Hydrolysis of big endothelin-1 by a serine protease in the membrane fraction of human lung.

The hydrolysis of human big endothelin 1-38 (bigET-1) was investigated in the membrane fractions from three human lung specimens. The hydrolysis products were identified by HPLC or by amino acid analysis, peptide sequencing and mass spectrometry, and the contractile effects of synthetic bigET-1, synthetic ET-1 and the major metabolite were tested on isolated rabbit pulmonary arteries. The dominating hydrolysis product was identified as bigET1-31, formed by a chymostatin-sensitive enzyme. Soybean trypsin inhibitor also suppressed bigET1-31 formation, while two other serine protease inhibitors, 3,4-dichloroisocoumarin and aprotinin, had no (or a limited) inhibitory effect. Through a partly phosphoramidon-sensitive enzymatic activity, endothelin-1 (ET-1) was formed independently of bigET1-31. On isolated pulmonary arteries, bigET1-31 had a contractile effect similar to that of synthetic bigET-1, with pEC50% values of 7.3 +/- 0.1 (n = 6) and 7.1 +/- 0.1 (n = 8), respectively. The pEC50% value of ET-1 was 9.2 +/- 0.3 (n = 6). These results indicate that human pulmonary membranes, besides hydrolysing bigET-1 to ET-1, also express serine protease activity that is responsible for the formation of the biologically active product, bigET1-31.