Slow moving proteinase. Isolation, characterization, and immunohistochemical localization in gastric mucosa.

Human gastric mucosa contains three immunochemically distinguishable aspartic proteinases, pepsinogen I (pepsinogen A), pepsinogen II (pepsinogen C, progastricsin), and a nonpepsinogen proteinase also termed slow moving proteinase (SMP). The properties of SMP, and in particular its relationship to another aspartic proteinase, cathepsin D, were examined in this study. Slow moving proteinase and cathepsin D were isolated, respectively, from gastric mucosa and human spleen. Antiserum specific to each proteinase was prepared in rabbits. Rabbit anti-SMP did not recognize cathepsin D, and conversely, anticathepsin D did not react with SMP. Immunohistochemical studies localized SMP to surface epithelial cells in both the fundic and pyloric gland areas of the stomach. In contrast, cathepsin D was found mainly in mononuclear cells in the lamina propria and in parietal cells. Slow moving proteinase exhibited considerably lower Km values for its interaction with two chromogenic substrates than did cathepsin D. An even greater distinction between the two enzymes was found with the protein inhibitor from Ascaris lumbricoides; the activity of SMP was inhibited very strongly, whereas that of cathepsin D was not affected. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions, SMP consisted of two subunits with apparent molecular weights of 42,500 and 41,000. The last two properties characterize a less-well-known aspartic proteinase, cathepsin E. We conclude that SMP is not cathepsin D, but that it may be cathepsin E.