Electrical Charge of Serum and Urinary Albumin in Normal and Diabetic Humans

The isoelectric points of albumin purified by pseudo-ligand chromatography on Affi-Gel Blue were determined simultaneously in serum and urine of 11 normal subjects and 25 diabetic patients, subdivided in groups according to their urinary excretion rates of albumin. Serum albumin was constituted by a single homogeneous peak at 4.7 (pI) in normal subjects, whereas the levels for diabetic patients covered this band and some other microheterogeneous levels, ranging from 3.5 to 7 pI. By affinity chromatography with Concanavalin A-Sepharose and immunoelectrophoretic techniques, all these micro-heterogeneous bands were characterized as glycosyl albumin. In normal subjects and diabetic patients whose urinary excretion rate of albumin was normal or increased only slightly (10 to 100 micrograms/min), the pattern of urinary albumin included a main band with normal pI (4.7) and some remarkable amounts of more anionic bands (pI between 4.0 and 4.7) if compared to the native protein, which was characterized as glycosyl albumin. Such a difference was not detected in urines of diabetic patients with clinical nephropathy. These results indicate that the non-enzymatic glycosylation of albumin is a main determinant of the excretion of this protein into urine, in spite of the anionic electrical charge. We describe also the renal selectivity properties in humans that may be viewed as a model for the study of renal disease, but the role of such a mechanism in early diabetic nephropathy remains unknown.