title: Functional characterization of albumin binding to the apical membrane of OK cells.
abstract: 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 binding data showed one specific binding site characterized by half-maximal binding (Michaelis constant, (Km) at 20 mg/l (300 nmol/l) and maximal binding capacity (Bmax) of 0.61 microgram/mg cellular protein. Excess of unlabeled albumin (BSA) inhibited binding at low concentrations of FITC-albumin completely but only partially at high concentrations. FITC-albumin binding was reversible and pH dependent. Km increased about sixfold when pH decreased from 7.4 to 5.0. The inhibitory effects of conalbumin, alpha-lactalbumin, and transferrin were significantly smaller 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 albumin in the proximal tubule.
authors.affiliation: Department of Physiology, University of Würzburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Gekle
authors.forename: S
authors.initials: S
authors.lastname: Mildenberger
authors.forename: R
authors.initials: R
authors.lastname: Freudinger
authors.forename: S
authors.initials: S
authors.lastname: Silbernagl
mesh_terms: D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000166:cytology / Q000378:metabolism*; D007700:Kinetics; D009893:Opossums; D012710:Serum Albumin, Bovine / Q000378:metabolism* / Q000494:pharmacology
chemical_list: C039217:fluorescein isothiocyanate bovine serum albumin; D012710:Serum Albumin, Bovine; D016650:Fluorescein-5-isothiocyanate