Formation of a Stable Ascorbic Acid 2-Glucoside by Specific Transglucosylation with Rice Seedα-Glucosidase

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Abstract
The enzymatic transglucosylation to synthesize a chemically stable form of l-ascorbic acid (AA) was further investigated by using commercially available enzymes. Among various glycosidases examined, only rice seed α-glucosidase could produce a nonreducing and stable glucoside of AA, which was identified as 2-O-α-d-glucopyranosyl-l-ascorbic acid (AA-2G). The enzyme showed the same regioselective transglucosylase activity as rat intestinal α-glucosidase that had been demonstrated by us to be effective in this reaction, although these two enzymes had different pH optima for maltose hydrolysis. The substrate specificity of rice α-glucosidase for AA-2G formation was considerably different from that of rat α-glucosidase. However, both α-glucosidases had high specificity for the α-1,4-glucosidic linkage but not α-1,6, suggesting that they can catalyze a preferential transglucosylation to the 2-position but not to the 6-position of AA. Thus, these results allows us to produce a sufficient amount of AA-2G with rice seed α-glucosidase for its application to medicinal and other uses.