Angiotensin I converting enzyme (kininase II) of the brush border of human and swine intestine

Mucosal brush border of human and swine small intestine is rich in angiotensin I converting enzyme or kininase II (ACE). The brush border of the intestinal mucosa was purified by centrifugation over a discontinuous glycerol gradient. Transmission electron micrographs showed that 90 per cent of the isolated vesicles had a trilaminar membrane structure and glycocalyx, characteristic of intestinal brush border. No significant contamination by other subcellular particles was evident. In the final purified preparation, the brush border marker enzymes sucrase, trehalase and alkaline phosphatase were enriched 23-, 18- and 17-fold from human intestine and 27-, 26- and 20-fold from swine tissue. ACE was highly concentrated in the human and swine brush border. The specific activity of ACE in the human and swine brush border fractions was enriched 17- and 7.6-fold over the crude homogenate. Kininase activity was demonstrated by bioassay. Captopril, the orally active specific inhibitor of ACE, inhibited the enzyme; its I 50 was 3 × 10 −9. Antibody to swine kidney ACE cross-reacted with swine intestinal enzyme as shown in rocket immunoelectrophoresis, indicating that the enzymes from kidney and from intestine have common antigenic determinants and that the enzyme is concentrated on the brush border membrane. Because of the abundant presence of ACE in the intestine, interference in the functions of this enzyme may occur with chronic captopril therapy.