Purification properties and specificity of cathepsin D from Cyprinus carpio

Cathepsin D was purified 750-fold from a homogenate of Cyprinus carpio muscles. The purified enzyme has a molecular weight of 36,000, is inhibited by pepstatin and is active between pH 2.7 and 3.7 when tested on hemoglobin as the substrate. It consists of two isoenzymes with pIs of 5.65 and 6.1, respectively. The mode of cleavage of the beta chain of oxidized insulin was determined by analysis of the N-terminal amino acids of the cleaved peptides. The major points of cleavage of the beta chain of oxidized insulin are 56% at Tyr16-Leu17 and 40% at Phe25-Tyr26. The minor points of cleavage are at Leu15-Tyr16, Phe24-Phe25, Gly23-Phe24, Leu11-Val12, Ala14-Leu15 and Gln4-His5.