Binding of E. coli heat-stable enterotoxin to rat intestinal brush borders and to basolateral membranes

We studied the binding of E. coli heat-stable enterotoxin (STa) to rat brush borders (BB) and to basolateral membranes (BLM) using a biologically active monoiodinated radioligand [( 125I]STa) and highly enriched BB and BLM preparations free of other significant organelle contamination. Binding of [125I]STa to BB was specific; time-, temperature-, and pH-dependent; saturable; and partially reversible. Nonlabeled toxin competitively inhibited the binding of radioligand to BB in a dose-related manner. Scatchard analysis revealed a single class of receptors with an apparent affinity constant of 8.7 +/- 1.5 X 10(8) l/mol. Binding was not affected by amino acids, sugars, and lectins. Proteolytic enzymes significantly decreased binding, although several did so by modifying the radioligand. Trypsin inhibited binding without modifying the radioligand thus supporting the proteinaceous nature of the receptor. Since the enrichment in binding activity in the BB over the homogenate was significantly lower than the enrichment in sucrase activity, we concluded that binding activity is probably associated with other membranous domains, but direct examination revealed no binding activity on basolateral membranes.