Glyoxylate Cycle Enzyme Activities in the Cyanobacterium Anacystis Nidulans1

The enzymes of the glyoxylate cycle, isocitrate lyase (EC.4.1.3.1) and malate synthase (EC.4.1.3.2), were measured in cell‐free extracts from the cyanobacterium Anacystis nidulans Drouet during photoautotrophic growth in medium aerated with ordinary air (0.03% CO2). Isocitrate lyase had an average specific activity of 112 nmoles·min−1·mg protein−1 whereas malate synthase had an average specific activity of 12.5 nmoles·min−1·mg protein−1. Unpurified isocitrate lyase showed classical Michaelis kinetics with a Km of 8 mM. Isocitrate lyase activity was strongly inhibited by numerous cellular metabolites at 10 mM concentration. The previously reported low specific activity for isocitrate lyase may be due to metabolite inhibition caused by growth in high CO2 concentrations. The activities reported for isocitrate lyase and malate synthase suggest the operation of the glyoxylate cycle in Anacystis nidulans under CO2‐limiting growth conditions.