trans

182 PROTEIN METABOLISM
in vivo, in infected mice. The activity of cysteine
proteinase inhibitors against T. cruzi,
and also against other parasitic protozoa,
observed in cultured systems or in animal
models of infection, correlates with the direct
inhibition of the proposed targets. Most of the
inhibitors are also able to inhibit the mammalian
cathepsins, yet mammalian cells are
not adversely affected by concentrations
which effectively kill the parasite. This selective
effect may be due to the redundancy of
proteolytic activities in higher eukaryotic cells
compared with parasitic protozoa. In T. cruzi,
cruzipain inhibitor-resistant parasites were
not also resistant to the established drugs,
nifurtimox and benznidazole, used for treatment
of Chagas disease, indicating that their
mechanisms of action are different. These
studies point to parasite proteinases as valid
targets for the development of much needed
alternative drugs against tropical diseases.
AMINO ACID METABOLISM
The disposal of amino nitrogen:
aminotransferases and glutamate
dehydrogenase
Aminotransferases and glutamate dehydrogenases
have a central role in amino acid
metabolism, both in catabolic and biosynthetic
processes. Aminotransferases catalyze
the exchange of the amino and keto groups
between amino acids and -ketoacids, and
act frequently as a first step in amino acid
degradation, by transferring the amino group
to -ketoglutarate to yield L-glutamate.
L-glutamate is the substrate of the glutamate
dehydrogenases, which deaminate glutamate
to -ketoglutarate and ammonia, with
the concomitant reduction of NAD or NADP
(Figure 8.1). Mammals have a glutamate
dehydrogenase that can use both coenzymes,
whereas protozoa, like plants, fungi and bacteria,
have usually one or two coenzymespecific
enzymes. Aminotransferases have
been reported in all protozoa, and frequently
several enzymes, with different specificities,
are present. For example, T. vaginalis contains
at least four aminotransferases: one specific
for ornithine and lysine; a second for
the branched amino acids; a third for aspartate
and aromatic amino acids; and a fourth
for alanine. The most frequently identified
enzymes are alanine aminotransferase and
aspartate aminotransferase; the presence and
usually high levels of the former enzyme are
associated with the production of L-alanine,
which is a frequent final product of glucose
catabolism in protozoa (see below).
Glutamate dehydrogenases also are found in
most parasitic protozoa. T. cruzi has two
enzymes, one NADP-linked and the other NADlinked.
The NADP-linked enzyme is strictly
cytosolic, and has been proposed to have a
biosynthetic role. However, enzyme levels are
developmentally regulated, and are higher in
the gut stages present in the insect, where
amino acids can be expected to be the main
source of energy. NADP-linked glutamate dehydrogenases
have also been characterized from
T. vaginalis, Plasmodium chabaudi, G. lamblia,
and P. falciparum. The T. cruzi NAD-linked
glutamate dehydrogenase is mitochondrial,
although there seems to be also a cytosolic isoform,
and it probably has a catabolic function.
The functions of the glutamate dehydrogenases
are probably different in different protozoa. The
NADP-linked enzyme appears to function in
NADPH generation in P. falciparum, whereas in
T. cruzi and G. lamblia it appears to be involved
in NADPH reoxidation and is essential, together
with alanine aminotransferase, for the production
and excretion of alanine. The NADP-linked
glutamate dehydrogenase from P. falciparum
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA