trans

206 PURINES AND PYRIMIDINES
Amitochondriates
Giardia lamblia and Trichomonas vaginalis
G. lamblia cannot synthesize pyrimidines
and deoxynucleosides, and must be able to
access a broad range of host nucleosides.
Saturable transport activities have been
described for both nucleobases and nucleosides
in G. lamblia. These have been classified
as type 1, specific for thymidine, uridine, and
possibly their nucleobases; type 2, a broadspecificity
transport activity for purine and
pyrimidine nucleosides and deoxynucleosides
but which discriminates against nucleobases;
and type 3, a low-affinity nucleobase
transporter. A type 2 activity has also been
described in T. vaginalis. Currently, none of
these transport activities has been analyzed at
the molecular level, although the sequence for
a porin-like transporter that was identified
by functional complementation of nucleoside
transport-deficient Escherichia coli has been
deposited in GenBank.
synthesizing purines de novo, it is likely that
these parasites possess multiple nucleobase
and nucleoside transport mechanisms. Thus,
conclusions based on uptake in whole parasites
are complicated by the presence of overlapping
activities.
Cestoda
In the cestode Hymenolepis diminuta, three
transport foci have been delineated for nucleobases.
These comprise a thymine–uracil carrier,
a transport activity that appears specific
for purine nucleobases, and a third type of
activity that is apparently specific for hypoxanthine.
Little is known about nucleoside
transporters in these parasites. Uridine uptake
appears to be carrier-mediated and is inhibited
by both purine and pyrimidine nucleosides.
Whether there are several permeases or
one broad-specificity carrier has not been
determined. Likewise, the effect of nucleobases
on uridine transport has not been determined.
Entamoeba histolytica
A saturable adenosine transport activity inhibited
by other nucleosides has been demonstrated
in E. histolytica. Sequences homologous
to ENT-type transporters have been deposited
within GenBank, but these genes have not been
functionally validated.
Helminths
There are no molecular clones available to study
purine and pyrimidine transporters in worms.
Most of our knowledge about nucleoside and
nucleobase transport systems in helminths is
based upon incorporation studies undertaken
several decades ago with intact parasites. Since
worms are multicellular and also incapable of
Trematoda
Most information on nucleoside and nucleobase
transport in trematodes is based upon
early work with Schistosoma mansoni. Uptake
of radiolabeled pyrimidine bases is nonsaturable
and unaffected by high exogenous
concentrations of pyrimidines. Uptake of purine
nucleobases is apparently carrier-mediated. It
is possible that this transporter also may recognize
the purine nucleosides adenosine and
guanosine.
Nematoda
Relatively little is known about the transport
mechanisms in nematodes, although they
can salvage both purine and pyrimidine
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA