| Abstract: | An extract of rat neutrophils was found to contain a high hemoglobin-hydrolyzing activity at pH 3.2, about 70% of which does not cross-react with anti-rat liver cathepsin D antibody. A neutrophil non-cathepsin D acid proteinase was successfully isolated from cathepsin D and characterized in comparison with the properties of rat liver cathepsin D. The neutrophil enzyme differed from cathepsin D in chromatographic and electrophoretic behaviors as well as immunological cross-reactivity, and its molecular weight was estimated to be 98,000 by gel filtration on Toyopearl HW 55. These findings strongly suggest that the neutrophil enzyme could be classified as cathepsin E. The enzyme, now designated rat cathepsin E, had an optimal pH at 3.0-3.2, preferred hemoglobin to albumin as substrate, and was markedly resistant to urea denaturation. Rat cathepsins D and E cleaved the insulin B-chain at six and eight sites, respectively; five sites were common for both enzymes. Possible relations among cathepsin E and cathepsin D-like or E-like acid proteinases reported so far were discussed. |
