Intracellular localization of membrane-bound endothelin-converting enzyme from rat lung

Intracellular localization of membrane-bound endothelin-converting enzyme (ECE) was examined in rat lung by sucrose-gradient ultracentrifugation coupled with organelle marker studies. Lung microsomal fraction was prepared and fractionated by ultracentrifugation through a linear sucrose gradient. Sedimentation profiles of marker enzymes for plasma membrane, Golgi, lysosome, and mitochondria showed that these organelles were measurably separated from each other. A major portion of phosphoramidon-sensitive ECE activity was distributed with a single peak at the ~1.05-1.2 M sucrose region, where it appeared to be cosedimented with membrane vesicles that contained the two different marker enzymes for Golgi apparatus. These microsomal vesicles also seemed to contain the majority of endogenous immunoreactive ET-1 found in the lung. These findings suggest that a majority of the ECE activity in rat lung may be responsible for the intracellular conversion of big ET-1 during its transit through the secretory pathways.