trans

AMINO ACID SOURCES 179
matrix proteins by adults. Adult Dirofilaria
immitis contains a 42 kDa aspartic proteinase,
which is present in a number of different tissues,
and a subtilisin-like serine proteinase,
furin. Metalloproteinases and cysteine proteinases
are present in secretions and extracts
of L3 and L4 larvae, and cysteine proteinase
inhibitors inhibit the third to fourth larval
stage molt.
Proteinase inhibitors
Mammals possess proteinase inhibitors capable
of inhibiting all catalytic classes; most are
class-specific, but a few, like 2 -macroglobulin,
have broad specificity. These proteinase inhibitors
have important regulatory functions,
and the disruption of the balance between a
proteinase and its inhibitor(s) is frequently the
cause of disease. During co-evolution with their
hosts at least some parasites seem to have
developed proteinase inhibitors that can protect
them from destruction by host proteolytic
activities, such as proteinases released from
leukocytes in the case of bloodstream parasites,
or digestive juices in the case of intestinal
parasites.
Little is known about proteinase inhibitors
in parasitic protozoa. Recently a serpin (serine
proteinase inhibitor) has been cloned and
sequenced from T. gondii. This inhibitor
belongs to the Kazal family, inhibits trypsin,
chymotrypsin, elastase and tryptase, and is
located in the dense granules of the tachyzoites.
Cystatin-like proteinase inhibitors act on C1
family cysteine proteinases, and have been
detected, but not characterized, in several
protozoan parasites.
Far more is known about proteinase
inhibitors from helminths. Nematodes contain
a number of serine proteinase inhibitors,
some of which belong to the serpin family, the
others to a different family of small proteinase
inhibitors, for which the name smapins has
been recently proposed. Related inhibitors also
are found in trematodes, such as Schistosoma
spp. Much less is known about cysteine proteinase
inhibitors.
Little is known of the actual functions of
these inhibitors, although they are presumably
important in the host–parasite relationship.
A microfilarial serpin specifically inhibits
two human neutrophil enzymes, cathepsin G
and elastase, and may protect the parasite in
the bloodstream from neutrophil attack. With
reference to smapins, their function seems
clear in at least two cases: in Ascaris they protect
against the proteolytic environment of the
host’s gut, and in the hookworm Ancylostoma
caninum they act as anticoagulants, inhibiting
several of the serine proteinases involved in
the blood clotting cascade.
Functions of parasite proteolytic enzymes
Digestive functions
Proteinases from different parasites participate
in the digestion of host proteins for nutrition.
Protein digestion is usually extracellular in the
helminths, with the proteinases secreted from
the gut epithelium in those worms with a digestive
tract. In protozoa most protein digestion
occurs in lysosomal compartments. Hemoglobin
degradation has been extensively studied in
two bloodstream parasites, Plasmodium spp.
and Schistosoma spp. P. falciparum has three
major proteolytic systems in the food vacuole,
a lysosome-like organelle responsible for the
massive hemoglobin digestion characteristic of
malarial trophozoites: the falcipains, the plasmepsins,
and falcilysin. Cysteine proteinase
inhibitors kill malarial parasites, both in vivo
and in vitro, with the concomitant accumulation
of hemoglobin in the digestive vacuole,
suggesting a major role for cysteine proteinases
in hemoglobin degradation. Specific inhibition
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA