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Cell-free preparations have been made from Thiobacillus A2 that are capable of the complete oxidation of thiosulphate to sulphate and of coupling the associated electron transport to ATP synthesis with a P/O ratio of about one. Spectrophotometric measurements and the effect of electron transport chain inhibitors on cytochrome reduction and oxygen uptake indicate that thiosulphate oxidation is coupled directly to cytochrome c reduction and does not involve cytochrome b. Complete inhibition of phosphorylation by, 2,4-dinitrophenol suggests that ATP synthesis was effected exclusively by electron transport phosphorylation accompanying re-oxidation of cytochrome c. Enzymes for thiosulphate oxidation to sulphate and the reduction of cytochrome c were located in the supernatant fraction after centrifuging at 130000 g, but oxygen uptake required the 130000 g pellet fraction, which provided membrane-bound cytochrome c and cytochrome oxidase.
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