Inhibition by alloxan of mitochondrial aconitase and other enzymes associated with the citric acid cycle

Abstract Considerable variations were found in the in vitro effect of alloxan on mouse liver enzymes associated with the citric acid cycle. The following approximative alloxan concentrations induced 50% inhibition of enzyme activity: 10 −6 M for aconitase, 10 −4 M for NAD-linked isocitrate dehydrogenase, glutamate dehydrogenase, α-ketoglutarate dehydrogenase, succinyl-CoA synthetase and fumarase, and 10 −3 M for citrate synthase and NADP-linked isocitrate dehydrogenase. Pyruvate dehydrogenase, succinate dehydrogenase and malate dehydrogenase were not inhibited by 10 −3 M alloxan. The inhibition of aconitase was competitive both when using mouse liver and purified porcine heart enzyme. The K i values for the purified enzyme in the presence of 5 μM alloxan were 0.22 μM with citrate, 4.0 μM with cis -aconitate and 0.62 μM with isocitrate as substrate. The high sensitivity of aconitase for inhibition by alloxan probably plays a prominent role for the toxic effects of alloxan.