Octopine dehydrogenase and phosphoarginine kinase in squid mantle: cooperation of two enzymes at the arginine branchpoint in cephalopod muscle

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Abstract
Octopine dehydrogenase (ODH, EC 1.5.1.11) and phosphoarginine kinase (PAK, EC 2.5.3.3) from squid mantle (Loligo opalescens) were purified and obtained uncontaminated by each other and free of lactic dehydrogenase and malic dehydrogenase. Kinetic data were analyzed for substrates and cosubstrates for forward and reverse reactions.Octopine, NAD+, ATP, and ADP are inhibitors of the ODH forward reaction (octopine forming); arginine and NADH inhibit the reverse reaction. ATP and ADP inhibition disappears upon addition of Mg2+ to the assay medium. Arginine phosphate inhibits the PAK forward reaction (phosphoarginine forming), whereas arginine and NADH inhibit the reverse reaction.We suggest that PAK and ODH function is two-phased and cooperative, with PAK being active before ODH, PAK working in the reverse reaction, and ODH in the forward reaction, during burst muscle work, and PAK being active before ODH during recovery. ODH activity is probably regulated by pyruvate and NADH availability in the forward direction and by arginine deinhibition in the reverse direction.Evidence is presented for the suggestion that present concepts of the relationship between phosphagen and octopine should be revised to assume that resting concentrations of free arginine in cephalopod muscle are kept at a minimum.

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