Purification, subunit structure and inhibitor profile of cathepsin A

Cathepsin A (EC 3.4.16.1), a lysosomal carboxypeptidase, has been purified 1374-fold from pig kidney. Purification steps included concanavalin A-Sepharose and phenyl-Sepharose chromatography and chromatofocusing. The specific activity (16.9 U/mg) of the purified enzyme was significantly higher than previously reported values. The enzyme preparation appeared homogeneous when analyzed by non-denaturing polyacrylamide gel electrophoresis and was free of detectable protease contamination. The molecular mass (M(r) = 97,000), isoelectric point (5.0), and sensitivity to inhibitors were consistent with reported properties of cathepsin A. However, the previously reported three-peptide chain structure was not observed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence or absence of 2-mercaptoethanol demonstrated that the enzyme is composed of two M(r) 47,000 subunits, each of which dissociate in the presence of 2-mercaptoethanol into two polypeptide chains of 19,000 and 31,000.