The respiratory enzyme system of Azotobacter vinelandii.

(1) Respiratory particles of Azotobacter vinelandii grown on nitrogen-free medium contained ubiquinone (Q-8), flavoprotein, cytochromes c4 + c5, b1, a1, a2 and o. Growth with urea as nitrogen source resulted in a loss of cytochromes a2 and o. (2) The aerobic steady-state reductions of cytochromes c4 + c5, b1 and a2 were 39-41%, 18-25% and 0% for the succinate, L-malate or NADH oxidases and 80-100% in the anaerobic state. The anaerobic reduction of cytochrome b1 was 56-69% in the "urea" particles. (3) The concentrations of the electron carriers were unaffected by aeration levels on nitrogen-free medium. With transfer from urea to nitrogen-free medium cytochrome a2 was linearly synthesised with no lag in growth. (4) The redox state and kinetics of ubiquinone were examined by solvent extraction and direct spectrophotometry, with excellent correlation. Ubiquinone was 35-44% reduced in the aerobic steady-state and 72-77% reduced in the anaerobic state, but the ubiquinone reductase rates were only 40-60% of the oxidase rates. The anaerobic reduction was only 65-69% in the "urea" particles. (5) 96% acetone extracts ubiquinone and cytochromes a1 and a2. Low oxidase activities remained in the extracted particles and there was no stimulation with ubiquinone. Assay of extracted and unextracted particles with artificial electron acceptors showed specific stimulation with added ubiquinone. (6) Succinate, L-malate or NADH oxidases were 70-80% inhibited by 60 microg. piericidin A/mg. protein and activity was restored by ubiquinone (Q-3). Cytochromes c4 + c5 and b1 were more reduced in the aerobic steady-states by partial piericidin A inhibition. (7) Electron micrographs showed that the respiratory particles were flat membrane fragments of 35-75 mmicro and studded with spherical knobs of 30-48°A diameter. Some differentiation between the "nitrogen-free" and "urea" particles was noticed. (8) The results were discussed in the light of a branched electron transport system.