trans

300 HELMINTH SURFACES
protease inhibitor. All of these mechanisms,
and others, may be important.
Functional biology
Digestion
Digestive enzymes, such as alkaline and acid
phosphatases, disaccharidases, and esterases
are associated with the outer tegumental membrane.
However, it is not known if these enzymes
are derived from the parasite or the host. These
enzymes may participate in extracorporeal
digestion of nutrient macromolecules, i.e. ‘contact
digestion’, although this process has not
been established conclusively in cestodes.
Nutrient absorption
Cestodes derive most of their energy from
glucose, which is absorbed at high rates
across the tegument, primarily by an active
transport system. Glucose is absorbed against
a steep concentration gradient by cysticerci
and adults, and the process is inhibited by
agents that block ATP synthesis, including
p-chloromercuribenzoate, iodoacetate and 2,4-
dinitrophenol. Glucose uptake is also blocked
by phlorizin, a specific inhibitor of Na -coupled
glucose transport in mammals and many other
organisms. Galactose and several other monosaccharides
competitively inhibit glucose
absorption, indicating the presence of a common
carrier system for hexoses. Though the
mechanisms of carbohydrate absorption in cestodes
and mammals are generally similar, substrate
selectivity is greater in cestodes, and only
glucose and galactose are accumulated. Based
on immunohistochemical data, high levels of
a protein recognized by antibodies to a vertebrate
Na -dependent glucose transporter
(SGLT) are found in the external cyst wall of
T. solium neurocysticerci, and also in the apical
membrane of the tegument of adult worms.
The gene encoding this SGLT homolog has
not been cloned, so it has not been possible
to characterize its transport properties and
pharmacology.
Cestodes also may absorb glucose by facilitated
diffusion, though this process is less
thoroughly delineated than active transport.
However, genes for two facilitated glucose
transporters, TGTP1 and TGTP2, related to the
mammalian transporter GLUT1, have been
identified in T. solium. Both proteins are localized
in cytons and in the apical membrane of
the tegument. When cRNAs for these transporters
are functionally expressed in Xenopus
oocytes, they confer functional glucose transport
that is inhibited by natural stereoisomers
of D-glucose and D-mannose, but not by
L-glucose or ouabain. The significance of this
facilitated glucose transport system in cestodes
must be reconciled with pharmacological
data showing that phloretin, a known
inhibitor of facilitated glucose transport in
other organisms, does not inhibit glucose
transport in intact cestodes.
Glycerol is absorbed by H. diminuta by passive
diffusion at high concentrations (0.5 mM)
and by a carrier-mediated process at lower
concentrations. At low concentrations, absorption
of glycerol is non-linear, dependent on
temperature and pH, and competitively inhibited
by -glycerophosphate. The existence of
two distinct glycerol carriers is suggested by
studies which show that only about half the
saturable component in H. diminuta is Na -
sensitive and inhibitable by 1,2-propanediol.
In H. diminuta, two distinct carrier-mediated
transport systems for fatty acid absorption
have been partially characterized, one specific
for short-chain fatty acids and the other for
longer-chain molecules. Acetate absorption
by H. diminuta is mediated by the short-chain
carrier at low concentrations, but passive diffusion
occurs at high concentrations.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS