Cellular Superfund Site: Not your usual superoxide source?
Authors: Davenport R. John 
Published: August 2003

Like a factory smokestack that spews toxic waste, a protein assembly line in mitochondria spurts out poisonous oxygen-containing compounds. Scientists have blamed two stations along the line for producing these so-called reactive oxygen species (ROS) (see
"The Two Faces of Oxygen"
). New results suggest that a third, unexpected source might produce a particular ROS called superoxide--at least in a short-lived, oxygen-sensitive
Caenorhabditis elegans
mutant.

To manufacture adenosine triphosphate, four protein complexes in the mitochondria hand off electrons to each other in a specified order. Ideally, two electrons combine with oxygen at the end of the chain to make water. Occasionally, however, electrons encounter oxygen higher up the chain, and trouble ensues. At the early stops, the electrons board carrier proteins one at a time. When only one electron is loaded, the carriers can donate these single, unpaired electrons to oxygen; this reaction produces ROS. One such ROS is superoxide, a negatively charged oxygen molecule that carries one of these highly reactive electrons; superoxide damages proteins, DNA, and membranes. Scientists hold two protein amalgams responsible for superoxide production: The third member of the chain, called complex III, makes the majority of this molecular vandal, and the first--complex I--also contributes.

In the current work, Senoo-Matsuda and colleagues analyzed superoxide production in nematodes with defective
mev-1
, a gene that encodes a complex II protein. This strain suffers from a shortened life-span and accumulates more ROS-associated chemical damage than do normal worms, a problem that's enhanced when researchers grow the worms in high concentrations of oxygen. To find out if superoxide might be the culprit, the team measured the amount of this troublesome molecule in mitochondria isolated from normal worms and worms with the
mev-1
mutation. Concentrations of superoxide mirrored the amount of macromolecular injury in both
mev-1
and normal strains under low and high oxygen conditions.

Further results hint that complex II could be churning out the superoxide in these mutants. Treatment with antimycin, a complex III inhibitor, spurs normal mitochondria to produce unusually large amounts of superoxide: Gumming up the complex slows the normal progression of electrons through complex III; as a result, many electrons escape early and form ROS. By contrast, mitochondria from worms with faulty
mev-1
show no such increase in superoxide production upon treatment with the drug. This result suggests that the mutant makes superoxide at a step distinct from complex III's action. Comparison of the
mev-1
protein sequence with that of a related protein in
Escherichia coli
reveals that the genetic alteration perturbs a region that binds to ubiquinone, a small molecule that normally receives two electrons from complex II. Perhaps the mutation weakens the association between
mev-1
and ubiquinone and allows ubiquinone to escape complex II with only one electron, which could then combine with oxygen to form superoxide. The results are consistent with the idea that complex II produces superoxide in these mutants, although proof requires further experiments.

Even if complex II generates superoxide in the short-lived mutant, it is unclear whether the finding applies to normal worms. But the new results hint that ROS can come from sources other than just the usual suspects. Additional studies of the
mev-1
mutation might clarify the molecular twists that can turn a wisp of ROS into a cloud.

--R. John Davenport; suggested by Chang-Su Lim

N. Senoo-Matsuda, K. Yasuda, M. Tsuda, T. Ohkubo, S. Yoshimura, H. Nakazawa, P. S. Hartman, N. Ishii, A defect in the cytochrome
b
large subunit in complex II causes both superoxide anion overproduction and abnormal energy metabolism in
Caenorhabditis elegans
.
J. Biol. Chem.
276
, 41553-41558 (2001).
[Abstract]
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