The specificity of a bovine fibroblast cathepsin D. I. Action on the S-sulfo derivatives of the insulin A and B chains and of porcine glucagon

The specificity of a cathepsin D-like enzyme from bovine fibroblasts (dental pulp) has been examined using porcine glucagon, the insulin A chain ( S- sulfo ), and the insulin B chain ( S- sulfo ) as substrates. Peptides obtained from exhaustive digests were separated by chromatography and high-voltage electrophoresis and, after elution and hydrolysis, identified by quantitative amino acid analysis. The character of susceptible peptide bonds and the relative rate of their hydrolysis led to the following conclusions: 1. 1. The residues most favorable for hydrolysis are large and hydrophobic. 2. 2. Fully charged amino acids were never found as carboxyl or amino donors of the hydrolyzed bond. 3. 3. Susceptible bonds were at least two residues removed from the N- or C-terminal end of the substrate peptide. 4. 4. Some bonds possessing the above characteristics were not cleaved suggesting hindrance by “residual conformation”.