Of production by homogenates and isolated membranes of E. coli has been examined. Approximately one-fourth of the O2-generated by extracts in the prescence of NAD (P) H is attributable to the membranes. The autoxidizable membrane component is a member of the respiratory chain, since O2-production is NADH-specific, amplified by cyanide, and absent from membranes lacking the respiratory NADH dehyd-rogenase. Other respiratory substrates (succinate, I -phosphoglycerol, D-lactate. and L-lactate) supported Or production at efficiencies between 3 and 30 O2-released per 10.000 electrons transferred, under conditions of substrate saturation. Membranes from quinoneless mutants quantitatively retain the ability to evolve O2- indicating that the dehydrogenases are the sites of O2-production. Relative O2-production was greater at low substrate concentrations, probably reflecting the facilitation of unpairing of electrons that may occur when enzymes with multiple redox centers are only partially reduced. Respiration rate, cell volume, rates of membraneous and cytosolic O2-production, and SOD levels were used to calculate a steady-state concentration of O2-between 10--10 and 10--9 M in well-fed, aerobic, SOD-proficient cells.
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