Effects of Divalent Cations on 5′-Mononucleotidase of Bovine Cerebral Cortex

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Abstract
A 5′-mononucleotidase preparation partially purified from bovine cerebral cortex was investigated with regard to its interaction with divalent metals. Magnesium, manganese, and zinc ions partly restored the activity lost by either acid or alkali treatment. Prolonged dialysis against an EDTA solution also eliminated the activity, which was restored by magnesium ions. The alkali-treated nucleotidase incorporated a larger amount of radioactive manganese than the untreated one did. The extra manganese incorporated by the treated nucleotidase roughly corresponded to the amount needed to bind to the enzyme molecule in a one-to-one ratio. The Km for magnesium (1.5 × 10−5 M) was independent of the type of substrate and was much lower than those for the substrates; the latter Km's were independent of the magnesium concentration. The facts suggest that the metal binds to the enzyme independently of the substrates. The inhibition patterns by nucleoside triphosphates were sigmoidal, and similar patterns were obtained with inorganic polyphosphates and chelators. The inhibition was a noncompetitive type, and the activity partially lowered by EDTA was not affected by ATP. The sigmoidal nature of the nucleotide inhibition, therefore, could be accounted for mainly by the removal of necessary metals for the enzyme activity instead of a cooperative mechanism.

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