Kinetic Characterization of Tissue‐Specific Isozymes of Octopine Dehydrogenase from Mantle Muscle and Brain of Sepia officinalis

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Abstract
Octopine dehydrogenase, the terminal enzyme of anaerobic glycolysis in the cuttlefish, Sepia officinalis, displays kinetically distinct tissue‐specific isozymic forms. An initial survey of octopine dehydrogenase from eleven tissues of Sepia revealed that only brain octopine dehydrogenase showed significant substrate inhibition by pyruvate. This property, which is characteristic of H‐type lactate dehydrogenase, was used as the basis for a study comparing and contrasting the kinetic properties of the mantle muscle and brain isozymes of octopine dehydrogenase. Compared to the mantle muscle enzyme, brain octopine dehydrogenase displayed: (a) a higher apparent affinity for the reactants of the reverse reaction (Km for octopine was 10‐fold lower than that of the muscle enzyme), (b) stronger substrate inhibition by both pyruvate and octopine, (c) greater inhibition by the product of the forward reaction, octopine, (d) an increased activity with the hypoxanthine derivative of NADH, and (e) an inhibition of enzyme activity in an incubate containing NAD+, pyruvate, and arginine (indicative of the formation of an inhibitory complex, enzyme‐pyruvate‐arginine‐NAD+). The kinetic properties of the mantle muscle and brain isozymes of octopine dehydrogenase and the differences between these two forms appear analogous to the well‐known properties of the M4 versus H4 isozymes of lactate dehydrogenase. Mantle muscle octopine dehydrogenase, with kinetic properties resembling the M form of lactate dehydrogenase, appears geared for the rapid synthesis of octopine under conditions of muscular work. Brain octopine dehydrogenase, like H‐type lactate dehydrogenase, displays properties which may poise the enzyme for a major role in the oxidation of octopine in vivo.