Wera S, Bollen M, Moens L, Stalmans W
Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, Belgium.
Biochem J. 1996 Apr 1;315 ( Pt 1)(Pt 1):91-6. doi: 10.1042/bj3150091.
During a 30 min incubation at 25 degrees C in the presence of 5-10 mM glucose 6-phosphate, pure glycogen-bound glycogen synthase b from dog liver was progressively converted into a form that was fully catalytically active in the presence of 10 mM Na2SO4 plus 0.5 mM glucose 6-phosphate. The latter enzyme was unlike synthase a (which does not require glucose 6-phosphate for activity), and unlike synthase b (which is strongly inhibited by sulphate). The conversion was insensitive to various inhibitors of Ser/Thr-protein phosphatases and alkaline phosphatases, and was therefore termed 'pseudo-activation'. Kinetically, pseudo-activation increased the V(max) 4-fold without affecting the K(m) for the substrate UDP-glucose. Pseudo-activation appeared to be an irreversible process, but several lines of evidence argue against a limited proteolysis. Pseudo-activation of glycogen synthase occurred also readily in a rat liver cytosol, but it was not observed with purified synthase from skeletal muscle. These observations have important implications for the assay of liver gycogen-synthase phosphatase; the possible physiological implications remain to be explored.
在25摄氏度下，于5 - 10 mM葡萄糖6 - 磷酸存在的条件下孵育30分钟，来自狗肝脏的纯糖原结合型糖原合酶b会逐渐转化为一种形式，该形式在10 mM硫酸钠加0.5 mM葡萄糖6 - 磷酸存在时具有完全的催化活性。后一种酶不同于合酶a（其活性不需要葡萄糖6 - 磷酸），也不同于合酶b（其受到硫酸盐的强烈抑制）。这种转化对丝氨酸/苏氨酸蛋白磷酸酶和碱性磷酸酶的各种抑制剂不敏感，因此被称为“假激活”。从动力学角度来看，假激活使V(max)增加了4倍，而不影响底物UDP - 葡萄糖的K(m)。假激活似乎是一个不可逆的过程，但有几条证据反对有限的蛋白水解作用。糖原合酶的假激活在大鼠肝脏胞质溶胶中也很容易发生，但在从骨骼肌中纯化得到的合酶中未观察到。这些观察结果对肝脏糖原合酶磷酸酶的测定具有重要意义；其可能的生理意义仍有待探索。
