Purification and properties of the glucose-6-phosphate-dependent form of human placental glycogen synthase.

Glycogen synthase in the glucose-6-phosphate (glucose-6-P)-dependent form was purified over 10,000-fold from an extract of term human placenta. The purified enzyme shows a single protein band on polyacry1amide-gel electrophoresis in the presence of sodium dodecyl sulfate. The enzyme activity in the presence of glucose-6-P is increased by the single addition of Mg²⁺, Ca²⁺, or Mn²⁺ and is reduced by the addition of either sulfate or phosphate. Addition of either Mg²⁺, Ca²⁺, or Mn²⁺ relieves the inhibition by sulfate or phosphate. The enzyme activity in the absence of glucose 6-P is greatly increased by the addition of MnSO₄, CoSO₄, and NiSO₄ and is increased to a lesser extent by MgSO₄, CaSO₄, and FeSO₄. The activation of the glucose-6-P-dependent form of the enzyme by these metal sulfates in the absence of glucose-6-P has never been reported. MnSO₄, which shows homotropic cooperativity, is the best activator among the various metal sulfates tested. The human placental glucose-6-P-dependent form of glycogen synthase (D form) can be converted to the glucose-6-P-independent form (I form) of the enzyme by incubating the partially purified glycogen synthase, which is copurified with synthase phosphatase, with Mn²⁺. This conversion can be reversed by the addition of cyclic AMP-dependent protein kinase. The synthase D to synthase I converting system from human placenta is unique in its stringent requirement for Mn²⁺.