Angiotensin-converting enzyme in the rabbit stomach wall. Identification in the membrane fraction by affinity purification.

To examine the potential role of angiotensin-converting enzyme as a physiological regulator of neuropeptide activity in the alimentary tract, we purified and partially characterized this enzyme from the crude membrane fraction of rabbit gastric muscles. A single-step purification from detergent-solubilized gastric membranes by affinity chromatography, using lisinopril-Sepharose, yielded an electrophoretically homogeneous peptidase composed of 180-kilodalton polypeptide. The purified enzyme represented approximately 2% of [Leu5]enkephalin-degrading activity of the original membrane preparation, and 275-fold purification was achieved. The gastric angiotensin-converting enzyme hydrolyzed synthetic substrates specific to the lung enzyme. It hydrolyzed angiotensin I and several other bioactive peptides, by removing their carboxyl-terminal dipeptides. The activity was completely inhibited with 10(-6) mol/L captopril. The hydrolysis of enkephalin was enhanced twofold by addition of 0.3 mol/L NACl to the assay buffer. These properties were comparable with those reported for the rabbit lung enzyme. Therefore, it was concluded that rabbit gastric muscle tissue contains membrane-bound angiotensin-converting enzyme. The results suggest a role for this enzyme as a local inactivator of bioactive peptides in the stomach wall.