A rabbit antiserum raised against the hexapeptide endothelin (16–21) shows different binding affinities for endothelin-1, -2 and -3

A antiserum raised against the C-terminal hexapeptide ET16−21 common to ET-1, -2 and -3 was produced and characterized with respect to its binding properties for ET-1, -2, -3, ET16−21, the C-terminal octapeptide ET14−21, its derivative Phe21-ET14−21 and human big-ET-1. The antibody reacted with the peptides with decreasing binding affinities in the order: ET-1>ET-2≥ET-3≥ET16–21=ET14–21>>Phe21-ET14–21. It showed no crossreactivity with human big-ET-1. Similar results were obtained using [125I]ET-1, -2 or -3 as tracer. Substitution of Trp21 by Phe decreased the binding affinity of ET14–21 about 10 fold. Thus, the immunologically recognized sequence of the peptides is C-terminal and Trp21 seems to be important for high binding affinities. The significant differences in binding affinity observed for ET-1, -2, -3 and ET16–21 are consistent with an interaction of the C-terminal part of the endothelins with the bicyclic N-terminal part.