trans

AMINO ACID SOURCES 173
as pentamidine and some derivatives, on the
other hand, competitively inhibit arginine
transport, and diamidines may be incorporated
through this arginine transporter. The
transport systems for arginine have also been
characterized in Trypanosoma cruzi and in
Trichomonas vaginalis.
The intraerythrocytic stages of Plasmodium
falciparum require glutamine, the most abundant
amino acid in plasma, and this requirement
is satisfied through a parasite-induced
increase in the permeability of the red blood cell
to glutamine. Normal erythocyte membranes
are relatively impermeable to this amino acid.
Cysteine is essential for growth of Giardia
lamblia, which cannot synthesize cysteine
de novo or from methionine. This parasite has
two transport systems for cysteine, one nonsaturable
and probably representing passive
diffusion, and a second specific for
SH-containing amino acids. Cysteine also is
essential for growth of T. brucei bloodstream
trypomastigotes, but the transport system
responsible for its uptake has not been characterized.
G. lamblia has a functional L-alanine
transporter, probably an antiport system catalyzing
the exchange of alanine, serine, glycine
and threonine. This antiport seems to function
for both the influx and efflux of alanine.
Parasitic protozoa frequently contain high
concentrations of free amino acids. L-alanine
is usually the most abundant, as reported in
some trypanosomatids, G. lamblia and T. vaginalis.
In contrast, glutamate, leucine, proline
and valine are the most abundant amino acids
in Entamoeba histolytica.
Helminths have amino acid pools that are
usually larger than those in vertebrates, and
these pools frequently consist of only a few
amino acids, normally related to TCA intermediates,
such as alanine, glutamate, proline,
serine and glycine. In addition, a number of
non-protein amino acids, such as 3-alanine,
2-, 3- and 4-aminoisobutyrate, 3- and 4-aminobutyrate,
citrulline, ornithine, taurine, creatinine,
homoserine, norleucine and norvaline,
have been detected in helminths.
Proteolysis
The proteolytic enzymes in parasites have
been widely studied during the last decade. In
addition to their expected role in the nutrition
of the parasite at the expense of host proteins,
proteinases are relevant to important aspects
of the host–parasite relationship, to the point
of being considered virulence factors for some
parasites. In some cases, proteinases are major
parasite antigens, and thus participate in the
development of the immune response of the
host. In other cases, proteinases act to protect
the parasite against the immune response, by
degrading immunoglobulins bound to surface
antigens. Proteinases are fundamental for the
invasion of tissues by some parasites, either
from outside the host’s body, as in Schistosoma
spp., or from the digestive tract into organs
like the liver, in the case of E. histolytica.
Proteolytic enzymes may participate in the
penetration of intracellular parasites into the
mammalian cell, and/or in the release of
the parasites from the infected cell, to invade
new cells and perpetuate the infection. Proteolytic
degradation for nutritional purposes
can be either intracellular (as in the malaria
parasites) or extracellular, in the digestive
tract (in some helminths).
Proteolytic enzymes
The general name of peptidases is applied to
all the enzymes able to hydrolyze the peptide
bond. Exopeptidases hydrolyze peptide bonds
located at one end of the polypeptide chain.
Aminopeptidases start hydrolysis from the
N-terminus, and carboxypeptidases cut from
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA