Mg2+‐ or Ca2+‐Activated ATPase in Squid Giant Fiber Axoplasm

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Abstract
A divalent cation‐activated ATPase in axoplasm from the squid giant axon is described. The enzyme requires Mg2+ or Ca2+, has a K+ optimum of 60 mM, and has a pH optimum of 7.5. Several nucleotide triphosphates other than ATP can serve as substrates. The enzyme is inhibited by excess ATP or Mg2+. The enzyme is enriched in a rapidly sedimenting fraction of the axoplasm, and is eluted in the exclusion volume of a Sepharose 4B column, suggesting that it is associated with a highly aggregated structure. Comparison of the properties of the enzyme with those of myosin and Na+–K+‐ATPase suggests that it differs from both of these enzymes. The enzyme has many similarities to vertebrate nerve ATPases previously described. The demonstration of the presence of this ATPase in squid axoplasm proves the neuronal localization of the enzyme.