Glycosylation of human albumin in diabetes mellitus II. Extensivein vitro modification by trioses and hexoses as revealed by isoelectric focusing

Molecular characteristics of methylglyoxal-modified bovine and human serum albumins. Comparison with glucose-derived advanced glycation endproduct-modified serum albumins

The amino acid modification, gel filtration chromatographic, and electrophoretic characteristics of bovine and human serum albumins irreversibly modified by methylglyoxal (MG-SA) and by glucose-derived advanced glycation endproducts (AGE-SA) were investigated. Methylglyoxal selectively modified arginine residues at low concentration (1 mM); at high methylglyoxal concentration (100 mM), the extent of arginine modification increased and lysine residues were also modified. Both arginine and lysine residues were modified in AGE-SA. Analytical gel filtration HPLC of serum albumin derivatives suggested that the proportion of dimers and oligomers increased with modification in both low and highly modified MG-SA and AGE-SA derivatives relative to unmodified serum albumins. In SDS-PAGE analysis, dimers and oligomers of low-modified MG-SA were dissociated into monomers, but not in highly modified MG-SA. MG-SA had increased anodic electrophoretic mobility under nondenaturing conditions at pH 8.6, indicating an increased net negative charge, which increased with extent of modification; highly modified MG-SA and AGE-SA had similar high electrophoretic mobilities. MG-SA derivatives were fluorescent: the fluorescence was characteristic of the arginine-derived imidazolone N delta-(5-methyl-4-imidazolon-2-yl)ornithine, but other fluorophores were also present. AGE-SA had similar fluorescence, attributed, in part, to glucose-derived imidazolones. AGE formed from glucose-modified proteins and AGE-like compounds formed from methylglyoxal-modified proteins may both be signals for recognition and degradation of senescent macromolecules.

Electrophoretic, chromatographic and immunological studies of human urinary proteins

Urinary proteins from both sexes were analyzed by high resolution two-dimensional gel electrophoresis (2-DE). For well reproducible 2-DE patterns, the samples were concentrated and desalted in one step by vacuum dialysis. A reference map for urine proteins was established by the analysis of urine from 10 healthy persons. Proteins in urine that share immunogenicity with serum proteins were identified by use of antibody to whole human-serum protein in an affinity-column fractionation of urine and differential analysis of the adsorbed (serum component) and unadsorbed (non-serum component) fractions. For identification of individual proteins, coelectrophoresis, immunoblotting and affinity chromatography with corresponding antibodies were used. Proteins identified in the map, besides known serum proteins, included: the subunit of Tamm-Horsefall protein, the secretory component of IgA, constant breakdown products of alpha 1-antitrypsin and retinol-binding protein, the five isoforms of the beta chain of human chorionic gonadotropin and the subunit of prostatic acid phosphatase. In addition, we could demonstrate three proteins which are markedly pronounced in female urine, especially pregnant women. To get more information about the native properties of various urinary proteins, they were separated into four main peaks according to their sizes using fast protein liquid chromatography equipment. Possible interpolypeptide disulfide bonds were studied using a nonreducing 2-DE system. 2-DE in combination with other methods seems to be a valuable tool for the characterization of urinary proteins in defined renal or extra-renal diseases. An example is given by analyzing the immune complexes from seven patients with a urinary tract infection.

Glycation of brain actin in experimental diabetes

Actin is a neuronal protein involved in axonal transport and nerve regeneration, both of which are known to be impaired in experimental diabetes. To determine if actin is subject to glycation, we rendered rats diabetic by injection of streptozotocin. Two or 6 weeks later brains were removed and a preparation of cytoskeletal proteins was analyzed by two-dimensional polyacrylamide gel electrophoresis. Brains from diabetic animals contained an extra polypeptide that migrated close to actin and reacted with monoclonal antibody C4 against actin. It was also found in a preparation of soluble synaptic proteins from diabetic rat brain, indicating that it was at least partly neuronal in origin. This polypeptide could be produced by incubation of cytoskeletal proteins from brains of nondiabetic rats with glucose-6-phosphate in vitro. The appearance of this glycated actin in diabetic animals was prevented by administration of insulin for a period of 6 weeks. We could not detect any effect of glycation in vitro on the ability of muscle G-actin to form F-actin filaments and its significance for the function of actin remains to be determined. The finding that glycation of platelet-derived actin from diabetic patients was significantly increased implies that the abnormality may also occur in clinical diabetes.

Urinary protein analysis

The differentiation and analysis of urinary proteins has substantially contributed to our knowledge of physiological and pathophysiological processes during glomerular filtration and tubular catabolism of plasma constituents. By use of high-resolution biochemical separation techniques, several urinary polypeptides could be identified as plasma proteins, tissue antigens, tubular enzymes and protein breakdown products. With regard to clinical application, the separation results of conventional gel chromatography and agarose electrophoresis were surpassed by fast protein liquid chromatography and polyacrylamide electrophoresis in one- and two-dimensional systems. In contrast to early one-dimensional polyacrylamide gel electrophoresis (PAGE) methods using homogeneous gels on a macro scale, modern gradient slab gels achieve better resolution over the entire relative molecular mass range of urinary proteins. For clinical demands, the use of micro-scale gradient gels, either laboratory-made or pre-cast, together with an improved Coomassie Brillant Blue staining, offers several advantages, including rapidity, sensitivity and economy. Isoelectric focusing and two-dimensional PAGE, combined with sensitive silver staining and immunoblotting methods, have proved to be valuable tools for the identification and characterization of urinary proteins in defined renal and extra-renal diseases. The quantitative determination of urinary indicator proteins such as albumin and alpha 1- and beta 2-microglobulin can be regarded as a reasonable complement to the pattern diagnosis, especially in the long-term course of renal diseases.

An improved method of high-voltage isoelectric focusing on cellulose acetate membranes

An apparatus and the procedure for cellulose acetate isoelectric focusing were refined and optimized for routine use in clinical laboratories. The space in the electrophoretic chamber was minimized to maintain high humidity during the run. The water content of the thin cellulose acetate membranes in the chamber was well controlled during isoelectric focusing and the effect of atmospheric carbon dioxide was also excluded. The temperature of the membranes was kept below 2 degrees C, even under conditions of high electric field strength for faster isoelectric focusing of high-molecular-weight proteins. No special training was required for technicians in clinical laboratories to obtain reproducible isoelectric profiles of human serum proteins.

Reaction of human serum albumin with aldoses

The reaction of human serum albumin (HSA) with aldoses (C3-C6) and acetaldehyde has been studied. U.v. and fluorescent spectra of the HSA-glyceraldehyde and HSA-GlcN adducts reveal yellow chromophores absorbing at 300-350 nm and emitting at 435 nm. However, even limited reaction of HSA with acetaldehyde induced perturbation in the Trp microenvironment. C.d. spectra of the adducts show an average 20% decrement in mean residual ellipticity [theta], which is independent of the extent of the reaction and the aldose used. It is concluded that most of the reactions with aldoses occur at the surface of the HSA molecule. With the exception of the GlcN adduct, the HSA adducts rearrange to produce pyrrole rings on the protein surface. I.e.f. analysis shows that the pI values of the modified HSA are almost linearly correlated with the chain length of the reacting aldose: from pI 4.2 for HSA-glyceraldehyde up to pI 5.0 for HSA-GlcN.