Schistosomes ingest and lyse host blood cells, releasing
the haemoglobin (Hb) into their gut. AR Timms and E Bueding found an
acid protease activity in Schistosoma mansoni which was capable
of hydrolysing Hb; they suggested that host Hb degradation provided
the major amino acid source for the synthesis of parasite proteins.
From 1979 on, Hb degradation by schistosomes was considered mostly due
to cysteine proteinase (CP) activity. Several S. mansoni and
S. japonicum CPs have been reported to be possibly involved in
the degradation of this substrate which includes cathepsin B (Sm31,
Sj31 antigens), cathepsin L and an asparaginyl endopeptidase (Sm32,
Sj32 antigens). However, a proteinase-processing, rather than a direct
Hb-digesting role for the Sm32 have been suggested by JP Dalton and PJ
Brindley. On the other hand, cathepsin L has been mainly located in
the reproductive system of the worms and it is present in smaller
amount than cathepsin B in the adult worm vomitus of several
Schistosoma species, suggesting a minor role of this enzyme in
the digestion of Hb. An important proportion of the Hb degradation
exerted by S. mansoni extracts occurs in the absence of thiols
between pH 3.5 and 4.5 and this activity is inhibited by pepstatin A
(a classic aspartyl proteinase inhibitor) but not by thiol-, serine-
and metalloproteinase inhibitors. Using mercury-labeled pepstatin, BJ
Bogitsh and KF Kirschner localized an aspartyl proteinase in the cecal
lumen and to the gastrodermis of S. japonicum.
Immunocytochemical studies using heterologous antiserum to bovine
cathepsin D indicated that the S. japonicum cathepsin D-like
enzyme is also localized to the dorsal and lateral surfaces of the
tegument and tubercles of male worms. A cDNA encoding this proteinase
was isolated and the native enzyme biochemically characterized at pH
3.5.
