SUMMARY: The addition of either sulphide or thiosulphate to an aerobic suspension of a Thiobacillus was followed within 10 sec. by the synthesis of intracellular ATP. The ATP formation accompanying sulphide oxidation was strongly inhibited by 0.1mM-2,4-dinitrophenol, but that accompanying thiosulphate oxidation was unaffected. The presence of carbon dioxide had no effect on ATP formation or on the steady-state concentration attained. 2,4-Dinitro-phenol did not significantly affect oxygen uptake nor the kinetics of thiosulphate oxidation but it inhibited carbon dioxide fixation with either sulphide or thiosulphate as substrate. It is concluded that sulphurcompound oxidation can be coupled to two different types of phosphorylation, only one of which is sensitive to 2,4-dinitrophenol. In the presence of 2,4-dinitrophenol and with thiosulphate as substrate, carbon dioxide fixation was not limited by the availability of ATP but might be limited by the availability of reduced nicotinamide adenine dinucleotides.


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