Biochemical characterization of cathepsin D from the mussel Lamellidens corrianus

A lysosomal cathepsin D (EC 3.4.23.5) was purified to homogeneity from the soft tissues of the fresh water mussel (Lamellidens corrianus) by pepstatin A affinity chromatography. The purified enzyme is a glycoprotein and migrates as a single protein species in native PAGE and shows a single band in SDS-PAGE corresponding to a molecular mass of ~ 43 kDa. Under both these conditions cathepsin D hydrolyzes hemoglobin as shown by zymogram analysis. The purified enzyme cross-reacts with an antiserum to purified starfish (Asterias rubens) cathepsin D. Additionally, the enzyme was recognized by the starfish lysosomal enzyme targeting receptors (mannose 6-phosphate receptors: MPR 300 and 46) in ligand blot analysis. The KM value of the purified enzyme with hemoglobin is 1.5 mM. pH and temperature optimum for the enzyme are 3.5 and 60 °C respectively.