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

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 degrees C. Kinetic studies demonstrated the presence of saturable binding with an apparent Kd of 2.1 micrograms of f-Alb/ml and a maximal binding of 2.3 micrograms/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 degrees C for 30 min. EDTA, Ca2+ and Mg/4 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.