The possible presence of natural β-d-glucosidase inhibitors in jujube leaf extract

•The enzymatic production of isoquercitrin from jujube leaf using hesperidinase was optimized.•Expected β-d-glucosidase activity was not observed under any conditions.•The presence of an unknown natural β-d-glucosidase inhibitor in jujube leaf is suspected.•Natural inhibitors of β-d-glucosidase are needed for food-grade applications. Isoquercitrin is a phenolic compound well-known for having greater health benefits than quercitin, its aglycone derivative, and other related glycosides. However, isoquercitrin is rarely found in nature. Here, we optimized the conditions for the enzymatic transformation of isoquercitrin from rutin that was extracted from jujube leaf using the hesperidinase, enzyme complex containing β-d-glucosidase and α-l-rhamnosidase. The maximum productivity (2.57±0.16mg/mL) was experimentally found under the following conditions: 47.3°C, 52.16h, and pH 5.31, which agreed well with the predicted value (2.65mg/mL). However, the achievement of this maximum yield was due to the absence of β-d-glucosidase activity. Further investigations using a β-d-glucosidase assay and reaction measurements under various conditions revealed that the β-d-glucosidase activity was not blocked by denaturation or known inhibitory factors. Currently, there are no recognized β-d-glucosidase inhibitors present in the jujube leaf; however, our observations strongly suggest that an unidentified β-d-glucosidase inhibitor exists in jujube leaf extract.