On the Mechanism of Action of Isocitrate Lyase

Abstract

1 The enzymes citrate lyase and isocitrate lyase catalyse similar reactions in the cleavage of citrate to acetate plus oxaloacetate and of isocitrate to succinate plus glyoxylate, respectively. 2 Nevertheless, the mechanism of action of each enzyme appears to be different from each other. Citrate lyase is an acyl carrier protein-containing enzyme complex whereas isocitrate lyase is not. The active form of citrate lyase is an acetyl-S-enzyme but that of isocitrate lyase is not a corresponding succinyl-S-enzyme. 3 In contrast to citrate lyase, the isocitrate enzyme is not inhibited by hydroxylamine nor does it acquire label if treated with appropriately labelled radioactive substrate. 4 Isotopic exchange experiments performed in H₂¹⁸ with isocitrate as a substrate produced no labelling in the product succinate. This was shown by mass-spectrometric analysis. 5 The conclusion drawn from these results is that no activation of succinate takes place on the enzyme through transient formation of succinic anhydride or a covalently-linked succinyl-enzyme, derived from this anhydride