cations, in contrast inhibited development of NBT-reducing activity. PQQ also developed NBT-reactivity in the presence of serotonin and albumin. A reaction scheme is proposed which explains these findings. It is suggested that the NBT-reducing activity of plasma which is not caused by glycation of plasma proteins, arises from PQQ adducts inherent to plasma. This NBT-reducing activity corresponds to approximately 10 micrograms PQQ/ml plasma.

Efficacy and tolerance of a miconazole-benzoyl peroxide cream combination versus a benzoyl peroxide gel in the topical treatment of acne vulgaris

62-65% reduction in the number of lesions was obtained in the treatment of 52 patients with acne vulgaris in this randomized single-blind study comparing a 5% benzoyl peroxide/2% miconazole cream (BPO-MCZ) with a 5% benzoyl peroxide gel (BPO). While BPO gel was equally effective in male and female patients (66 and 73% reduction of lesions), the BPO-MCZ cream was significantly more effective in the latter (50 vs. 74% mean reduction of lesions). Tolerance was significantly better with the BPO/MCZ cream preparation, particularly in females.

Evaluation of the fructosamine test for the measurement of plasma protein glycation

Repeated estimation of plasma protein glycation by the fructosamine assay gave more variable results than expected from analytical variability (coefficient of variation approximately 2%). Fructosamine results obtained on plasma samples drawn at different times of the day differed by up to 1 mmol/l, corresponding to a coefficient of variation of greater than 10%. As a consequence, the information concerning averaged glycaemia of a fructosamine determination is subject to an uncertainty of 7.8 mmol/l. Fructosamine concentrations were linearly related to the protein concentration. Correction for the protein concentration decreased this variability; however, factors other than protein concentration, such as lipid content, also influence results of fructosamine determinations.

[HbA1c determination in capillary blood]

A practical procedure is described for the determination of HbA1c in capillary blood collected in micro-hematocrit-capillaries. These blood samples can be mailed to the analytical laboratory prior to consultation with the physician. The results of such determinations in 23 juvenile diabetic patients obtained during a two-year observation period are presented.

Nonenzymatic glycosylation of albumin in vivo. Identification of multiple glycosylated sites

Nonenzymatic glycosylation of albumin in vivo occurs at multiple sites. Glucose gets attached to Lys-199, Lys-281, Lys-439, and Lys-525 as well as to some other lysine residues. The principal glycosylated site is Lys-525. Approximately 33% of the overall glycosylation occurs at this site. This site specificity is remarkable and is postulated to be a consequence of local catalysis of the nonenzymatic glycosylation reaction. It appears that positively charged amino groups in the protein catalyze the Amadori rearrangement at specific sites. The principal glycosylated site, Lys-525, lies in a Lys-Lys sequence; other glycosylated sites lie in a Lys-Lys, Lys-His, and Lys-His-Lys sequence or are near disulfide bridges, which are likely to place amino groups of more remote parts of the protein closer to these sites. The occurrence of nonenzymatic glycosylation at most of the identified sites in albumin from diabetic patients is explained by the concept of local acid-base catalysis of the Amadori rearrangement.

Nonenzymatic glycosylation of albumin in vivo. Identification of multiple glycosylated sites

Nonenzymatic glycosylation of albumin in vivo occurs at multiple sites. Glucose gets attached to Lys-199, Lys-281, Lys-439, and Lys-525 as well as to some other lysine residues. The principal glycosylated site is Lys-525. Approximately 33% of the overall glycosylation occurs at this site. This site specificity is remarkable and is postulated to be a consequence of local catalysis of the nonenzymatic glycosylation reaction. It appears that positively charged amino groups in the protein catalyze the Amadori rearrangement at specific sites. The principal glycosylated site, Lys-525, lies in a Lys-Lys sequence; other glycosylated sites lie in a Lys-Lys, Lys-His, and Lys-His-Lys sequence or are near disulfide bridges, which are likely to place amino groups of more remote parts of the protein closer to these sites. The occurrence of nonenzymatic glycosylation at most of the identified sites in albumin from diabetic patients is explained by the concept of local acid-base catalysis of the Amadori rearrangement.

Albumin-directed antibodies in diabetes: demonstration of human serum albumin-directed IgM autoantibodies

Sera of 406 individuals, 174 Type 1 (insulin-dependent) diabetic patients, 125 non-diabetic family members and 107 unrelated control subjects, were screened for the presence of antibodies against glycated albumin. In none of these sera could such antibodies be detected. However, antibodies directed towards monomeric, unmodified human serum albumin were detected in 13 sera. These albumin autoantibodies were of the IgM class, and occurred in sera from nondiabetic persons (0.9-1.6%) and with a five-fold higher frequency in sera from diabetic patients (5.2%). The presence of albumin antibodies was neither related to the presence of diabetic late complications, islet cell antibodies, HLA-status nor duration of diabetes. The albumin antibodies were also found in sera from persons carrying antibodies against mumps (17%) or Epstein-Barr virus.

[HbA1c and HbA1 determination in diabetes control]

Objective assessment of the time-averaged blood glucose concentration of the preceding weeks in diabetic patients by means of glycosylated hemoglobin measurements has proved to be of value in the management of this disease. The present article summarizes the principle of this determination and information which is important for the interpretation of results.

Real and artefactual erythrocyte swelling in hyperglycaemia

The mean erythrocyte volume of patients with acute diabetic decompensation was determined by Coulter measurement and found to be elevated above normal (mean increase 5.5 mu3). Experiments in vitro revealed this to be an artefact associated with Coulter determination. A more reliable estimate of in vivo erythrocyte volume can be obtained from centrifugated haematocrit and erythrocyte count. With this method, true erythrocyte swelling parallel to glucose concentration was observed when erythrocytes were exposed to isotonic glucose-NaCl solutions. This volume increase resulted from decreased sodium concentration and was in the order of 0.5-1.0 mu3 per mmol/l of sodium. Glucose was osmotically ineffective. Similar volume changes were documented in a diabetic patient parallel to his daily variations of blood glucose. In severe diabetic decompensation, dehydration usually prevents an increase in erythrocyte volume. We conclude that hyperglycaemia is associated with erythrocyte swelling if total serum tonicity remains within the normal range.

Effect of temperature on quantifying glycated (glycosylated) hemoglobin by cation-exchange chromatography

As a consequence of nonideal chromatographic conditions, values for stable glycated hemoglobin (HbA1c) determined by cation-exchange chromatography in a commercial minicolumn system (y) or by "high-performance" liquid chromatography (x) differ markedly, yielding the regression line y = 0.82x + 0.6. With use of the protocol specified by the manufacturer, 20% of the HbA1c peak is not collected in the HbA1c fraction. Increasing the ionic strength of the eluting buffer by increasing the operating temperature to 28 degrees C increases the rate of elution from the minicolumn, making results of the two methods more closely comparable (y = 0.98x - 0.22). Because at a given pH the elution volume is determined primarily by the ionic strength, close limits on the composition of the eluting buffer are set by the temperature-dependence of its ionic strength. At a specified temperature and pH the position of a peak can be judged to within a volume of 1 mL if the conductivity of the eluent does not vary by more than +/- 0.05 mS.

Effect of temperature on quantifying glycated (glycosylated) hemoglobin by cation-exchange chromatography

As a consequence of nonideal chromatographic conditions, values for stable glycated hemoglobin (HbA1c) determined by cation-exchange chromatography in a commercial minicolumn system (y) or by "high-performance" liquid chromatography (x) differ markedly, yielding the regression line y = 0.82x + 0.6. With use of the protocol specified by the manufacturer, 20% of the HbA1c peak is not collected in the HbA1c fraction. Increasing the ionic strength of the eluting buffer by increasing the operating temperature to 28 degrees C increases the rate of elution from the minicolumn, making results of the two methods more closely comparable (y = 0.98x - 0.22). Because at a given pH the elution volume is determined primarily by the ionic strength, close limits on the composition of the eluting buffer are set by the temperature-dependence of its ionic strength. At a specified temperature and pH the position of a peak can be judged to within a volume of 1 mL if the conductivity of the eluent does not vary by more than +/- 0.05 mS.

Mass spectral and HPLC analysis of biological compounds with diphenylborinic acid

Diphenylborinic acid is shown to react with a variety of polyfunctional molecules of biological interest to give products which are easily identifiable by mass spectrometry. The diphenylborinate adducts of amino acids possess extraordinary stability and are separable by reverse phase chromatography. The stability of products is discussed with regard to the nature and stereochemical arrangement of the functional groups involved in complex formation.

Nonenzymatic glycosylation of basement membrane collagen in diabetes mellitus

For a better understanding of the processes leading to diabetic microangiopathy, type IV collagen from kidneys of patients with long-term diabetes was compared with the collagen from kidneys of sex- and age-matched controls. Type IV collagen from diabetic kidneys revealed no abnormalities in amino acid composition, hydroxylation of proline and lysine, enzymatic glycosylation of hydroxylysine, and immunological reactivity with several monoclonal and polyclonal, anti-type IV collagen antibodies. However, ketoamine-linked hexose, resulting from the nonenzymatic condensation of glucose with lysyl or hydroxylysyl residues, was 1.7-fold higher in diabetic type IV collagen. The stoichiometry of this modification was estimated to be 1-2 residues of hexose per triple helical molecule (Mr 380,000). This small amount of ketoamine-linked hexose might hardly have an effect on the function and turnover of type IV collagen, unless it is bound to a crucial site along the collagen molecule. The nonenzymatic glycosylation of collagen might therefore be a mere consequence of the metabolic disturbances, rather than the primary cause for the late complications of diabetes mellitus.

