Schistosomes express two forms of cathepsin D.

We report here a cDNA and its deduced amino acid sequence encoding a cathepsin D-like, aspartic protease expressed by adult stages of the human blood fluke Schistosoma mansoni. The cDNA encodes a short signal peptide, a pro-enzyme peptide of 37 amino acid residues, and a mature enzyme of 377 residues which has strong homology with mammalian cathepsins D. This aspartic protease, although 84% identical in amino acids of the mature enzyme region to the previously reported cathepsin D from the Asian schistosome S. japonicum, differs remarkably from the S. japonicum enzyme in having a carboxyl terminal extension of 43 amino acid residues. These cathepsins D of schistosomes may play pivotal roles in the degradation of hemoglobin obtained by the parasites from ingested host erythrocytes.