Proteolysis of human hemoglobin by schistosome cathepsin D

Schistosomes feed on human blood. They employ proteases to degrade hemoglobin from ingested erythrocytes, using the residues released for amino acid metabolism. However, the identity and the role of the participating protease(s) are unclear and controversial. Confocal microscopy localized schistosomal cathepsin D to the parasite gastrodermis, and revealed elevated protease expression in females. At sub-cellular level, cathepsin D was localized to superficial digestive vacuoles of the gut and to cisternae of the gastrodermal rough endoplasmic reticulum. Schistosome cathepsin D, expressed in insect cells, autoactivated at pH 3.6 to a ∼40 kDa form that cleaved the substrates o-aminobenzoyl-Ile-Glu-Phe-nitroPhe-Arg-leu-NH 2 and hemoglobin. The NH 2-terminal residues of mature cathepsin D of Schistosoma japonicum and Schistosoma mansoni were Asn1 and Gly1, respectively, revealing that the proregion peptide was comprised of 35 residues. The proteases cleaved hemoglobin at pH 2.5–4.6, releasing numerous fragments. S. Japonicum cathepsin D cleaved at 13 sites, S. mansoni cathepsin D at 15 sites. Early cleavage sites were αPhe33-Leu34 and βPhe41-Phe42, while others included αLeu109-Ala-110 and βLeu14-Trp15, demonstrating a preference for bulky hydrophobic residues at P1 and P1′. Most of the schistosomal cathepsin D cleavage sites were discrete from those of human cathepsin D. The gastrodermal location, elevated expression in females, acidic pH optima, similar substrate preferences in two species, and the discrete substrate preferences compared with human cathepsin D together provide compelling support for the hypothesis that schistosomal cathepsin D plays an integral role in hemoglobin proteolysis, and might be selectively targeted by drugs based on protease inhibition.