Regulation of isocitrate lyase from autotrophic and photoheterotrophic cultures of Gloeomonas sp.

Isocitrate lyase (EC. 4.1.3.1) has been partially purified from extracts of the unicellular green alga Gloeomonas sp. grown either photoautotrophically in CO2 or photoheterotrophically with acetate resulting in a 50-fold increase in specific activity. Enzyme preparations from both cultures were eluted from DEAE-cellulose columns at almost the same position. The enzymes showed classical Michaelis kinetics with Km values of 1.14 and 1.4 mM and responded in a similar manner to metabolites. Glycol-late, 3-phosphoglycerate, ribulose diphosphate, α-ketoglutarate, succinate, malate, fumarate, and ADP inhibited the enzyme at concentrations of 10 mM. Inhibition by oxaloacetate occurred at 1 mM. Succinyl-CoA (1 mM) activated the enzyme to 155% of the control. These results are discussed in relation to the autotrophic and photoheterotrophic mode of nutrition in algae.