Dextranases are enzymes that hydrolyze the α 1,6 glucosyl backbone chain of dextran and isomaltooligosaccharides. Dextranases are classified as endolytic enzymes (endodextranase) or exolytic enzymes (exodextranase) by their reaction modes. Furthermore, dextranases are divided into several families on the basis of the similarity in their amino acid sequences. 1,2) Most endodextranases belong to glycoside hydrolase family (GH) 49 and 66, and exodextranases belong to GH13,15, 27, 49 and 87. 3) The dextranase from Streptococcus mutans ATCC 25175 (SmDex) is an endodextranase (EC 3.2.1.11; 6 α D glucan 6 glucanohydrolase) that is classified under GH66. GH66 enzymes are composed of endodextranase and cycloisomaltooligosaccharide glucanotransferase 4) (EC 2.4.1.248; (1 6) α D glucan:(1 6) α D glucan 6 α D (1 6α D glucano) transferase (cyclizing)). In GH66 members, twenty nine enzymes have been reported and sixteen enzymes have been characterized at present.There are two types of chemical modification reagents based on their inactivation mechanisms, 5) i) two molecules reaction type (Equation (1)) and ii) intermediate formation type (Equation (2)) as follows:where E, R, E R and E R are intact enzyme, chemical modification reagent, inactivated enzyme bound by reagent covalently, and reversible intermediate complex, respectively. In Equation (2), k and KR are the rate constant of irreversible inactivation and dissociation constant of intermediate complex, respectively. Inactivation of the suicide substrate (mechanism based inactivator) follows the intermediate formation type mechanism. 6) Suicide substrate itself is an inert compound that does not attack any free amino acid. When this compound is incorporated into the catalytic site of an enzyme, its inert active species is activated by enzyme reaction to form the covalent linkage with amino acid residue, resulting in the inactivation of the enzyme. 6) Epoxyalkyl based suicide substrate is an effective material to label the catalytic residues and has been applied in a number of glycosidases. 7 17) ω Epoxyalkyl α glucopyranosides have already been reported to show irreversible inhibition to the isomalto dextranase (IMDex) (EC 3.2.1.94; 6 α D glucan isomaltohydrolase) from Arthrobacter globiformis T6 with the suicide substrate mechanism. 18) IMDex is an exodextranase which produces isomaltose from the non reducing terminal of dextran, suggesting a possibility that ω epoxyalkyl α glucopyranoside also becomes a suicide substrate for endodextranase. Therefore, 3 ,4 epoxybutyl α D glucopyranoside (E4G), 4 ,5 epoxypentyl α D glucopyranoside (E5G), and 5 ,6 epoxyhexyl α D J. Appl. Glycosci., 57, 269 272 (2010)

Starch is an abundant carbohydrate widespread in plants. This high energy polysaccharide is preserved in the storage tissues, such as seeds. In the germination of plant seeds, the starch is degraded by several hydrolytic enzymes (α amylase, β amylase, debranching enzyme and α glucosidase), followed by conversion to the biological materials and energy necessary for growth. 1) In plant seeds, starch exists as practically insoluble polysaccharide called starch granules or raw starch. Therefore, the pathway of starch degradation in the germination stage has been generally considered as follows. 2)α Amylase is a key enzyme attacking the starch granules initially to liberate soluble dextrin. Produced dextrin is hydrolyzed by the further action combined with α amylase, β amylase and debranching enzyme to form oligosaccharides such as maltose. Finally, α glucosidase converts the oligosaccharides to glucose. Among these starch degrading enzymes α amylase has been considered to be the exclusive enzyme capable of hydrolyzing the starch granules, while the other enzymes were not. However, it had been reported that plant α glucosidases exhibited the ability to attack soluble starch effectively, 3,4) which suggested that α glucosidase hydrolyzed not only oligosaccharides such as maltose but starch as well in plant seeds. Recently, barley, 5,6) millet 7,8) and rice 9,10) α glucosidases were found to be capable of degrading the starch granules. The combination of α glucosidase and α amylase exhibited the synergism of degradation of starch granules. 5,6,11) It is of interest to learn the α glucosidase mediated starch metabolism in the germination stage. The first section of this article introduces our molecular level analysis on the degradation of starch granules catalyzed by plant α glucosidase.In the second section, we describe the multiple formation mechanism of rice α glucosidases observed in the ripening and germination stages. α Glucosidases, which are synthesized in the ripening stage and preserved in dry seeds, are important in starch metabolism, since these enzymes hydrolyze starch granules before attacking by α amylase. Certain α glucosidase isozymes, which are expressed in the early stage of germination, also hydrolyze starch granules. It is valuable to learn the expression systems of α glucosidases in the dry seeds and germinating seeds. In both seeds, there were several enzymes, of which the expression feature differed by variety of rice, requiring more precise analysis to understand the contribution of each enzyme to starch metabolism. The second section of this article introduces the mechanism for the formation of rice α glucosidase isoforms and isozymes as well as their characteristics elucidated using purified en- Abstract: In germination of plant seeds, storage starch is principally degraded by the combination of amylolytic enzymes. As starch is an insoluble granule, a conventional view of the degradation pathway is that the initial attack is performed by α-amylase having the starch granule-binding ability. Pla...