trans

324 HELMINTH SURFACES
are typically transported via a circulatory system
from the tissue of origin to an excretory
organ, where they are eliminated in a fluid
(e.g. urine). Nematodes possess an intestine
and exhibit defecation. However, nematodes
lack a recognizable kidney (though see tubule
system, below), and soluble waste products
are apparently not concentrated prior to elimination.
Late larval and adult stages of parasitic
nematodes are primarily anaerobic, and
oxidize only a small fraction of ingested carbohydrates
to CO 2 prior to excretion. Instead,
they produce and excrete low molecular weight
organic anions, such as acetate, lactate and
butyrate. The pseudocelom and PCF serve
together as a circulatory system, transporting
these molecules from muscle and reproductive
tissue to the hypodermis, intestine and
possibly the tubule system for excretion into
the environment.
Nematodes derive most of their energy from
the degradation of glucose (typically stored as
glycogen or trehalose) or other sugars, including
fructose. They excrete the end-products of
energy metabolism, which would be toxic if
allowed to accumulate. Little is known about
the pathways through which CO 2 or alcohols
are excreted from nematodes, even though
these processes are found in most animal parasitic
species. In vertebrates, CO 2 is exported
from tissues via a Cl /HCO
3 exchanger. No
predicted C. elegans protein has been assigned
this function. Higher alcohols, such as ethanol,
are excreted from some nematodes, including
C. elegans, presumably by simple diffusion.
Glycerol, another major excretory product, is
transported out of cells by aquaporin-type
channels in other organisms. No data are available
on whether nematode aquaporins can
transport glycerol. In contrast to the absence of
information on the mechanisms underlying
CO 2 or alcohol elimination, many studies on
organic acid excretion from adult stages of
animal-parasitic nematodes have been published.
The physiology of organic acid excretion
in these species will be the focus of the remainder
of this discussion.
In A. suum, the H concentration in cells
and extracellular fluids is regulated to a level
that maintains the pH close to 7.0. In adult
A. suum and other animal parasites, organic
acid products of intermediary metabolism
have pK a s that range from 2.0 (lactic acid) to
4.8 (branched-chain fatty acids, such as
2-methylvaleric acid). At pH 7.0, at least 99%
of the organic acids produced by carbohydrate
metabolism exist in the dissociated form, i.e.
as H and the conjugate organic anion. This
point is important: non-dissociated organic
acids are quite lipophilic, and can exit tissues
via simple diffusion. However, organic anions
are poorly soluble in lipids and almost certainly
require a protein-mediated transport process
for excretion.
Animal parasites lower the pH of incubation
media to levels that are equivalent to the pK a of
acids they excrete, supporting the concept that
they excrete organic anions and H into their
environment. The molecular mechanisms that
mediate H excretion through the hypodermis
and intestine have not been defined, though it
appears to occur through a different process
than organic anion excretion. In vertebrates,
protons are excreted by a Na /H antiporter at
pH 7.0, and by an HCO 3 /Cl exchanger at
pH 7.0. The electrical driving force for H
extrusion in the first case is provided by the
steep, inward Na gradient across vertebrate
cell membranes. Orthologous H antiporters
have not yet been identified in nematodes.
Less well characterized transport systems, such
as an electrogenic H -translocating ATPase in
vertebrate distal nephron, and H conducting
cation channels in vertebrate renal brush border
cells and snail neurons, also play a role in
H excretion. Based on sequence homology,
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS