Radioimmunoassay of immunoreactive C-terminal big-endothelin(22-38)

Endothelin, a potent endogenous vaso-constrictor, is derived from its biosynthetic precursor by proteolytic degradation. The last step cleaves big-endothelin(1-38) to endothelin(1-21) and a C-terminal fragment. We have developed a radioimmunoassay for this peptide, based on an antiserum recognizing the C-terminal sequence big-endothelin(22-38) and cross-reacting with intact big-endothelin. To test for the presence of immunoreactive big-endothelin(22-38) in plasma, samples were extracted by passage through C18 silica cartridges, and desorption with methanol/water volume fraction 0.8. The yields of this purification and concentration procedure were about 70%. In 15 healthy persons we found concentrations of 1-11 pmol/l, which is about 10-fold higher than the reference levels reported for endothelin(1-21) or big-endothelin(1-38). Thus, endothelin-derived peptides containing the big-endothelin(22-38) sequence, but different from intact big-endothelin, do circulate in human blood. If a sufficiently tight correlation to levels of circulating endothelin(2-21) can be proven, this would facilitate studies on the physiological role of endogenous endothelin.