Big endothelin-1 (big ET-1) is converted to 21-amino acid residue endothelin-1 (ET-1) via a specific cleavage at Trp21-Val22 by endothelin converting enzyme (ECE). This conversion is an essential step to produce bioactive ET-1 and represents a regulatory site in the biosynthesis of this potent vasoconstrictor. ECE-1a, a unique membrane-bound enzyme, processes big ET-1 more efficiently than other big ET isoforms, which mainly differ in the C-terminal tail (residues 22-38). In this study, each of the highly conserved residues, Val22, Pro25, Pro30, Gly32, Leu33, and Gly34 were replaced with Ala in the preproendothelin-1 (PPET-1) cDNA using site-directed mutagenesis. The mutant and wild-type cDNAs were transiently transfected into Chinese hamster ovary cells along with ECE-1a cDNAs, and concentrations of the resulting recombinant peptides, ET-1 and big ET-1, in the transfection media were then measured. The concentration of immunoreactive ET-1 in the media from Val22, Pro25, Pro30, Gly32, and Leu33 mutant PPET-1-transfected cells was 4- to 6-fold lower than that of wild type and (Gly34-->Ala)PPET-1. Moreover, with the exception of Gly34 there was a corresponding increase in the concentrations of immunoreactive big ET-1 in the media from mutants. Similar results were obtained when His27, Val28, and Ser35 of big ET-1( )were substituted with the corresponding residues in big ET-2 and big ET-3. These findings suggest that the C-terminal tail has an important role in the intracellular processing of big ET-1 by ECE-1a. Herein we also report that a recombinant big ET-1 analog we previously generated and characterized, (Ala21)big ET-1, inhibits ECE-1a activity in a dose-dependent (K i=1 microM) and competitive manner.