Cathepsin D: Cleavage of soluble collagen and crosslinked peptides

Introduction Cathepsin D is a lysosomal proteinase of wide distribution [ 11. It has been shown to occur extracellularly in connective tissues [2] where, notwithstanding its low pH optimum, it has been implicated in the destruction of proteoglycans [3]. Previous authors reported little [4] or no [5] effect on gelatin and no cleavage of the fl or y components of soluble collagens [.5]. During studies on the effects of cathepsin D on non-collagen components of the NaCl-insoluble fraction of bovine skin [6] a specific dissolution of collagen was noted, with maximal effect occurring near pH 4. Cleavage of intermolecular crosslinks was considered the most plausible mechanism for such an effect and we therefore decided to examine this possibility using as model substrates, soluble collagen, gelatin, CNBr peptides of collagen and a peptide containing a crosslink, made available through a related investigation. This paper reports that there is a cathepsin D-susceptible peptide bond within the extra-helical carboxy-terminal sequence of the al(I) chain, located on the aminoterminal side of the (S -hydroxy-) a-amino adipic acids -semialdehyde residue which is involved in the formation of intermolecular crosslinks. Abbreviations: CNBR, cyanogen bromide; SDS, sodium dodecylsulphate; EDTA, ethylenediaminetetra-acetic acid *The CNBr peptide alCB6 contains a residue of lysine or hydroxylysine which may be converted to the aldehyde and become involved in crosslinking [ 151. In this work it was isolated after reduction to the alcohol ElsevierfNorth-Holland Biomedical Press