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.