The pyruvate branch point in squid brain: competition between octopine dehydrogenase and lactate dehydrogenase

Abstract

Octopine dehydrogenase (EC 1.5.1.11) and lactate dehydrogenase (EC 1.1.1.27) from the brain of the squid, Symplectoteuthis oualaniensis, were studied kinetically. Lactate dehydrogenase had a higher affinity for pyruvate (K_m = 0.37 mM) than octopine dehydrogenase (K_m = 0.86 mM), whereas their affinities for NADH were similar (K_m values for NADH were about 0.01 mM). The affinity of lactate dehydrogenase for lactate was low (K_m = 14 mM), whereas the affinity of octopine dehydrogenase for octopine was high (K_m = 0.2 mM). NAD⁺ strongly inhibited octopine dehydrogenase (K_i = 0.04 mM), and had a less pronounced effect on lactate dehydrogenase (K_i = 0.5 mM). The kinetic characteristics of the enzymes suggest that brain lactate dehydrogenase could function to maintain redox balance under stress conditions, whereas brain octopine dehydrogenase would function mainly in the oxidation of exogenous octopine to arginine and pyruvate.