Rapid binding of beta 2-microglobulin to renal brush-border membranes

125I-labelled human beta 2-microglobulin binding to rat renal brush-border membranes was assessed by an in vitro assay under near physiological incubation conditions (i.e. low content of albumin). Binding rate was 55 pmol/min per mg protein in the presence of 200 nM of beta 2-microglobulin and degradation rate was negligible versus binding rate. The binding rate was in reasonable agreement with the in vivo reabsorption rate, supporting the hypothesis of proteins binding to the luminal membrane during the process of reabsorption. Mild solubilizing treatment (Triton 0.1%) of brush border after beta 2-microglobulin binding yielded the labelled molecule associated with a high-molecular-weight component. Aminopeptidase activity and binding ability were to a certain extent co-purified during the course of the brush-border preparation, suggesting that most of the beta 2-microglobulin binding sites were localized in the brush-border membranes.