Quantitation of glycosylated hemoglobin by boronate affinity chromatography

Total hemoglobin glycosylation and the contribution of glycosylation at the N-terminus of the beta-chains and at "non"-beta-N-terminal positions were quantitated by use of boronate affinity and ion exchange chromatography. Glycohemoglobin (y) was found to correlate linearly (y = 1.92x + 0.53; r = 0.96) with HbA1c (x) and to contain approximately 50% beta-N-terminally glycosylated hemoglobin. This result is in agreement with the binding on boronate agarose of the various hemoglobin components resolved by cation exchange chromatography. An amount of glycohemoglobin similar to that of HbA1c was isolatable from HbA. A slope of less than 2 results because HbA1c is retained only to 93% and the intercept of the regression line reflects the partial adherence (65%) of HbA1a + b to the resin. These results confirm the occurrence of significant "non"-beta-N-terminal glycosylation and show that under optimal chromatographic conditions total glycohemoglobin can be determined with boronate affinity chromatography.

Measurement of receptor induced changes in intracellular free calcium in human platelets

The intracellular free calcium concentration plays a key role as second messenger in the regulation of cellular reactions. Post-receptor events can be investigated by measurement of free calcium concentration in cells. Our experience in measuring the intracellular free calcium concentration in platelets with the use of the fluorescent indicator quin-2-tetraacetoxymethyl- ester is described. Possible pitfalls in the preparation procedures of the platelets are discussed as well as critical steps and the limitations of the quin-2-method. The methodological approach is demonstrated by the presentation of an investigation with the adenylate cyclase inhibitors adenosine-5'-diphosphate and epinephrine as well as their interrelationship. The potentiation effect of epinephrine to adenosine 5'-diphosphate on platelet function such as aggregation is accompanied by a potentiation of the effect of these two platelet activators in elevating the intracellular free calcium concentration. Adenosine 5'-diphosphate elevates intra-platelet free calcium alone, whereas epinephrine acting through stimulation of the alpha2-receptor needs another permissive factor to immediately elevate the intra-platelet free calcium.

Quantitation of glycosylated hemoglobin. Elimination of labile glycohemoglobin during sample hemolysis at pH 5

A simple method for the elimination of labile glycohemoglobin in the chromatographic quantitation of glycosylated hemoglobin is described. Use is made of the instability of Schiff base adducts in acidic solution. Erythrocytes are lysed with a pH 5 buffer. At this pH dissociation reaches completion during sample preparation.

Glycosylation of variant hemoglobins in normal and diabetic subjects

The extent of in vivo glycosylation of variant hemoglobins was examined in individuals with S-, C-, and D-trait. Chromatographic estimates of glycosylation for nondiabetic individuals with S-trait were significantly lower than those for nondiabetic black subjects with normal hemoglobin (P < 0.001). However, chemical determinations of glycosylation (thiobarbituric acid or TBA technique) were similar for these groups (P > 0.10). The chromatographic elution pattern of hemoglobin S (HbS) was determined, and on this basis an adjustment procedure was performed for chromatographic data. A regression line was calculated for the relationship between chromatographic and colorimetric estimates of glycosylated hemoglobin in S-trait individuals with and without diabetes. The slope of this line was significantly different (P < 0.001) from that for the relationship in individuals with normal hemoglobin. However, after adjustment of chromatographic values from S-trait individuals, the slopes were similar (P < 0.10). Findings from individuals heterozygous for HbC and D were similar to those for individuals with S-trait. These data indicate that the extent of glycosylation of HbS, C, and D is similar to that of HbA in both the normoglycemic and hyperglycemic range. The TBA technique is the most direct method for determining the extent of glycosylation in individuals with HbS, C, or D. However, adjustment of column chromatographic values is feasible.

Glycosylated hemoglobins: increased glycosylation of hemoglobin A in diabetic patients

The components of the hemoglobin-A1 fraction--hemoglobins A1a--c--arise from nonenzymatic glycosylation of hemoglobin A at the beta-chain N-terminal amino groups and can be resolved from hemoglobin A by cation exchange chromatography. Glycosylation can also occur at the alpha-chain N-terminals as well as the epsilon-amino groups of lysine residues of both alpha- and beta-chains; this results in glycosylated species appearing in the hemoglobin-A fraction. In this study, we determined the extent of hemoglobin-A glycosylation using a colorimetric chemical method specific for the detection of ketoamine-linked hexoses in proteins. We demonstrate increased glycosylation of the main hemoglobin-A fraction in diabetic patients, which correlates significantly (r = 0.72, P less than 0.001) with the hemoglobin-A1 percentage determined by column chromatography in the corresponding hemolysates. This finding provides the basis for the application of this chemical procedure to the measurement of total glycosylation of hemoglobin.