Biomedical Engineering Reference
In-Depth Information
substrate specificity at positions P3, P2, P2
0
, and P3
0
for the four plasmepsins IV
and also reported studies of inhibitors based on the discoveries by Beyer et al.
51
In the following, additional studies of plasmepsins from species other than
P. falciparum will be presented. Humphreys et al.
52
presented an analysis of the
plasmepsin IV from the P. berghei species that infects rodents. This was studied
because mice or rats would provide a convenient model system to study malaria
and the effects of selective plasmepsin inhibitors as potential treatments for
infection. This report also included information on the four genes in P. falci-
parum. Humphreys et al. showed that PbPMIV cleaved a synthetic substrate
previously used for study of other plasmepsins and that this activity could be
inhibited by pepstatin. Spaccapelo et al.
53
found that plasmepsin IV deficient
P. berghei have significantly reduced virulence in mice; this could be related to
the fact that this species has only the single plasmepsin in the food vacuole, thus
reducing the redundancy and possibly making PM IV more important for the
survival of that organism.
Sharma et al.
54
reported on the isolation and analysis of PvPM4 directly
from parasite samples. Following ion exchange, size exclusion, and HPLC, the
protein appears to be pure. A casein/gelatin hydrolysis assay was utilized to
show activity and inhibition by pepstatin, consistent with expectations. In
addition, hemoglobin cleavage was observed. Other class-specific protease
inhibitors did not affect the P. vivax enzyme, confirming the nature of the
catalytic activity.
Two reports have appeared describing the plasmepsin IV from P. chabaudi,a
second species that infects rodents. Martins et al.
55
analyzed the genes in
P. chabaudi and P. yoelii, and found differences at specific points that could
influence specificity. In a follow-up paper (in 2006), the same authors
56
pre-
pared the recombinant P. chabaudi protein and found some interesting inhi-
bitor selectivity. They also reported that HIV protease inhibitors blocked the
enzyme with K
i
values in the 30-190 nM range. These observations will be
described later in the section on inhibitors.
Li and colleagues
57
reported on a plasmepsin from the species P. gallinaceum
during the insect stage of the malaria life cycle, described earlier as the sexual
stage of malaria. It was suggested by their results that this plasmepsin, shown to
be PgPMIV, in involved in digestion of chitin-binding proteins in cooperation
with a chitinase also produced by P. gallinaceum. This digestion is essential to
allow the parasite to cross the midgut peritrophic matrix in order to develop
into an oocyst on the midgut basal lamina. This raises the possibility of tar-
geting inhibitors to the mosquito host of malaria in an effort to block the life
cycle at a stage outside the human host.
Bhaumik et al. recently summarized the current information on the four
vacuolar enzymes in P. falciparum
28
with details for 29 entries in the Protein
Databank (their table I
28
). They also included a sequence alignment of the four
enzymes from P. falciparum with the plasmepsin IVs from P. vivax, P. ovalae,
P. malariae, and P. knowlesi. This review will discuss further information in the
Bhaumik et al. paper later in the discussion of inhibitors, as they also cataloged
the various structures that have been co-crystallized with the plasmepsins.
