Microvillar endopeptidase, an enzyme with special topological features and a wide distribution

The endopeptidase present in the kidney microvillar membrane (EC 3.4.24.11) has been purified by immunoadsorbent chromatography from the pig. Three physically different forms have been obtained. The toluene-trypsin solubilized form has hydrophilic properties. The detergent and detergent-trypsin forms are amphipathic. Only a small change in apparent relative molecular mass of the subunit is produced by trypsin, indicating that little of the polypeptide is removed by the proteinase. Although apparently immunologically identical, the intestinal form has slightly different molecular properties, possibly attributable to differences in glycosylation. In spite of the failure of papain and other proteinases to release the endopeptidase from the membrane, reconstitution of the purified enzyme in liposomes has shown that it is a stalked dimeric protein, thus resembling other hydrolases in this membrane. In addition to its main locations in kidney and intestinal microvilli, there is clear evidence from inhibitor and immunological studies that the enzyme has a wide distribution including membrane fractions prepared from spleen, lung aorta and myocardium.