trans

216 PURINES AND PYRIMIDINES
FIGURE 9.7 Giardia lamblia purine salvage and interconversion pathways. ★ indicates a deoxynucleoside
kinase activity. Enzyme identities are listed in Table 9.1.
with either hypoxanthine or guanine. These
data provide powerful suggestive evidence that
the sole route of purine salvage in T. foetus is
through TfHGXPRT and that the parasite lacks
other routes of purine salvage (e.g. APRT or
AK), although such activities have been proposed.
Other purine enzymes that have been
detected include AD, GD, and PNP.
Giardia lamblia, Trichomonas vaginalis, and
Entamoeba histolytica
The purine salvage pathways of G. lamblia,
T. foetus, and E. histolytica all differ dramatically
from those of the Apicomplexa, Kinetoplastida,
and T. foetus in that they cannot interconvert
AMP, IMP, and GMP. Thus, none of these parasites
would be expected to contain ASS, ASL,
IMPDH, GMPS, GMPR, or AMPD activities, and
they should not be able to survive or proliferate
in an environment without a source of both
adenine and guanine rings. Furthermore, G.
lamblia and T. vaginalis, but not E. histolytica,
do not appear to have a mechanism to generate
deoxynucleotides de novo. Thus, they also
require sources of environmental purine and
pyrimidine deoxynucleosides.
G. lamblia cannot salvage hypoxanthine,
xanthine, or inosine but can incorporate adenine,
guanine, and their corresponding ribonucleosides
(Figure 9.7). However, adenine and
adenosine are only incorporated into the
adenylate pool, while guanine and guanosine
are metabolized exclusively to guanylate
nucleotides. Thus, G. lamblia has no HPRT or
XPRT activities, and IMP is not the common
salvage intermediate for AMP and GMP synthesis.
Radioisotope incorporation experiments
have revealed that adenosine and guanosine
are first cleaved to their respective bases,
which are, in turn, phosphoribosylated by
APRT and GPRT, respectively (Figure 9.7). Both
GlAPRT and GlGPRT have been cloned and the
recombinant enzymes shown to exhibit strict
substrate specificities for their nucleobase
substrates. The three-dimensional structure
of GlGPRT has also been solved and provides
some important insights into the restricted
base specificity of the enzyme. Despite the
lack of evidence for direct phosphorylation
of naturally occurring nucleosides, a number
of analogs appear to be phosphorylated by
intact G. lamblia. Perhaps there exists a NPT
activity. There is no detectable RR activity in
G. lamblia, and exogeneous ribonucleosides
are not converted to DNA. Thus, G. lamblia
seems to obligatorily scavenge purine, as well
as pyrimidine, deoxynucleosides from the
environment, a process that requires direct
phosphorylation by kinase enzymes. A deoxynucleoside
kinase activity has been detected
in G. lamblia extracts.
Metabolic labeling studies have demonstrated
that T. vaginalis and E. histolytica also
do not interconvert AMP, IMP, and GMP, but
the pathways differ from those in G. lamblia in
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA