Biomedical Engineering Reference
In-Depth Information
In adult-stage helminths, metalloproteases have been suggested to be central
to nutrient acquisition due to their presence in the parasite intestine/gastro-
dermis and associated secretions. In hookworms, transcripts for these enzymes
have been found in tissue lining the intestinal tract
5,12
and the ES component.
6
Metalloproteases have also been localized to the oesophageal glands and may
have a role in parasite attachment to the intestinal lumen by aiding in digestion
of the intestinal mucosal plug lodged in the buccal capsule of the adult para-
site.
38
A neprilysin-like zinc metalloprotease from N. americanus (Na-MEP-1)
is involved in blood-meal digestion and was shown to hydrolyse host globin
fragments, but only after cleavage with Na-APR-1.
12
Although seemingly cri-
tical in the parasite attachment and feeding process, the vaccine potential of
hookworm metalloproteases remains as yet unexplored. This is due to the
diculty in expressing substantial quantities of recombinant MEP-1 in prop-
erly folded form.
12
A major component of the highly protective H. contortus gut antigen com-
plex, H-gal-GP, is a family of four membrane-bound zinc metalloproteases,
designated MEP-1-4.
39
These enzymes belong to the M13 zinc metalloendo-
peptidase family (EC 3.4.24.11), which are also known as neutral endopepti-
dases or neprilysins.
40
After separation by gel filtration from the rest of the
H-gal-GP complex, vaccination of sheep with a combination of all four MEPs
significantly reduced H. contortus egg counts by 45-50% compared to non-
vaccinated animals. Similarly, MEP-3 alone or MEP-1, -2, and -4 in combi-
nation each reduced egg counts by 33%.
39
MEP-3 would appear to be the most
effective member of this metalloendopeptidase family; the restriction of MEP-3
expression to the blood-feeding fourth larval and adult stages of this parasite
40
supports this observation. No significant immunity has been observed with
insoluble bacterial forms of these recombinant proteases, suggesting that
conformational epitopes are required for protection.
39
8.2.4 Aminopeptidases and Exopeptidases
In addition to the proteases mentioned above, which mainly have endopepti-
dase and oligopeptidase activities, it has been suggested that aminopeptidases
and proteases with exopeptidase functions play a vital role in the blood
digestion process in hematophagous parasites. They are thought to exert their
activities downstream of the endopeptidases, removing terminal amino acids
and dipeptides for transport into the intestinal cells.
12
A glycosylated gut membrane protein with aminopeptidase activity, known
as H11, has been identified from H. contortus, and sequence analysis revealed
that it is similar to mammalian microsomal aminopeptidases that mediate the
terminal events of digestion processes by digesting small peptides.
41
H11 is the
most effective immunogen isolated from a parasitic nematode to date, inducing
high levels of protection in its native form, with 490% reductions in H. con-
tortus worm and egg burdens compared to controls.
42,43
H11 has been immu-
nolocalized exclusively to the intestinal brush-border of adult H. contortus, and
enzyme activity is inhibited by H11 antisera in vitro.
44
Parasite-derived H11
