Effect of glycation of albumin on its binding to renal brush-border membrane vesicles: influence of aging in rats

Abstract

Aging is associated with the loss of preferential urinary excretion of Amadori-product glycated albumin. We have measured the binding of 125I-labeled glycated albumin to the renal brush-border membrane vesicles from young and old rats to determine whether a specific receptor-mediated endocytosis system may be involved. 125I-Glycated albumin was specifically bound by renal brush-border membrane vesicles in a time- and temperature-dependent manner; the binding was concentration-dependent, saturable and reversible. Scatchard plots gave an apparent dissociation constant K m of 488 ± 17 nM, and a number of binding sites N of 33.5 ± 3.4 pmol/mg protein/min in membrane vesicles from young (3 months old) rats; the binding of native [ 125I]albumin, gave a K m of 1194 ± 200 nM ( P < 2%) and N of 82.4 ± 16.3 pmol/mg protein/min ( P < 3%). Vesicles from 10-month-old rats had a similar K m (619.6 ± 135.3 nM) and N (21.91 ± 2.98 pmol/mg protein/min), while those from older (30 months old) rats had significantly increased K m (1344 ± 237 nM, P < 3%) and N (81.3 ± 10.9 pmol/mg protein/ min, P < 1%) for 125I-glycated albumin binding. 125I-Glycated HSA was not displaced by unlabeled native HSA in less than 100-fold excess and native [ 125I]HSA was only displaced by a 10-fold excess of unlabeled glycated HSA. The binding of native [ 125I]HSA was partly inhibited (85%) by unlabeled glycated HSA. Thus, there appear to be two different binding sites, one for glycated and the other for native albumin, lying close together; and the glycation site on albumin is the discriminatory recognition factor.