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- In the early stage of CO2-fixation by Thiobacillus thiooxidans,which was incubated aerobically in the presence of sulfur, mostpart of the fixed carbon was found in the phosphate ester fraction.
- The fixation was inhibited by NaF, picolinic acid, PCMB, azide,dipyridyl, o-phenanthroline, monoiodoacetic acid, and arsenite,each in the concentration range where the sulfur oxidation wasnot affected strongly.
- The crude extract of this organismcould fix CO2 in the presenceof ATP, R-5-P and Mg++. Most partof the fixed carbon was foundin PGA.
- The crude extract showedthe CO2-fixation coupled with the H2S-oxidationin the presenceof ADP.
- An appreciable reduction of PGA could not be detectedin thepresence of reducing systems, involving TPNH and DPNH.
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- The cells of Thiobacillus thiooxidans, which had been in contactwith sulfur or sulfide in air (or CO2-free air), could fix addedCOa very rapidly after replacing air with nitrogen. This fixationis designated as the postoxidative fixation.
- "Preoxidation"of the sulfur compounds is mandatory for theoccurrence of thepostoxidative fixation.
- The cells which had preliminarilyoxidized sulfide could notshow the CO2-fixation, when theywere placed under an anaerobiccondition in the absence of thesulfur compound.
- These results indicate that sulfur compoundsmay have an importantrole as the electron donor for the reductionof CO2, besidestheir role as the substrate of respiration tosecure energyfor the fixation of CO2
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- Thiobacillus thiooxidans, isolated in our laboratory, was foundto oxidize sulfur, but not thiosulfate. Tetrathionate is alsooxidized slightly. Its ability to oxidize sulfur is inactivatedeven by such a mild treatment as keeping the cells in a frozenstate.
- Inhibitory action of alcohols on the sulfur oxidationincreasesas the length of carbon chain of alcohols increases.Carboxylicacids do not inhibit the sulfur oxidation at pH abovetheirpK, while they strongly inhibit the reaction at pH belowthepK.
- The sulfur oxidation is inhibited by cyanide, azide,diethyldithiocarbamateand carbon monoxide, and the inhibitionby carbon monoxide isnot reversed by light. These results suggestthe presence ofmetal enzymes in the sulfur oxidation system.The terminal enzymeof this reaction appears to be differentfrom the usual cytochromeoxidase.
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