trans

212 PURINES AND PYRIMIDINES
FIGURE 9.5 Toxoplasma gondii purine salvage and interconversion pathways. The key enzyme, adenosine
kinase, is indicated by a thick line. Enzymes are designated numerically and can be identified from Table 9.1.
each in a variety of mechanistic states, i.e.
apoenzyme, substrate- and product-bound
forms. These structures have provided considerable
insights into the mechanisms by which
PRT enzymes stimulate catalysis.
T. gondii deficient in either TgAK or
TgHGXPRT have been obtained by both chemical
and insertional mutagenesis. Neither
enzyme alone is essential for parasite viability
or proliferation. It has not been possible to
generate a double mutant devoid of both TgAK
and TgHGXPRT activity, suggesting that
T. gondii accommodates only these two routes
of purine salvage. Given the discrepancies
in the specific activities of the two enzymes
in extracts, it can be reasonably conjectured
that AK and HGXPRT represent the primary
and secondary purine salvage mechanisms
of this parasite. Thus, the purine salvage
pathway of T. gondii can be primarily distinguished
from that in P. falciparum by its high
AK activity.
Eimeria tenella
Unlike P. falciparum and T. gondii, which can
be cultured, E. tenella is not amenable to simple
biochemical or genetic manipulations.
There is little recent information on purine
salvage by this parasite. Radiolabel incorporation
experiments suggest that purine salvage
by E. tenella is similar to that in T. gondii.
Both sporozoites and merozoites incorporate
adenine, adenosine, hypoxanthine, and inosine
into adenylate and guanylate nucleotides,
but guanine and xanthine are only converted
to guanylates. This implies that guanine and
xanthine cannot satisfy the purine needs of
the parasite. The metabolic labeling results indicate
that GR activity is likely absent, whereas
the branchpoint pathways from IMP to AMP
and GMP are intact. An HGXPRT activity has
also been isolated and affinities for the three
oxypurine substrates determined.
Kinetoplastida
Leishmania spp.
The purine salvage pathway of Leishmania is
interwoven and complex. The key enzymes of
purine salvage are PRT enzymes. L. donovani
express three different PRT activities: HGPRT,
APRT, and XPRT (Figure 9.6). XPRT is an unusual
enzyme. Mammalian cells, which contain both
HGPRT and APRT enzymes, lack XPRT. All three
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA