Alpha-isopropylmalate synthase as a marker for the leucine biosynthetic pathway in several clostridia and in Bacteroides fragilis.

Alpha-Isopropylmalate synthase (EC 4.1.3.12) is present in extracts of Bacteroides fragilis, Clostridium thermoaceticum, Clostridium formicoaceticum, Clostridium pasteurianum, and Clostridium kluyveri with specific activities (micromol alpha-isopropylmalate formed per min and g protein) of 8.6, 8.9, 2.4, 1.9, and 0.3, respectively. The product alpha-isopropylmalate was identified by gas chromatography combined with mass spectroscopy. The presence of 5mM leucine in the growth medium represses the synthesis of alpha-isopropylmalate synthase in C. thermoaceticum by 40 and 70%. The enzyme from c. pasteurianum was partially purified to a specific activity of 1413. All studied enzyme properties are similar to those of the enzymes from aerobic bacteria. It is suggested that in these anaerobic bacteria the alpha-isopropylmalate pathway is present in addition to the pathway via the ferrodoxin-dependent, reductive carboxylation of branched chain fatty acids.