• 作者： 楊兆鸞; 蘇仲卿
• 作者服務機構： 中央研究院生物化學研究所
• 中文摘要： 觀察四種基質濃度改變對竹筍蔗糖合成酵素催化反應之影響，蔗糖合成或分解反應，雙倒數曲線圖都相交於一點，此結果顯示反應依照連續雙雙機構。UDPG果糖及蔗糖的Km值分別1.92，0.98和19mM，UDP的Km值隨第二基質蔗糖增加而強烈地減少；當蔗糖濃度高於40 mM，UDP的表面Km值小於5〞M“由平衡透析法顯示蔗糖和果糖能與酵素直接結合。放射性果塘與蔗糖問同位素交換反應數據配合連續反應機構。從產物抑制研究，果糖被發現是蔗糖的競爭性抑制者，UDPG和UDP彼此間不是競爭性抑制者，這些排除遵循Theorell-Chance機構的可能性。綜合以上證據，可以提出竹筍蔗糖合成酵素是依照與蔗糖首先結合而最後放出果糖的「次序性」雙雙機構進行反應。
• 英文摘要： The effect of the changes of concentrations of four substrates on therate of reaction at steady-state catalyzed by bamboo shoot sucrose synthetasewas investigated. For either sucrose synthesizing or sucrose cleaving reac-tion, the double reciprocal plots intersected on one point. These results in-dicate that the reaction proceeds according to a sequential bi bi mechanism.Km values for UDPG, fructose and sucrose were 1.92, 0.98 and 19 mM, re-spectively. The Km value of UDP decreased drastically as the concentra-tion of the second substrate sucrose increased; when the level of sucrosewas higher than 40 mM, the apparent Km for UDP was smaller than 5 ,uM.The double reciprocal plots of sucrose vs. initial rates at various UDP levelsintersected within the first quadrant, rendering the estimation of the Kmvalue of UDP by the secondary plot impossible. Sucrose and fructose canbind directly with the enzyme as revealed by equilibrium dialysis. Thedata of isotope exchange reaction between U-14 C-fructose and sucrose con-formed with the sequential mechanism. From the product inhibition study,fructose was found to be a competitive inhibitor of sucrose. UDPG andUDP were not competitive inhibitors with each other. These results ruledout the Theorell-Chance mechanism. Based on these results, it is proposedthat sucrose synthetase from bamboo shoot operates after an ordered bi bimechanism with the enzyme binding with sucrose first and releasing fructoselast. The regulatory property of sucrose exerted upon the enzyme with re-spect to the binding with sucrose and the second substrate UDP stronglysuggests that the enzyme in vivo catalyzes the reaction in favor of sucrosecleavage. From the results reported in the preceding paper（1），we have predict-ed that the sucrose synthetase obtained from the shoot of bamboo Lelebaoldhami can be an allosteric enzyme which operates according to an orderedbi bi mechanism with sucrose or fructose serving as the leading substrate.This paper reports the experimental results which are in support of theprediction.
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