Extraction and characterization of cathepsin D from sea cucumber (Stichopus japonicus) guts

The crude cathepsin D was obtained from sea cucumber ( Stichopus japonicus) guts in this paper.The crude cathepsin D was extracted by using 50 mmol/L Glycine-HCl buffer ( p H3.0) with material to solvent ratio of 1∶ 6 ( w/v) at 4 ℃ for 1 h.The activity and characterization of the enzyme were investigated by using acid denatured hemoglobin as a substrate.The results showed that the optimum p H of the crude cathepsin D was p H3.0 and was stable between p H3.0 ~ 7.0. The crude cathepsin D exhibited its optimal temperature at 50 ℃ and was stable at 4 ~ 40 ℃. The crude cathepsin D was activated by metal ions of Mg²⁺, Ca²⁺, Ni²⁺, Zn²⁺, Cd²⁺and K⁺, while inhibited by Fe³⁺ and Fe²⁺at concentration of 5 mmol/L.The crude cathepsin D was strongly inhibited by aspartic protease inhibitor pepstatin A. These results suggested that the crude cathepsin D obtained from S.japonicus guts was acid protease maily composed of aspartic protease under the condition of present study.