Proteolytic specificity of chicken cathepsin L on bovine β-casein

Abstract
The proteolytic specificity of chicken cathepsin L was studied using bovine β-casein as substrate. The peptide mixtures obtained after various times of hydrolysis were separated by RP-HPLC and ten peptides were identified. Chicken cathepsin L accepts proline residues in all positions except P1′. Looking at the amino acid residues on the amino side of the scissile bond we found three times the Tyr-Pro pair at P1′–P2′ positions and that the S1′ subsite can interact with modified amino acids such as phosphoserine.