THE INACTIVATION OF DIHYDROXY-ACID DEHYDRATASE IN ESCHERICHIA-COLI TREATED WITH HYPERBARIC-OXYGEN OCCURS BECAUSE OF THE DESTRUCTION OF ITS FE-S CLUSTER, BUT THE ENZYME REMAINS IN THE CELL IN A FORM THAT CAN BEREACTIVATED

The enzyme dihydroxy-acid dehydratase previously has been shown to be inactivated in vivo in Escherichia coli within minutes of exposure to hyperbaric O2. In this paper, we show its inactivation is due to the d estruction of its catalytically active [4Fe-4S] cluster. The inactivat ion is not followed by an appreciable decrease in the amount of dihydr oxy-acid dehydratase protein as determined by Western blots. Thus, the protein from the inactivated enzyme remains unproteolyzed in the cell s. Dihydroxy-acid dehydratase activity recovers after the cells treate d with hyperbaric O2 are returned to ambient oxygen. Since this recove ry in activity is not accompanied by a significant increase in dihydro xy-acid dehydratase protein and is not prevented by chloramphenicol, i t appears primarily to be due to reactivation of the previously inacti vated enzyme. The reactivation occurs by reconstitution of the enzyme' s Fe-S cluster. These results demonstrate that this enzyme can cycle b etween forms in which the Fe-S cluster is either present or absent. Th e facile ability to cycle between these two forms would be compatible with a regulatory role in addition to a catalytic role for this enzyme .