trans

MITOCHONDRIAL METABOLISM AND ENERGY GENERATION 349
enzymes involved in rhodoquinone biosynthesis
would be unique to helminths and obvious
targets for chemotherapy. Similarly, Complex II,
which comprises about 8% of the total
membrane protein in adult A. suum mitochondria,
contains a low potential cytochrome, b 558 ,
whose properties differ significantly from
those of cytochrome b 560 , commonly found in
Complex IIs isolated from aerobic mitochondria.
Cytochrome b 558 is reduced by NADH or
succinate and reoxidized by fumarate in the
presence of the appropriate A. suum respiratory
complexes. These data suggest that cytochrome
b 558 also may play an important role in determining
the direction of electron flow in these
anaerobic organelles.
In addition to the oxidative decarboxylation
of malate, intramitochondrial NADH also is
generated from the oxidative decarboxylation
of pyruvate to acetyl CoA, catalyzed by the
pyruvate dehydrogenase complex (PDC). The
PDC is a large multienzyme complex consisting
of three catalytic components: pyruvate
dehydrogenase (E1), dihydrolipoyl transacetylase
(E2), and dihydrolipoyl dehydrogenase
(E3) and, in eukaryotes, an E3-binding protein
(E3BP) and an associated E1 kinase and E1
phosphatase. The reoxidation of NADH generated
by pyruvate oxidation also may be coupled
to fumarate reduction or used to drive the
synthesis of branched-chain fatty acids. In
A. suum, no other reactions have been identified
which generate sufficient reducing power
to account for the quantities of reduced products
formed. During anaerobic metabolism,
intramitochondrial NADH/NAD and acyl
CoA/CoA ratios are elevated dramatically to
levels that potently inhibit the activity of PDCs
isolated from aerobic mitochondria. This inhibition
results from both end-product inhibition
and the stimulation of E1 kinase activity
that catalyzes the reversible phosphorylation
and inactivation of the complex. Given these
observations, it was initially surprising to find
a PDC with a functional kinase in adult A. suum
muscle mitochondria, since under the elevated
NADH levels present during microaerobiosis,
maximal flux through the complex would
be necessary to fuel the worms’ relatively inefficient
fermentative metabolism. In fact, PDC
activity in many facultative anaerobes, such as
E. coli, is downregulated during anaerobiosis,
and other enzymes better suited to function in
reduced environments, such as pyruvate:ferredoxin
oxidoreductase and pyruvate:formate
lyase are involved in pyruvate decarboxylation.
Similar enzyme systems have been identified
in anaerobic parasitic protozoa, such as Giardia
lamblia and Trichomonas vaginalis, which
also lack a PDC. In contrast, in some obligate
anaerobes, such as Enterococcus faecalis, the
PDC is dramatically overexpressed and more
resistant to end-product inhibition.
Not surprisingly, PDC activity in adult
A. suum body wall muscle appears to be regulated
to maintain activity under the reducing
conditions present in the host gut. For example,
the PDC in adult body wall muscle is abundant;
in fact the PDC is more abundant in
A. suum body wall muscle than in any other
eukaryotic tissue studied to date and
approaches 2% of the total soluble protein. The
PDC also is less sensitive to end-product inhibition
by elevated NADH/NAD and acetyl
CoA/CoA ratios than PDCs from aerobic organisms.
Interestingly, E3, the enzyme directly
responsible for the NADH sensitivity of the
complex, is identical in both aerobic secondstage
larvae (L2) and anaerobic adult muscle.
However, the PDC from adult muscle contains
an unusual ‘anaerobic’ E3 binding protein
(E3BP), which lacks the terminal lipoyl domain
found in E3BPs from all other sources. The
binding of E3 to this ‘anaerobic’ E3BP significantly
reduces the sensitivity of the E3 to
NADH inhibition, and helps to maintain PDC
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS