Thermostability of enzymes of the tricarboxylic acid cycle of Bacillus coagulans.

The thermostability of four enzymes of the tricarboxylic acid cycle has been studied in the facultative thermophile, Bacillus coagulans. Although isocitrate dehydrogenase appeared to be more temperature-sensitive in whole-cell extracts of cultures grown at 30 degrees C compared with that in cultures grown at 55 degrees C, this difference could be largely eliminated by the removal of cell-wall material. The specific activity of each of the enzymes examined was approximately threefold higher in cultures grown at 55 degrees C than in those grown at 30 degrees C. The maximum temperature, Arrhenius plot and effect of stabilizing agents for each enzyme were examined and found to be independent of growth temperature. Sodium chloride (10% w/v) was an effective protective agent for fumarase, aconitase and malate dehydrogenase. Protection from thermal denaturation of isocitrate dehydrogenase, aconitase and fumarase but not malate dehydrogenase was also given when the enzymes were heated in the presence of their substrates. These results are discussed in light of the generalized theories of facultative thermophily which have been proposed.