ET-1 is formed from its biological precursor Big ET-1, a 38 amino acid-long peptide that, after synthesis in the cytoplasm, is cleaved by endothelin conversion enzyme to yield active ET-1 (amino acids 1–21) and a C terminal fragment (amino acids 22–38) .
Big ET-1 has a circulating half-life of 23 min (Hemsen et al., 1995) compared with only 3.5 min for ET-1.