Cathepsin D: Specificity of Peptide‐Bond Cleavage in Type‐I Collagen and Effects on Type‐III Collagen and Procollagen

Cathepsin D, purified from bovine thymus, has a limited proteolytic effect on types I and III bovine collagens. The α1 (I) chain was cleaved in native or denatured collagen only within the carboxy‐terminal extrahelical sequence, the major site being between residues C6 (Leu) and C7 (Ser). The α2 chain was unaffected in native collagen but was slowly cleaved between residues 782 (Phe) and 783 (Leu) in the denatured form. Cleavages, at 45° C, in type III collagen occur within the extra‐helical amino‐terminal sequence, on the carboxyterminal side of the lysine residue involved in intermolecular cross‐linking. All three sites of action are within sequences of general hydrophobic character. The very restricted cleavage of peptide bonds in denatured collagens can be ascribed to the infrequent occurrence of groupings of more than two hydrophobic residues and to the high content of the conformation limiting residues proline and hydroxyproline. The previously demonstrated failure of cathepsin D to solubilize a representative proportion of type III collagen from the fibres of bovine skin collagen (P. G. Scott and C. H. Pearson (1978) Biochem. Soc. Trans. 6 , 1197–1199] may be explained by lack of ability of the enzyme to act on this collagen at 25° C, in such a manner as to separate molecules joined by intermolecular cross‐links involving the amino‐terminal extrahelical region of the molecule.