Endothelin converting enzymes in guinea-pig lung membrane fractions: purifications and characterizations

We have identified neutral proteolytic activities that convert big endothelin-1 (big ET-1) to ET-1 in guinea-pig lung membrane fraction. The active proteins have been solubilized and two distinct enzymes have been purified by combinations of sequential column chromatographies. One purified enzyme was a metalloenzyme based upon its sensitivity to chelating agent with a molecular mass of 108 kDa on SDS-PAGE. Another enzyme was also a metalloenzyme with a molecular mass of 162 kDa. Further investigations revealed that the 108 kDa enzyme was inhibited by phosphoramidon and also by thiorphan, and produced ET-1 with the Km value of 5.7 microM for big ET-1. The 162 kDa enzyme was also inhibited by phosphoramidon, but neither by thiorphan nor by captopril, and quantitatively produced ET-1 from big ET-1 with a Km value of 2.1 microM. These results indicate that the 108 kDa enzyme probably seems to be a neutral-endopeptidase (EC 3. 4. 24. 11.) or similar one, but the 162 kDa enzyme is a unique metalloprotease that converts big ET-1 to ET-1.