trans

TRANSPORT SYSTEMS 201
to be discovered in parasitic protozoa, although
it is probable that helminths possess CNT-type
proteins.
Apicomplexa
Plasmodium
Most of our knowledge about purine transport
in plasmodia is based upon observations on
the intraerythrocytic stage of P. falciparum.
Manipulation of the exogenous purine source
in the cultivation medium suggests that, while
hypoxanthine is the preferred purine source,
the parasite also can salvage the purine nucleobases,
adenine and guanine. It also can
salvage the purine nucleosides, adenosine,
guanosine, and inosine. This implies that the
parasite possesses the necessary machinery to
transport purines from the extracellular milieu.
Intraerythrocytic plasmodia reside in a parasitophorous
vacuole (PV) within the host erythrocyte.
Mature erythrocytes are unable to
synthesize the purine ring de novo and contain
limited amounts of endogenous purines,
so preformed purines must be salvaged from
the host plasma. Therefore, the parasite has
evolved an intricate purine translocation pathway
(Figure 9.2).
The uninfected erythrocyte possesses a
broad capacity equilibrative nucleoside transporter,
hENT1, in its plasma membrane.
Shortly after invasion the parasite appears to
induce a new nucleoside transport activity
in the infected erythrocyte, which can be
distinguished from hENT1 since it is nonsaturable,
transports both D- and L-adenosine,
and is impervious to the hENT1 inhibitor,
4-nitrobenzyl-6-thioinosine. This induced
activity shares many of the properties of the
‘new permeation pathway’ proposed by Kirk
and coworkers, since it is non-saturable, lacks
stereo-specificity, and is exquisitely sensitive
to anion transport inhibitors such as
furosemide.
Parasites are separated from the erythrocyte
cytosol by the parasitophorous vacuolar membrane
(PVM). Thus, purines must traverse the
erythrocyte membrane, cytoplasm and PVM.
A nutrient channel within the PVM, which
appears to accommodate small solutes of
1400 Da or 23 Å, may function as a molecular
sieve, enabling nucleosides and nucleobases
to freely permeate the parasitophorous
vacuolar space. In addition, the mature stages
of the parasite possess a tubovesicular membraneous
network, which is contiguous with the
PVM and has been implicated in nucleoside
translocation. This may serve to increase the
surface area of the PVM, allowing for greater
permeation of nutrients.
P. falciparum mRNA extracted from intraerythrocytic
stages and expressed in Xenopus
oocytes identified an adenosine transporter
FIGURE 9.2 Transport of nucleosides and/or nucleobases
by the intraerythrocytic malaria parasite for
purine salvage. NPP, new permeation pathway; TVM,
tubovesicular membrane; RBCM, red blood cell membrane;
PVM, parasitophorous vacuole membrane; PPM,
parasite plasma membrane; PfNT1, P. falciparum
nucleoside transporter 1; PVNC, parasitophorous vacuole
nutrient channel; hENT1, human erythrocyte
nucleoside transporter 1.
BIOCHEMISTRY AND CELL BIOLOGY