Glycosylation of human abumin in diabetes mellitus: Extensive microheterogeneity of serum and urinary species as revealed by isoelectric focusing

Albumin and glycosylated albumin from serum and urine of 10 diabetic patients have been studied by ultrathin‐layer isoelectric focusing andsilver staining. Both total serum albumin and its glycosylated form were microheterogeneous with a main band focused in the pH 4–5 range; in the case of glycosylated ablumin several cathodic bands with pI's up to 7 were found to form a discretely abundant population among the total species. Some similarites and some striking differences were observed for urinary albumin, which, beside the main unmodified albumin with pI ca. 4.8, was enriched in microheteregeneous more anodal bands (PI 4.0–4.7 species). All these anodal bands were tightly bound by concanavalin A‐Sepharose, suggesting their identity as glycosylatedalbumin. these observations indicate that the glycosylation of albumin is not an equimolar reaction, but it can involve many sites of attachement of carbohydrates. The also suggest that albumin species with higher net negative charge and higher content of carbohydrates are preferentially able to pass through the glomerular barrier, providing the descriptive basis of diabetic albuminuria. Their relevance to diabetic nephropathy is a goal for further work.