Guinea pig skeletal muscle 5′-nucleotidase

ATP, UTP, GTP, and CTP were found to be powerful competitive inhibitors of 5′-nucleotidase partially purified from guinea pig skeletal muscle, the concentrations required for 50% inhibition being 1.25 uM, 5.5 uM, 10 uM and 27.5 uM respectively with 5′-AMP as substrate. The enzyme does not require divalent cations. Furthermore magnesium, calcium and cobalt ions added in large excess with respect to nucleoside triphosphates did not completely relieve the inhibition, indicating that the complexes nucleoside triphosphates-divalent cations are also inhibitors. Using specific optical assays to follow the dephosphorylation of AMP, GMP and IMP it was found that the hydrolysis of each 5′-mononucleotide is competitively inhibited by other 5′-mononucleotides. The regulation of skeletal muscle 5′-nucleotidase supports the hypothesis of its role in the mechanism of muscular contraction.