The importance of nucleoside triphosphate inhibition of liver glycogen synthase phosphatase activity

The liver glycogen particle contains constitutive glycogen-synthase phosphatase activity which is inhibited by ATP-Mg in a concentration-dependent manner within the physiological range (I_0.5 = 0.1 mM). Therefore, we determined whether other nucleoside triphosphate-magnesium complexes also inhibit synthase phosphatase activity. UTP-Mg, CTP-Mg and GTP-Mg were all found to be inhibitory. The maximum inhibition was 85-90% which was greater than that for ATP-Mg. The I_0.5 for UTP-Mg was comparable to that of ATP-Mg but it was greater for CTP-Mg and for GTP-Mg. At in vivo physiological concentrations, both UTP and ATP are possible inhibitors of synthase phosphatase activity. In the presence of a saturating concentration of ATP-Mg, added UTP-Mg increased the inhibition suggesting the presence of at least two distinct nucleotide binding sites. Substitution of calcium for magnesium in an ATP complex had no effect on the I_0.5, but increased the maximum inhibition. The present studies also suggest that in the multistep conversion of synthase D to synthase I, ATP-Mg inhibition occurs early in the sequence. Addition of glycogen, a known inhibitor of synthase phosphatase activity, to a reaction mixture containing 3 mM ATP-Mg did not further inhibit synthase phosphatase activity when added at concentrations up to 22 mg/ml. The latter data suggest that the presence of a nucleoside triphosphate may desensitize the phosphatase to glycogen inhibition. ATP-Mg and, to a lesser extent, UTP-Mg and CTP-Mg all stimulated phosphorylase phosphatase activity but GTP-Mg did not.