Cathepsin D Inhibitor from Potato Tubers (Solanum tuberosum L.)

The interest in naturally occuring inhibitors of proteolytic enzymes has been increasing mainly because of their possible use as therapeutic agents. Very little is known, however, about inhibitors of aspartic proteinases of protein character, their number include a pepsin and gastricsin inhibitor isolated from the roundworm Ascaris suum and A. hominis,1,2 intact activation (propart) peptides3 and the IA3 inhibitor of yeast proteinase A.4 So far, no cathepsin D inhibitor of tissue origin has been described in spite of the fact that this major lysosomal aspartic proteinase5 plays an important role in many physiological or pathophysiological processes. We have been able to characterize a cathepsin D and trypsin inhibitor of protein character (PDI), first isolated from potato tubers in our laboratory in 1976,6 to determine its complete amino acid sequence and to align it with the sequences of the family of soybean trypsin inhibitor.7 In this study we report some of the basic structural characteristics of PDI homologs found in potatoes.