The Kell blood group glycoprotein (KEL) is a zinc endopeptidase with endothelin-3-converting enzyme activity which can preferentially cleave endothelin-3 (EDN3), an endothelium-derived vasoconstrictor peptide. EDN3 is cleaved at Trp21-Ile22 to form the bioactive peptide EDN3(97-117) (Lee et al. 1999). KEL forms a heterodimer with membrane transport protein XK (Russo et al. 1998).