MAMMALIAN LIVER GLYCOGEN SYNTHETASE - TRANSFORMATIONS OF ENZYME IN VITRO

The properties of liver glycogen synthetase were investigated, utilizing a new method of isolation that allows examination of the enzyme only a short time after excision of tissue. The enzyme differed significantly from that obtained by the lengthy centrifugal fractionation of a liver homogenate. The enzyme as isolated from the liver of fed mice, was found to be predominantly in the glucose 6-P dependent (D) form. A large increase in glucose 6-P independent (I) activity was evident if the 8000 g supernatant fraction was incubated at 0 ° before isolation of the synthetase. A small but significant increase in D activity was also observed under the same conditions. When the supernatant fraction was incubated at 25 °, instead of 0 °, a much higher rate of I activity increase was observed, but in this case it occurred partly at the expense of D activity. The I activity increase was prevented by 0.1 m NaF both at 0 ° and 25 °. The calculated Ka for glucose 6-P with a fresh preparation was found to be in the neighborhood of 0.5 mm, a value much higher than that reported for the particulate enzyme. Stimulation by glucose 6-P did not decrease Km for UDP-glucose, but led to a several-fold increase in Vm. No change in kinetic parameters for UDP-glucose or glucose G-P resulted from aging at 0 °. Inorganic orthophosghate at concentrations ranging from 1 to 10 mm failed to activate any of the preparations tested. Some speculation regarding the observed transformations of the enzyme is offered. © 1969.