Acetoin degradation in Bacillus subtilis by direct oxidative cleavage.

Acetate and acetaldehyde can be detected as products of the oxidative dissimilation of acetoin in Bacillus subtilis extracts. They arise as the result of the direct cleavage of acetoin without a previous oxidation to diacetyl. This can be deduced from the following observations: (a) no diacetyl was detected in acetoin dissimilation experiments in vitro and (b) methylacetoin, an acetoin analogue which can not be oxidized to the diketone, also undergoes oxidative splitting, yielding acetone and acetate. The splitting reaction requires thiamine pyrophosphate as a cofactor, suggesting that the oxidative step occurs, as known for similar reactions, by the electron transfer from hydroxyethylthiamine pyrophosphate to a proper acceptor, which in vitro can be replaced by dichlorophenolindophenol. In vivo the final product of the oxidation of hydroxyethylthiamine pyrophosphate is activated acetate. A mutant which lacks acetoin-cleaving activity can not reutilize the acetoin accumulated after growth in glucose. This corroborates the actual importance of the cleavage reaction for acetoin dissimilation. The enzyme diacetylmethylcarbinol synthase, thought to be responsible for the formation of diacetylmethylcarbinol from diacetyl, probably is identical to the enzyme catalyzing the cleavage of acetoin.