MECHANISM OF PRODUCTION OF EXTRACELLULAR SUPEROXIDE RADICAL

Paraquat mediates a superoxide dismutase-inhibita- ble reduction of cytochrome c by suspensions of Esch- erichia coli B. Glucose was most effective in providing electrons for this cytochrome c reduction, but other nutrients could serve in this capacity, provided the cells were preconditioned by growth on these nutrients. Pa- raquat reduction depended upon a NADPH:paraquat diaphorase, present in the cytosol. Reduced paraquat could diffuse across the cell envelope and react with dioxygen, in the suspending medium, thus generating 02- in that compartment. Most of the paraquat reduced in the cell, under the conditions used, reoxidized in situ and most of the 02- production was thus intracellular. The partitioning of reduced paraquat between intra- cellular and extracellular compartments, prior to re- action with dioxygen, depended upon intracellular pOz and any strategy which raised intracellular ~02 de- creased the efflux of reduced paraquat and thus de- creased extracellular 02- production. Extracellular 02- and HzOz did contribute to cell damage in proportion to the amount produced. 02- appeared to be unable to cross the cell envelope in either direction and the only Oz- which was effective in raising the rate of biosyn- thesis of the manganese-superoxide dismutase, was that generated within the cell.