In vitro hypoglycemic and antioxidant activities of steamed Polygonatum cyrtonema Hua with various steaming degrees: Relationship with homoisoflavonoids

Polygonatum cyrtonema Hua, an edible and medicinal plant in China, is usually steamed and then dried before use. Changes in homoisoflavonoids composition, radical scavenging activity, starch digestive enzyme inhibitory activity, cell glucose consumption and intracellular ROS formation of steamed P. cyrtonema Hua rhizome with various steaming degrees were investigated. Nine homoisoflavonoids were identified in raw and steamed rhizome extracts, and their contents were almost increased with increasing steaming degree. Steamed rhizome extract had increased activities to scavenge free radicals, reduce ferric iron, and inhibit α-amylase and α-glucosidase. The treatment with dark-steamed and deep-dark-steamed rhizome extracts restored glucose consumption in IR-HepG2 cells to normal level, and suppressed ROS level to that of Trolox treated group. Multivariate analyses (heat-map, correlation coefficient and PCA) indicated that the hypoglycemic and antioxidant activities of steamed rhizome depended on steaming degree, and were significantly correlated with disporopsin and 5,7,2′,4′-tetrahydroxy-6-methyl-homoisoflavanone (p < 0.05). Overall, these results suggested that steaming degree had remarkable impacts on homoisoflavonoids composition and bioactivities of P. cyrtonema Hua rhizome, and homoisoflavonoids greatly contributed to these activities.