Modification of 5′‐Nucleotidase Activity by Divalent Cations and Nucleotides

Abstract

The 5′-nucleotidase activity of the purified cytoplasmic fraction preparation of bovine brain does not depend on the presence of the divalent metal ions Mg²⁺, Ca²⁺, and Cu²⁺ in the incubation medium. The Zn²⁺ ion (0.5 mM) causes total enzyme inhibition. Although EDTA and 8-hydroxyquinoline inhibit the 5′-nucleotidase from this source, it has not been possible to show the existence of metal ions in the enzyme molecule. The inhibition of 5′nucleotidase by EDTA is progressive and irreversible; when the enzyme is not preincubated with EDTA, the inhibition is overridden by metal ions. The purines (except xanthine, 0.3 mM), pyrimidines, and their nucleosides do not affect the 5′-nucleotidase activity. The nucleoside di- and triphosphates are competitive enzyme inhibitors against 5′-AMP as substrate. The K₁ values of the diphosphates are lower than those determined for the corresponding triphosphates. The inhibition caused by the above nucleotides is reversed, partly or wholly, by Mg²⁺, depending on the molar ratio between the effectors. The inhibitory action of the -SH group reagents on the 5′-nucleotidase activity is weak and reversible.