On the dehydration of (R)-lactate in the fermentation of alanine to propionate by Clostridium propionicum

All the enzymes of the pathway of (S)-alanine fermentation to acetate and propionate were detected in cell-free extracts of Clostridium propionicum . Among these (S)-glutamate dehydrogenase (NAD), (R)-lactate dehydrogenase (NAD) and propionate CoA-transferase were purified to apparent homogeneity. Their structures were presumably alpha 6, alpha 2 and alpha 4, respectively. The latter enzyme was specific for short-chain monocarboxylic acids with a pronounced preference for (R)-lactate over the (S)-enantiomer. The key step of the pathway, the dehydration of (R)-lactate required acetyl phosphate and CoASH under anaerobic conditions. It was inhibited by hydroxylamine, arsenate, azide (1 mM each) or by 0.1 mM 2,4-dinitrophenol. Thus it closely resembled the dehydration of (R)-2-hydroxyglutarate in Acidaminococcus fermentans , although an activation was not necessary.