The amino acid sequence of a novel inhibitor of cathepsin D from potato.

The amino acid sequence of a cathepsin D inhibitor isolated from potato is described. It was determined by analysis of peptides generated by use of the glycine-specific proteinase PPIV. The order of the peptides was established by examination of tryptic peptides derived from the two cyanogen bromide peptides. The inhibitor comprises 187 amino acid residues, and has a calculated Mr of 20,450.

Cathepsin D inactivates cysteine proteinase inhibitors, cystatins.

The formation of inactive complexes in excess molar amounts of human cathepsins H and L with their protein inhibitors human stefin A, human stefin B and chicken cystatin at pH 5.6 has been shown by measurement of enzyme activity coupled with reverse-phase HPLC not to involve covalent cleavage of the inhibitors. Inhibition must be the direct result of binding. On the contrary the interaction of cystatins with aspartic proteinase cathepsin D at pH 3.5 for 60 min followed by HPLC resulted in their inactivation accompanied by peptide bond cleavage at several sites, preferentially those involving hydrophobic amino acid residues. The released peptides do not inhibit papain and cathepsin L. These results explain reported elevated levels of cysteine proteinases and lead to the proposal that cathepsin D exerts an important function, through inactivation of cystatins, in the increased activities of cysteine proteinases in human diseases including muscular distrophy.