The role of succinate as a precursor of propionate in the propionic acid fermentation.

The mechanism of the formation of propionate by the propionic acid bacteria has remained uncertain for many years but current evidence indicates its formation is by decarboxylation of succinate (review by van Niel, 1952). Johns (1951a) demonstrated that Micrococcus lactilyticus decarboxylates succinate very rapidly to propionate. Delwiche (1948) and Johns (1951b) have concluded that propionate is formed by a similar reaction with organisms belonging to the genus Propionibacterium; however, in this case the decarboxylation of succinate is much slower than with M. lactilyticus. Barban and Ajl (1951) studied the conversion of C"'02 and propionate-C4 to C4 dicarboxylic acids by propionic acid bacteria and found in carrier type experiments that the succinate acquired radioactivity much more rapidly than did the fumarate or malate. They concluded that CO2 and propionate are converted reversibly to succinate without prior conversion to fumarate and malate. Recently detailed studies of the mechanism of the decarboxylation of succinate to propionate and CO2 have been carried out by Whiteley (1953a, b, c) with extracts of M. lactilyticus. She has found that Mg++, adenosine triphosphate, and Coenzyme A stimulate the reaction. Similar results have been obtained by Delwiche et al. (1953, 1954). The mechanism shown in figure 1 illustrates some of the current views on the reaction. Delwiche et al. (1953, 1954), found with an enzyme preparation of Propionibacterium pentosaceum that propionate-2-C"4 is fixed in succinate