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Graphical analysis of inhibition kinetics for dextransucrase from Leuconostoc mesenteroides was done with typical inhibitors, competitive and noncompetitive. Based on the plots of Yonetani-Theorell and Semenza-Balthazar, mutual competition between the pairs of inhibitors of identical kinetic type was observed, while combination of competitive and noncompetitive inhibitors gave no significant mutual interactions. By the procedure ofNitta et aL, binding sites for competitive and noncompetitive inhibitors were shown to be distant from each other. Moreover, two noncompetitive inhibitors competed with each other for a single binding site on the enzyme. Although biphasic reciprocal plots may suggest rather complicated binding of various inhibitors, the results obtained by the three graphical methods are fully explained when competitive and noncompetitive inhibitors for substrate sucrose bind to the so-called donor-and acceptor-sites of dextransucrase, respectively. Dextransucrase (EC 2.4.1.5) from Leuconostoc mesenteroides catalyzes a glucosyl transfer reaction from sucrose to acceptor dextran and produces a high molecular weight dextran as shown in equation (1).1) Sucrose + (D-Glucose)n > D-Fructose + (D-Glucose)n + 1 (1) In our previous paper,2) we reported the purification of dextransucrase from L. mesenteroides strain NRRLB-1416, which contained only a small amount of endogenous dextran.

Effects of various compounds on the dextransucrase (EC 2.4.1.5) from Leuconostoc mesenteroides was evaluated based on the two catalytic activities of enzyme,that is the hydrolase activity for the substrate, sucrose, and the transferase activity of a D-glucosyl group to an acceptor molecule. Theeffectors were grouped into six categories by their activation or inhibition of the sucrase and transferase activities of dextransucrase. Type I-A inhibited both activities, type I-B inhibited the sucrase activity, and type I-C inhibited the transferase activity. Type A-Aactivated both the hydrolase and transferase, and type A-B activated only the transferase. Antagonistic modulation (type IA-A), was shown by methyl aD -glucoside and glycerol, which activated the sucrase and inhibited the transferase. A double reciprocal plot for dextran gave a biphasic pattern which led to Ki values for each limb. Based on the biphasic kinetics and the action of antagonistic effectors, the regulation of dextran synthesis wasdiscussed. Dextransucrase (EC 2.4.1.5) catalyzes the transfer of a D-glucosyl group from sucrose to a growing chain of the polysaccharide dex

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