Rodríguez P, Durante P
Laboratorio de Trombosis Experimental, Instituto Venezolano de Investigaciones Científicas, Caracas.
Biochem Med Metab Biol. 1990 Apr;43(2):147-58. doi: 10.1016/0885-4505(90)90020-2.
Human platelets exhibited significant glucosyltransferase activity, that transfer [14C]glucose from UDP-Glc to an endogenous protein acceptor. The enzyme protein:glucosyltransferase responsible for the catalysis was characterized and compared with glycogen:glucosyltransferase. We describe a partial separation of both activities, the ratio of protein:glucosyltransferase/glycogen:glucosyltransferase varied from 7:1 in a crude homogenate of platelets to 36:1 in the Sephadex G-100 column. This procedure failed to separate the protein:glucosyltransferase from its endogenous acceptor. Glucosylation of protein demonstrated dependence with respect to time and both protein and UDP-Glc concentration, and was saturated by very low concentration of donor and acceptor substrates. It was inhibited 76% by 5 mM Mn2+ concentration and was activated 23 and 11% by 5 mM concentrations of Ca2+ and Mg2+, respectively. With respect to glycogen:glucosyltransferase, when the effect of time, protein, and substrate concentration were determined under identical conditions, it did not show the same dependence. At 5 mM concentration, Mn2+, Ca2+, and Mg2+ were activators of the enzyme 43, 80, and 200%, respectively. On the basis of these characteristics, we conclude that the synthesis of glucoprotein and glycogen are catalyzed by two distinct enzymes. Addition of exogenous glycogen (range 0.002-1%) inhibited the protein:glucosyltransferase, whereas at 0.001-0.007% concentration it was acceptor substrate for glycogen:glucosyltransferase activity. At higher concentrations this activity was strongly inhibited. The concentration of glycogen in platelets could play a regulatory role in forming the glucoprotein and the glycogen molecules.
人血小板表现出显著的葡萄糖基转移酶活性，该酶可将[14C]葡萄糖从UDP-葡萄糖转移至内源性蛋白质受体。对负责催化的酶蛋白：葡萄糖基转移酶进行了表征，并与糖原：葡萄糖基转移酶进行了比较。我们描述了两种活性的部分分离，酶蛋白：葡萄糖基转移酶/糖原：葡萄糖基转移酶的比例在血小板粗匀浆中为7:1，在葡聚糖G-100柱中为36:1。该方法未能将酶蛋白：葡萄糖基转移酶与其内源性受体分离。蛋白质的糖基化表现出对时间以及蛋白质和UDP-葡萄糖浓度的依赖性，并且在极低浓度的供体和受体底物下达到饱和。5 mM的Mn2+浓度可抑制其76%，5 mM的Ca2+和Mg2+浓度分别可激活其23%和11%。关于糖原：葡萄糖基转移酶，在相同条件下测定时间、蛋白质和底物浓度的影响时，它并未表现出相同的依赖性。在5 mM浓度下，Mn2+、Ca2+和Mg2+分别是该酶的激活剂，激活率分别为43%、80%和200%。基于这些特性，我们得出结论，糖蛋白和糖原的合成由两种不同的酶催化。添加外源性糖原（范围为0.002 - 1%）可抑制酶蛋白：葡萄糖基转移酶，而在0.001 - 0.007%的浓度下，它是糖原：葡萄糖基转移酶活性的受体底物。在较高浓度下，该活性受到强烈抑制。血小板中糖原的浓度可能在糖蛋白和糖原分子的形成中起调节作用。
