Characterization of phosphoramidon-sensitive metalloproteinases with endothelin-converting enzyme activity in porcine lung membrane

Endothelin-1 (ET-1), a 21 amino-acid potent vasoconstrictor peptide, is produced from the biologically inactive intermediate big ET-1 via an endoproteolytic cleavage between Trp-21 and Val-22 by endothelin converting enzyme (ECE). cDNA sequence analysis predicts that the two other members of the endothelin family, ET-2 and ET-3, are also generated from the corresponding intermediates called big ET-2 and big ET-3, respectively. The metalloproteinase inhibitor phosphoramidon inhibited the conversion of big ET-1 into mature ET-1 both in vivo and in cultured endothelial cells, suggesting that ECE may be a neutral metalloproteinase. In this study, we solubilized and partially purified ECE from the membrane fraction of porcine lung. Using gel filtration chromatography, we separated two distinct ECE activities, designated M1 (apparent molecular mass approx. 300 kDa) and M2 (approx. 65 kDa). Optimum pH for the cleavage of big ET-1 by M1 and M2 was 7.0 and 7.5, respectively. M1 efficiently converted human big ET-1(1-38) to ET-1, but not human big ET-2(1-37) or human big ET-3(1-41)-amide. In contrast, M2 converted both big ET-1 and big ET-2, but not big ET-3. M1 was inhibited by phosphoramidon (IC₅₀ approx. 1 μM) but not by thiorphan or bacitracin. In contrast, M2 was inhibited by much lower concentrations of phosphoramidon (IC₅₀ approx. 0.3 nM), as well as by thiorphan and bacitracin. ECE activity in M1 was able to bind to a concanavalin A-agarose column and was eluted by α-methyl-d-glucoside, indicating that the ECE is glycosylated. From these results, M1 and M2 from the porcine lung membrane are similar to the candidate of ECE in endothelial cells and neutral endopeptidase in kidney (EC 3.4.24.11), respectively. Taken in conjunction with the previous finding that neither thiorphan nor bacitracin affected the conversion of endogenously synthesized big ET-1 in cultured endothelial cells, we conclude that physiologically relevant ECE found in the endothelial cells is more similar to M1 than to M2.