trans

PURINE METABOLISM 215
specificities of the nucleoside-cleaving activities
are different, and AD expression is
unknown.
Trypanosoma brucei
Metabolism of purines by T. brucei is qualitatively
similar to that by T. cruzi. Bloodstreamform
T.b. gambiense and T.b. rhodesiense and
procyclic T.b. gambiense all salvage bases and
nucleosides efficiently. The observation that
all four bases are incorporated into nucleotides
implies the existence of three distinct PRT
activities. Only TbHGPRT, both in its native
and recombinant forms, has been purified
and studied in some detail. The enzyme, like
its leishmanial counterpart, is localized to the
glycosome, is specific for hypoxanthine and
guanine, and recognizes allopurinol. T.b. gambiense,
T.b. rhodesiense and T.b. brucei all can
incorporate allopurinol into 4-aminopyrazolopyrimidine
nucleotides and RNA, implying
intact ASS and ASL activities. Purine nucleoside
salvage proceeds either through direct
phosphorylation via kinase or phosphotransferase
activities and by cleavage to the base.
AK activity has been detected. Nucleoside cleavage
activities in procyclic forms include two
NHs, one specific for purine ribonucleosides
and one for pyrimidine nucleosides, and an
adenosine/methylthioadenosine phosphorylase
activity. Bloodstream-form T.b. brucei
appears to contain two nucleoside phosphorylase
activities, one for inosine, adenosine, and
guanosine (IAGNH), and one specific for
adenosine and 5-methylthioadenosine. The
gene encoding TbIAGNH has been cloned, and
the properties of both the native and recombinant
IAGNH enzyme have been investigated in
detail. T.b. gambiense procyclic and T.b. brucei
bloodstream parasites lack both ADA and
AD activities but can deaminate guanine.
T. congolense and T. brucei appear to metabolize
purines similarly. The lack of AD appears to
be a defining difference between most trypanosomes
and Leishmania. The exception is
T. vivax, which has been reported to have AD.
Amitochondriates
Tritrichomonas foetus
The ability of T. foetus to salvage all four purine
bases, as well as adenosine, inosine, and
guanosine, into both adenylate and guanylate
nucleotides, indicates that the parasite contains
intact branchpoint and AMPD and
GMPR activities. TfIMPDH is a well-characterized
enzyme that differs from its mammalian
counterpart in a manner that could be therapeutically
exploited, and whose gene has been
cloned and crystal structure determined.
Competition studies for nucleoside uptake by
the corresponding nucleobases implies that
inosine is primarily cleaved to hypoxanthine,
whereas adenosine and guanosine can be
directly phosphorylated, but this model is
unconfirmed by enzymological studies. A single
phosphoribosyltransferase, TfHGXPRT, that
can phosphoribosylate all three 6-oxypurines,
has been studied in detail, and the crystal
structure of the recombinant enzyme has been
solved to 1.9 Å resolution. Wang and coworkers
have dissected the amino acid determinants
that govern substrate specificity of TfHGXPRT.
Specific site-directed mutations have converted
TfHGXPRT into an HGPRT which cannot recognize
xanthine and into an HGXAPRT which
can also recognize adenine. Structure-based
drug design approaches have also identified
novel classes of TfHGXPRT inhibitors that can
inhibit the enzyme at nanomolar concentrations
without affecting the human HGPRT.
These enzyme inhibitors also impede parasite
growth, and this growth inhibition could be
circumvented by supplementing the medium
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA