trans

144 ENERGY METABOLISM – TRYPANOSOMATIDAE
T. brucei. Transport across the plasma membrane
of the substrate of the pathway, glucose,
and its end-products, pyruvate, and under
anaerobic conditions, glycerol, are all mediated
by facilitated diffusion transporters. No information
is available about metabolite transporters
in the glycosomal membrane.
Glucose transport
T. brucei utilizes glucose, fructose and mannose
as substrates for the glycolytic pathway,
and the glucose transporter present in the
plasma membrane of bloodstream forms is
capable of transporting all three substrates.
The glucose transporter is considered not only
an ideal target for drugs that would directly
interfere with glucose transport, but the transporter
could also be instrumental in the import
of drugs into the trypanosome’s cytosol by
complexing them to sugar analogs. Detailed
studies on the transporters of T. equiperdum,
T. rhodesiense and T. brucei have been carried
out. They have revealed important differences
in specificity between parasite and host transporters
and these have been exploited for
the synthesis of specific inhibitors of the trypanosome
glucose transport, as well as for
targeting drugs to the trypanosome’s cytosol.
Unfortunately, sugar analogs containing various
bulky substituents at carbon positions that
were not required for recognition of the transporter
were still able to bind to the transporter,
but not to enter the cell by facilitated diffusion.
This inability of the glucose transporter to
transport such bulky analogs will render it less
suitable for this interesting type of approach.
Pyruvate transporter
The plasma membrane of T. brucei contains a
facilitated diffusion carrier which can be saturated
by pyruvate (K m 2 mM). The activity of
the carrier is sufficient to account for the rate of
pyruvate production by the cell and is specific
for pyruvate in that it does not transport lactate.
It is inhibited by a number of other monocarboxylic
acids including pyruvate analogs,
but not L-lactate, which makes it unique among
the monocarboxylate transporters. Transport
inhibitors like di-isothiocyanostilbene
disulfonate, quercetin and phloretin all inhibit
pyruvate transport. The pyruvate carrier is
rather insensitive to the monocarboxylate
carrier inhibitor -cyano-4-hydroxycinnamate
(K i 10 mM) but could be completely blocked
by UK5099 [-cyano(1-phenylindol-3-yl)acrylate]
(K i 55 M). Inhibition of pyruvate transport
results in the accumulation of pyruvate
within the trypanosomes causing acidification
of the cytosol and lysis of the cells, indicating
that carrier-mediated pyruvate export in the
bloodstream-form trypanosome is of vital
importance to their survival.
Glycerol transporter
In T. brucei bloodstream forms, glycerol is transported
across the plasma membrane by both
simple diffusion and by a saturable facilitateddiffusion
carrier. At low concetrations of glycerol,
the carrier, with a K m of 0.17 mM for
glycerol, is responsible for most of the transport
of glycerol, but at higher concentrations simple
diffusion predominates. Transport is neither
sodium-dependent nor proton-motive-force
driven. The transporter is sensitive to phloretin
and cytochalasin B and is also inhibited by the
substrate analog glyceraldehyde. In procyclic
insect forms glycerol is taken up by simple
diffusion only.
Hexokinase (HK)
Hexokinase, the first enzyme of the glycolytic
pathway, was one of the last enzymes for
which sequence information became available.
HK has a low percentage of identity (36%)
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA