Proteolysis of human hemoglobin by schistosome cathepsin D

Schistosomes feed on human blood. They employ proteases to degrade hemoglob in from ingested erythrocytes, using the residues released for amino acid m etabolism. However, the identity and the role of the participating protease (s) are unclear and controversial. Confocal microscopy localized schistosom al 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 gastrode rmal rough endoplasmic reticulum. Schistosome cathepsin D, expressed in ins ect cells, autoactivated at pH 3.6 to a similar to 40 kDa form that cleaved the substrates o-aminobenzoyl-Ile-Glu-Phe-nitroPhe-Arg-NH2 and hemoglobin. The NH2-terminal residues of mature cathepsin D of Schistosoma japonicum a nd Schistosoma mansoni were Asn1 and Gly1, respectively, revealing that the proregion peptide was comprised of 35 residues. The proteases cleaved hemo globin 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 s ites were alpha Phe33-Leu34 and beta Phe41-Phe42, while others included alp ha Leu109-Ala-110 and beta Leu14-Trp15, demonstrating a preference for bulk y hydrophobic residues at pi and P1'. Most of the schistosomal cathepsin D cleavage sites were discrete from those of human cathepsin D. The gastroder mal location, elevated expression in females, acidic pH optima, similar sub strate preferences in two species, and the discrete substrate preferences c ompared with human cathepsin D together provide compelling support for the hypothesis that schistosomal cathepsin D plays an integral role in hemoglob in proteolysis, and might be selectively targeted by drugs based on proteas e inhibition. (C) 2001 Elsevier Science B.V. All rights reserved.