trans

AMINO ACID METABOLISM 191
Methionine adenosyltransferase, the enzyme
which synthesizes S-adenosylmethionine
(SAM) from methionine and ATP, is present in
some parasitic protozoa, although a number of
parasites can take up SAM from the medium.
Methionine adenosyltransferase has been
detected in T. brucei, T. vaginalis and partially
characterized in Leishmania braziliensis panamensis
and Leishmania infantum. SAM decarboxylase
is also present in parasitic protozoa,
including T. b. brucei and T. b. rhodesiense, and
in T. vaginalis. Further, the gene encoding the
T. cruzi enzyme has been recently cloned,
sequenced and expressed in E. coli. It is only
25% homologous to the human SAM decarboxylase.
The P. falciparum SAM decarboxylase
has the unique characteristic of being present
as part of a bifunctional protein, the other half
possessing ODC activity. The N-terminal SAM
decarboxylase is connected to the C-terminal
ODC by a hinge region. SAM decarboxylases are
normally heterotetrameric proteins, with two
different subunits arising from self-cleavage
of the pro-enzyme. The tetrameric structure of
the SAM decarboxylase moiety, as well as the
dimeric structure of the ODC moiety, are conserved
in the bifunctional enzyme.
Spermidine synthase has been recently
cloned and sequenced from L. donovani, and
has 56% amino acid identity with the human
enzyme. Null mutants are polyamine auxotrophs,
a requirement that can be satisfied by
spermidine, but not by putrescine or other
diamines, nor by spermine. This latter finding,
consistent with that of L. donovani ODC deletion
mutants, implies that the pathway from
spermine to spermidine, present in mammalian
cells, is not present in these organisms.
Helminths seem to take up polyamines
from the medium; the biosynthetic enzymes
described above are apparently absent.
Helminths have, on the other hand, enzymes
for the interconversion of spermine back to
spermidine, and from spermidine back to
putrescine, that seem to be lacking in protozoa.
Another interesting aspect of helminth
polyamine metabolism is that, unlike most
other eukaryotes, including protozoa, they
usually contain much less putrescine than
spermidine and spermine.
Arginine and synthesis of nitric oxide
The production of nitric oxide (NO) from arginine
by nitric oxide synthase (NOS) is involved
in the protective responses of the host against
a number of parasites. On the other hand,
mounting evidence indicates that, at least in
some cases, the parasites themselves produce
NO, probably involved in their regulatory cascades.
NOS has been partially purified from
T. cruzi, is activated by Ca 2 , and is stimulated
by glutamate and N-methyl-D-aspartate. Cyclic
GMP appears to be controlled by L-glutamate
through a pathway mediated by NO. NOS seems
to be localized in the inner surface of cell membranes
and in free cytosolic clusters in the body,
flagellum and apical extreme. A soluble NOS
purified 2800-fold from L. donovani is a dimer
made up of 110 kDa subunits, requires NADPH
and is inhibited by EGTA. Several lines of evidence
suggest that it is similar to the mammalian
NOS isoform I. NOS have also tentatively
been identified in the intraerythrocytic forms of
P. falciparum, and in neuromuscular tissue of
the nematode Ascaris suum. The malarial NOS
activity, present in lysates of infected red blood
cells, is independent of Ca 2 , inhibited by three
well known NOS inhibitors, and has a molecular
weight of about 100 kDa, suggesting that it is different
from the isoforms present in mammals.
Threonine and leucine in lipid synthesis
The role played by threonine in producing
acetate for incorporation into fatty acids has
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA