trans

192 PROTEIN METABOLISM
been described (section on Threonine and
Serine). Recent studies with Leishmania mexicana
indicate that, whereas acetate is readily
incorporated into fatty acids both in promastigotes
and amastigotes, leucine was the only substrate
efficiently incorporated into sterols by
both parasite stages. Promastigotes of L. mexicana,
L. major and L. amazonensis produced
about 75% of their sterols from leucine, whereas
the same stage from other Leishmania spp.
(L. braziliensis, L. donovani and L. tropica) produced
much less, around 30%, of their sterols
from leucine. Endotrypanum monterogeii also
produced a large proportion of its sterols (77%)
from leucine, whereas T. cruzi produced only
8%, the rest arising from acetate. All these
Trypanosomatids produced fatty acids from
acetate. 13 C-NMR studies indicated that promastigotes
of L. mexicana incorporate the intact
carbon skeleton of leucine into the isoprenoid
pathway leading to sterol, without previous
conversion to acetyl-CoA, as in plants and animals.
However, the conversion to acetyl-CoA
occurs, since carbon from leucine is also incorporated
into fatty acids, which are optimally
synthesized from acetate. The presence of this
hitherto unknown pathway for sterol biosynthesis
in Trypanosomatids opens up a new
possibility for chemotherapy.
from Plasmodium lophurae and P. chabaudi.
Recently, the gene encoding the enzyme has
been cloned and sequenced from P. falciparum,
and exhibits 38–47% identity to SHMTs from
other organisms. In addition, most of the
other genes involved in folate synthesis have
been cloned and sequenced from P. falciparum.
The shikimate pathway is present
in Apicomplexa, and has previously been
reported in bacteria, algae, fungi and higher
plants, but is absent in mammals. This pathway
seems to be essential for the parasites
because of its involvement in folate synthesis,
through the formation of p-aminobenzoate,
and not for the synthesis of aromatic amino
acids. Growth of P. falciparum, T. gondii and
Cryptosporidum parvum is inhibited by the
herbicide glyphosate, and this inhibition is
reversed by addition of p-aminobenzoate.
A homolog of p-aminobenzoate synthase
has been recently detected in P. falciparum.
SHMT has also been detected in C. fasciculata
and T. cruzi. C. fasciculata has three isoforms,
localized in the mitochondrion, glycosome
and cytosol. All three isoforms are tetrameric,
like SHMTs from other organisms. The T. cruzi
enzyme differs from the others in being
monomeric, with an apparent molecular weight
of 69 kDa.
Serine and folate metabolism
Serine has an important role in folate metabolism,
through its participation in the reaction
catalyzed by serine hydroxymethyltransferase
(SHMT), which transfers the -carbon of serine
to tetrahydrofolate (THF) to give 5,10-
methylene tetrahydrofolate (CH 2 THF) and
glycine. CH 2 THF then transfers its one-carbon
unit to dUMP to give dTMP, or, with the previous
formation of methyl-THF, to homocysteine
to give methionine (Figure 8.8). SHMT
has been partially purified and characterized
Cysteine and synthesis of glutathione
Glutathione (GSH) is a tripeptide (-glutamylcysteinyl-glycine)
present at fairly high concentrations
in most cells, where it acts as
a protecting agent against oxidative damage.
In Trypanosomatids glutathione is present
primarily as trypanothione (bis-glutathionyl
spermidine). Tripanothione synthesis is mediated
by two enzymes, -glutamyl-cysteine synthetase
and glutathione synthetase, with the
utilization of two moles of ATP per mole of
GSH synthesized. Most parasites contain GSH,
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA