Endothelin-converting enzyme is a phosphoramidon-sensitive metalloprotease that cleaves big endothelin to the potent vasoconstrictor peptide, endothelin. The converting enzyme is expressed in endothelial cells in a variety of tissues and in some secretory cells. In the present study, phosphoramidon-sensitive endothelinconverting enzyme activity has been demonstrated by radioimmunoassay in the neuroblastoma cell line, SH-SY5Y, and in Bu17 and 06 glioma lines. The identity of the activity was confirmed by immunoblotting, revealing a polypeptide of '-~12OkDa in each of these lines, in D384 glioma cells, and in primary astrocytes. Immunofluorescence revealed the cell-surface location of endothelinconverting enzyme in the neuronal and glial cell lines and in primary astrocytes. Pretreatment of SH-SY5Y and Bul 7 cells with phosphoramidon resulted in an apparent concentration of the enzyme protein in an intracellular compartment. Immunoperoxidase-staining of rat brain sections located this metalloprotease to the pyramidal cells of the hippocampus. Endothelin-converting enzyme-i was revealed by in situ hybridisation in the neuronal and glial cell lines. Key Words: Endothelins-Metalloprotease-Neutral endopeptidase-Endothelin-converting enzyme-Phosphoramidon. J. Neurochem. 68, 570-577 (1997).The endothelin (El) peptide family was identified originally in endothelial cells (Yanagisawa et al., 1988), where they are synthesised from larger precur.-sor proteins, the preproendothelins (Masaki and Yanagisawa, 1992). The three isoforms, ET-1, ET-2, and ET-3, are also found in nonvascular tissues (Inoue et al., 1989). In the brain, the presence of ET-l and ET-3 has been demonstrated in endothelial, glial, and neuronal cells (MacCumber et al., 1990;Ehrenreich et al., 1991;Fuxe et al., 1991;Giaid et al., 1991), where they may have important roles as neuropeptides, and in the regulation of cerebral blood flow and astrocytic function (Barone et al., 1995). ETs also act as morphogens, for example, in the development of tissues derived from the neural crest (Baynash et al., 1994;Kurihara et al., 1994;Puffenberger et al., 1994).The generation of ETs from the intermediate big ET (Yanagisawa et al., 1988) involves a unique proteolytic processing event, catalysed by a phosphoramidonsensitive El-converting enzyme (ECE) (Opgenorth et al., 1992;Turner and Murphy, 1996). Iwo phosphoramidon-sensitive ECEs have been cloned, ECE-I (Ikura et al., 1994;Schmidt et al., 1994;Shimada et al., 1994Shimada et al., , 1995aXu et al., 1994) and ECE-2 (Emoto and Yanagisawa, 1995). ECE-1 is expressed more abundantly in all the tissues examined to date. ECE-1 and ECE-2 show an overall identity of 59% and are distinguished principally by their sensitivity to phosphoramidon, their pH optimum, and probably their subcellular localization (Emoto and Yanagisawa, 1995). Additional isoforms of ECE-1 (ECE-la and ECE-1/3), which differ only in their N-terminal tails generated by alternative splicing, have been detected in several species (Shimada et al., 1995b...