The influence of pH on the degradation of bovine myelin basic protein by bovine brain cathepsin D

The degradation of bovine myelin basic protein by bovine brain cathepsin D (EC 3.4.23.5) was studied over a pH range of 2.75–6.0. Throughout this pH range pepstatin, an inhibitor of cathepsin D, prevented the degradation. The degradation at a pH away from the optimum of pH 3.5 was predictably slower, but also resulted in more restricted cleavage. Above pH 4.5 bovine basic protein peptide 1–42 was not degraded further to peptide 1–36 as occurs at pH 3.5. Additionally, at pH 5.5 another fragment of basic protein, peptide 1–91, persisted indicating that under certain conditions basic protein as well as basic protein peptide 43ndash;169 may be cleaved in the molecular region of basic protein around the phenylalanyl-phenylalanine residues at position 88ndash;89. The small amount of peptides 1–91 and 92–169 detected at pH 5.5 suggests that the bond between residues 91 and 92 in intact basic protein is a minor cleavage site. The options and variation in cleavage around residues 88–92 of basic protein presumably result from pH-dependent changes in conformation in this region but could also be due to changes in conformation of cathepsin D. These results indicate that local tissue changes such as pH may affect not only the velocity of the reaction but also the nature of the product formed by the degradation of basic protein by brain cathepsin D.