FUNCTIONAL-CHARACTERIZATION OF ALBUMIN-BINDING TO THE APICAL MEMBRANEOF OK CELLS

We characterized binding of albumin to the apical membrane of opossum kidney (OK) cells using fluorescein isothiocyanate (FITC)-albumin (i.e ., bovine serum albumin, BSA) as substrate. Functional analysis of bin ding data showed one specific binding site characterized by half-maxim al binding (Michaelis constant, K-m) at 20 mg/l (300 nmol/l) and maxim al binding capacity (B-max) of 0.61 mu g/mg cellular protein. Excess o f unlabeled albumin (BSA) inhibited binding at low concentrations of F ITC-albumin completely but only partially at high concentrations. FITC -albumin binding was reversible and pH dependent K-m increased about s ixfold when pH decreased from 7.4 to 5.0. The inhibitory effects of co nalbumin, alpha-lactalbumin, and transferrin were significantly smalle r compared with BSA. We conclude that OK cells express a high-affinity binding site for albumin on the apical membrane. This binding site is pH sensitive, binds albumin in the physiological range, and could be responsible for the effective receptor-mediated reabsorption of albumi n in the proximal tubule.