Endogenous superoxide is a key effector of the oxygen sensitivity of a model obligate anaerobe.

It has been unclear whether superoxide and/or hydrogen peroxide play important roles in the phenomenon of obligate anaerobiosis. This question was explored using , a major fermentative bacterium in the human gastrointestinal tract. Aeration inactivated two enzyme families-[4Fe-4S] dehydratases and nonredox mononuclear iron enzymes-whose homologs, in contrast, remain active in aerobic Inactivation-rate measurements of one such enzyme, fumarase, showed that it is no more intrinsically sensitive to oxidants than is an fumarase. Indeed, when the enzymes were expressed in , they no longer could tolerate aeration; conversely, the enzymes maintained full activity when expressed in aerobic Thus, the aerobic inactivation of the enzymes is a feature of their intracellular environment rather than of the enzymes themselves. possesses superoxide dismutase and peroxidases, and it can repair damaged enzymes. However, measurements confirmed that the rate of reactive oxygen species production inside aerated is far higher than in Analysis of the damaged enzymes recovered from aerated suggested that they had been inactivated by superoxide rather than by hydrogen peroxide. Accordingly, overproduction of superoxide dismutase substantially protected the enzymes from aeration. We conclude that when this anaerobe encounters oxygen, its internal superoxide levels rise high enough to inactivate key catabolic and biosynthetic enzymes. Superoxide thus comprises a major element of the oxygen sensitivity of this anaerobe. The extent to which molecular oxygen exerts additional direct effects remains to be determined.