trans

GLYCOLYSIS 341
fluctuate between periods of glycogen degradation
and synthesis, and the replenishment
of glycogen reserves is not induced by a marked
decrease in the glycogen levels, but instead
occurs slowly and continuously. Therefore,
the role of the glycogen metabolism in blooddwelling
parasites like adult schistosomes,
that continuously reside in glucose-rich environments,
is still not clear. Since the replenishment
of the endogenous glycogen reserves in
adult S. mansoni is not induced by a marked
decrease in the glycogen levels, but occurs
slowly in vivo and continuously in each and
every worm pair, it is likely that glycogen is
degraded intermittently, for instance for muscle
contraction or tegumental membrane repair.
This metabolism of glucose ‘through’ glycogen
could help to maintain a low internal free glucose
pool and so promote sufficient glucose
import to deeper tissues through diffusion. In
this respect it is noteworthy that adult schistosomes
exhibit relatively high glucose 6-phosphatase
activity, which would allow glucose
derived from degraded glycogen stores in one
cell to be exported to other cells within the adult.
A balance between the rate of phosphorylation
of imported glucose by hexokinase and the rate
of glucose released from internal glycogen stores
by glucose 6-phosphatase may help maintain
an even distribution of glucose throughout
the worm, and promote continuous, energyindependent
movement of free glucose into the
worm down a concentration gradient.
Trehalose is a soluble -1-linked nonreducing
disaccharide of glucose, and in many
parasitic nematodes is more abundant than
glucose. Trehalose is synthesized by the combined
action of trehalose 6-phosphate synthase
and trehalose 6-phosphate phosphatase,
and is degraded to glucose by the action of
trehalase. These enzymes have been detected
in parasitic nematodes, but not extensively
characterized. Trehalose levels are usually
lower than those of glycogen, but trehalose
still may function as a critical carbohydrate
reserve in many nematodes and their eggs.
However, since the storage of glycogen is energetically
more favorable than trehalose, many
authors have suggested that trehalose might
also serve as an intermediate in glusose transport
to the tissues, as has been well documented
in insects. Trehalose is also clearly
involved in stress responses in many nematodes,
and may be important in resistance to
desiccation and in cryoprotection.
GLYCOLYSIS
Glycolysis is the degradation of glucose to yield
two molecules of pyruvate, and this almost
universal pathway is also used by parasitic
helminths for the catabolism of glucose, their
main substrate for energy generation. Glycolysis
occurs in the cytosol, and the compartmentalization
of glycolytic enzymes within a
membrane-bound organelle, like the glycosomes
of some protozoan parasites, has not
been observed in parasitic helminths. Based
on the end-products of the glycolytic pathway
and their further metabolism, parasitic
helminths can be classified into three types:
1. Those parasitic helminths that use the complete
glycolytic pathway and export the endproduct,
pyruvate, into the mitochondria
for further degradation via the tricarboxylic
acid cycle. This aerobic degradation occurs
mainly in larval stages of cestodes, trematodes
and nematodes (see section on the
Aerobic/Anaerobic Transition).
2. Those that use the complete glycolytic
pathway, but cytoplasmically convert its
end-product, pyruvate, to more reduced
end-products such as lactate and ethanol.
This fermentation process, so-called
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS