Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, Universitätstrasse 30, W-8580, Bayreuth, FRG.
Planta. 1993 Mar;189(3):329-39. doi: 10.1007/BF00194429.
Experiments were carried out to investigate whether sucrose synthase (Susy) catalyses a readily reversible reaction in vivo in potato (Solanum tuberosum L.) tubers, Ricinus communis L. cotyledons, and heterotrophic Chenopodium rubrum L. cell-suspension cultures. (i) The contents of sucrose, fructose, UDP and UDP-glucose were measured and the mass-action ratio compared with the theoretical equilibrium constant. In all three tissues the values were similar. (ii) Evidence for rapid turnover of label in the sucrose pool was obtained in pulse-chase experiments with potato discs and with intact tubers attached to the plant. The unidirectional rates of sucrose synthesis and degradation were considerably higher than the net flux through the sucrose pool in the tubers. (iii) Labelling of the glucosyl and fructosyl moieties of sucrose from [(14)C]glucose in the presence of unlabelled fructose provided evidence that Susy contributes to the movement of label into sucrose. Methods for estimating the contribution of sucrose-phosphate synthase and Susy are presented and it is shown that their relative contribution varies. For example, the contribution of Susy is high in developing tubers and is negligible in harvested tubers which contain low Susy activity. (iv) The absolute values of the forward (v(+1)) and backward (v(-1)) reaction direction of Susy are calculated from the kinetic labelling data. The estimated values of v(+1) and v(-1) are comparable, and much higher than the net flux through the sucrose pool. (v) The estimated concentrations of the substrates and products of Susy in tubers are comparable to the published K m values for potato-tuber Susy. (vi) It is concluded that Susy catalyses a readily reversible reaction in vivo and the relevance of this conclusion is discussed with respect to the regulation of sucrose breakdown and the role of Susy in phloem unloading.
实验旨在研究蔗糖合酶（Susy）在活体土豆（Solanum tuberosum L.）块茎、蓖麻（Ricinus communis L.）子叶和异养藜（Chenopodium rubrum L.）悬浮细胞培养物中是否催化易于逆转的反应。（i）测量了蔗糖、果糖、UDP 和 UDP-葡萄糖的含量，并将质量作用比与理论平衡常数进行了比较。在所有三种组织中，这些值都相似。（ii）在土豆圆盘和与植物相连的完整块茎的脉冲追踪实验中，获得了蔗糖池中文标签快速周转的证据。蔗糖合成和降解的单向速率远高于块茎中蔗糖池的净通量。（iii）在存在未标记果糖的情况下，[(14)C]葡萄糖标记的蔗糖的葡糖基和果糖基部分提供了 Susy 有助于标签进入蔗糖的证据。介绍了估计蔗糖磷酸合酶和 Susy 贡献的方法，并表明它们的相对贡献不同。例如，Susy 的贡献在发育中的块茎中很高，在收获的块茎中可以忽略不计，因为收获的块茎中含有低活性的 Susy。（iv）从动力学标记数据计算 Susy 的正向（v(+1)）和反向（v(-1)）反应方向的绝对值。估计的 v(+1)和 v(-1)值相当，远高于蔗糖池的净通量。（v）块茎中 Susy 的底物和产物的估计浓度与已发表的马铃薯块茎 Susy 的 K m 值相当。（vi）结论是 Susy 在体内催化易于逆转的反应，并就蔗糖分解的调节以及 Susy 在韧皮部卸载中的作用讨论了这一结论的相关性。
