GLYCOGEN-SYNTHETASE AND THE CONTROL OF CRYOPROTECTANT CLEARANCE AFTERTHAWING IN THE FREEZE-TOLERANT WOOD FROG

During in vivo freezing at -2.5 degrees C, the activity of glycogen sy nthetase in Rana sylvatica liver (but not skeletal muscle) is sharply decreased, the content of the G6P-independent, active ($) under bar I form falling to 24% of the control value (5 degrees C) within 5 minute s post-exotherm, a pattern opposite to the freeze-induced activation o f phosphorylase that supports cryoprotectant synthesis. Upon thawing, however, synthetase ($) under bar I activity rose to high levels (4.2- fold greater than control) to facilitate glucose clearance and its res torage as liver glycogen. Kinetic properties of liver synthetase ($) u nder bar I and ($) under bar D forms are presented, the ($) under bar I form showing a much lower K-m for UDPG (0.19 mM compared with 0.49 m M for ($) under bar D) although K-m values for glycogen were the same (0.43-0.45 mg/ml). The ($) under bar D form was dependent upon glucose -6-P for activity but with a K-a value of 1.7 mM, this form is probabl y functionally inactive in vivo.