Tan A W, Tan A H, Nuttall F Q
Biochim Biophys Acta. 1980 Aug 7;614(2):328-38. doi: 10.1016/0005-2744(80)90222-3.
We have previously shown that the synthase D (UDPglucose:glycogen 4-alpha-D-glucosyltransferase, EC 1.4.1.11) present in the liver of the adrenalectomized fasted rat was not converted to synthase I by synthase phosphatase from normal animals, suggesting the presence of a non-substrate form of synthase D (Tan, A.W.H. and Nuttall, F.Q. (1976) Biochim. Biophys. Acta 445, 118--130). The enzymatic properties of this synthase D have now been examined. Using optimal assay conditions, the total amount of synthase D activity in the adrenalectomized fasted rats was similar to that of normal fed rats when 1% glycogen was included in the homogenizing buffer. However, the two enzymes appeared to have different affinities for the substrate, UDPglucose and the modifier, glucose-6-P. The changes in kinetic properties were not due to differences in glycogen or to a dialyzable modifier in the extracts. Synthase D from adrenalectomized fasted and from normal fed rats was partially purified. After DEAE-cellulose chromatography, modification appeared to have occurred such that the enzyme from the adrenalectomized fasted rat had properties similar to that of the normal fed rat. The enzymes were cold-labile, had different properties from enzymes in the crude extract and they were both converted to synthase I by synthase phosphatase. We conclude from these studies that the phosphorylation site in the synthase is in a flexible region of the protein. Changes in the ability of the synthase D to interact and be dephosphorylated by synthase phosphatase can occur readily in vivo and in vitro. The molecular basis for the modification remains unknown.
我们之前已经表明，肾上腺切除的禁食大鼠肝脏中存在的合成酶D（UDP葡萄糖：糖原4-α-D-葡萄糖基转移酶，EC 1.4.1.11）不会被正常动物的合成酶磷酸酶转化为合成酶I，这表明存在一种非底物形式的合成酶D（Tan，A.W.H.和Nuttall，F.Q.（1976年）《生物化学与生物物理学报》445，118 - 130）。现在已经研究了这种合成酶D的酶学性质。在匀浆缓冲液中加入1%糖原时，使用最佳测定条件，肾上腺切除的禁食大鼠中合成酶D的总活性与正常喂食大鼠的相似。然而，这两种酶对底物UDP葡萄糖和调节剂6-磷酸葡萄糖似乎具有不同的亲和力。动力学性质的变化不是由于糖原的差异或提取物中可透析的调节剂引起的。对肾上腺切除的禁食大鼠和正常喂食大鼠的合成酶D进行了部分纯化。经过DEAE - 纤维素色谱分离后，似乎发生了修饰，使得肾上腺切除的禁食大鼠的酶具有与正常喂食大鼠的酶相似的性质。这些酶对冷不稳定，与粗提取物中的酶具有不同的性质，并且它们都被合成酶磷酸酶转化为合成酶I。我们从这些研究中得出结论，合成酶中的磷酸化位点位于蛋白质的一个柔性区域。合成酶D与合成酶磷酸酶相互作用并被其去磷酸化的能力变化在体内和体外都很容易发生。修饰的分子基础仍然未知。
