A receptor for formaldehyde-treated serum albumin on human placental brush-border membrane

Abstract

Formaldehyde-treated serum albumin (f-Alb) is known to be taken up and degraded by sinusoidal liver cells via receptor-mediated endocytosis. We report that 125I-labeled f-Alb ( 125I-f-Alb) binding to human placental brush-border membranes also occurs. This binding reached equilibrium within 40 min at 37°C. Kinetic studies demonstrated the presence of saturable binding with an apparent K d of 2.1 μg of f-Alb/ml and 0 maximal binding of 2.3 μg/mg of membrane protein at pH 7.5. Maximal binding was observed at between pH 7.5 and 8.0. 125I-f-Alb binding to the membranes was little inhibited by a 1000-fold molar excess of ovalbumin, human apo-transferrin and native bovine serum albumin. No binding was observed with membranes which had been pretreated with proteinase or trypsin. This f-Alb receptor was extremely heat-stable, since the binding was not abolished even by pretreatment of the membranes at 78°C for 30 min. EDTA, Ca 2+ and Mg 2+ had no effect on 125I-f-Alb binding, so the binding was independent of divalent cations. These data suggest that a receptor specific for f-Alb exists on human placental brush-border membranes of syncytial trophoblasts.