trans

326 HELMINTH SURFACES
or carrier protein. While there is no functional
evidence for an NH 4 transporter in nematodes,
a homolog of the yeast NH 4 permease, MEP1,
has been identified in the C. elegans genome.
No information is yet available on the expression
or function of this putative transporter.
NH 3 diffuses freely across membranes, and
most nitrogen is probably excreted from
nematodes in this form, driving the conversion
of additional NH 4 to NH 3 . NH 3 excretion
across the cuticle–hypodermis complex would
be facilitated by the acidic microenvironment
maintained within the aqueous pores of the
cuticle, which would effectively ionize NH 3 as
it enters the cuticle pores. This process would
ensure low concentrations of NH 3 relative to
NH 4 in the aqueous pores, providing a driving
force favoring diffusion of the uncharged
species out of the worm. Among helminths,
this principle has been demonstrated experimentally
only for S. mansoni. At least some
members of the aquaporin family can also
transport urea, and it is possible that an aquaporin
contributes to nitrogen excretion from
nematodes.
Some parasites, including A. suum, T. spiralis
and Nippostrongylus brasiliensis, excrete low
levels of short-chain aliphatic amines. Excreted
amines were once thought to neutralize the
acidic microenvironment of nematodes. The
pK a s of these amines, however, fall between
10.05 and 10.65. Therefore, it is unlikely that
they contribute substantially to the buffering
capacity in the pH range of gastrointestinal
worms, as they would be almost completely
protonated at pH values below 7.5. An organic
cation transporter cloned from C. elegans provides
a potential route for the excretion of
aliphatic amines, but no information is available
on its expression. The classic substrate
for this transporter is tetraethylammonium,
but it also transports choline and a variety of
basic drugs.
Role of the cuticle–hypodermis complex in
drug absorption
The cuticle is an important site for the absorption
of anthelmintics and other small organic
molecules. For example, accumulation of levamisole
by A. suum can be accounted for solely
by transcuticular diffusion. For the filariae,
B. pahangi and Dipetalonema viteae, absorption
indices for a wide range of non-electrolytes
show no obvious relationship with lipophilicity.
When other physicochemical parameters,
such as molecular weight, dipole moment and
total energy are considered along with log K,
quantitative predictions can be made about
the absorption of non-electrolytes by filarial
nematodes. This indicates that absorption is
influenced by non-lipid components in the
cuticle, and contrasts sharply with data from
the trematode, S. mansoni, for which absorption
kinetics can be predicted by the single
variable log K.
A. suum and H. contortus have also been
used to determine how physicochemical properties
are related to absorption rates of drugs.
As in filariae, log K alone does not accurately
predict absorption rate, and collagen and lipid
components of the cuticle each present a distinct
barrier to diffusion of organic molecules.
Diffusion across the collagen barrier is highly
dependent on molecular size and charge of the
permeant. The permeability of neutral solutes
decreases with increasing size, and positively
charged molecules penetrate faster, and anions
slower, than neutral solutes of comparable
size. These results indicate that the cuticle of
A. suum contains functional, negatively charged
aqueous pores that are about 15 Å in radius.
The functional pores that mediate permeability
through the cuticle of gastrointestinal parasites
must consist of tortuous paths through
the crosslinked fibers of the cuticle. Electrondense
solutes do not migrate through recognizable
channels in A. suum cuticle.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS