TRACER EXPERIMENTS ON THE MECHANISM OF ACETATE FORMATION FROM CARBON DIOXIDE BY BUTYRIBACTERIUM RETTGERI

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Abstract
Tracer expts. carried out with B. rettgeri indicate that the mechanism by which this organism converts labeled CO2 to doubly labeled acetate does not involve a reverse di- or tricarboxylic acid cycle, the glycine-serine reaction, or any other process in which methyl-labeled pyruvate is an intermediate. Expts. with C14 -labeled lactate show that CO2 originates mainly from the lactate carboxyl C, and acetate from the 2 and 3 positions. These reactions are more or less reversible; CO2 rapidly exchanges with the carboxyl C of lactate, and acetate slowly exchanges with the other positions. The carboxyl C of acetate is converted to CO2 without a corresponding conversion of the methyl C, but this exchange reaction is far too slow to account for the observed incorporation of CO2 in the carboxyl position. The distr. of C14 between the C of acetate derived from labeled CO2 is independent of the incubation time. C14-formaldehyde is incorporated into every C atom of the fermentation products of glucose. The methyl C atoms of lactate and acetate are labeled more strongly than the adjacent C atoms. Evidence was obtained for formation of formaldehyde from CO2. The significance of these results is discussed.

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