Antioxidant Potentials and Inhibitory Activity of Orange & Purple Color Sweetpotatoes (Ipomoea batatas L.) toward Rat Intestinal alpha-Glucosidase using p-Nitrophenyl-alpha-D-glucopyranoside (PNP-G) as a Substrate

Antioxidant compounds in food play an important role as a health promoting factor. Scientific evidence suggests that antioxidants reduce the risk for chronic diseases including cancer and heart disease. The trolox equivalence antioxidant capacity (TEAC), 2, 2-diphenyl-1-picrylhydrazyl (DPPH), and Folin-Ciocalteu are just a few electron transfer antioxidant capacity assays widely used today. Polyphenols and other naturally occurring compounds has become the target of investigation for the treatment of non-insulin dependent diabetes in recent years. Specifically, naturally occurring compounds and plant extracts have been tested for inhibition of the enzyme α-glucosidase (AGH). The antioxidant and antidiabetic potentials of the orange and purple fleshed sweetpotato genotypes were examined. The anti-diabetic activities were tested for inhibitory activity using the enzyme α-glucosidase obtained from rat intestine using the substrate p-nitrophenyl-α-D-glucopyranoside (PNP-G). The antioxidant activity was investigated with three different screening methods: the 2, 2’-azinobis (ABTS), DPPH, and oxygen radical absorbance capacity (ORAC). It was observed that the antioxidant activities of sweetpotato extracts in the hydrophilic fraction have a significant antioxidant effect using each method. The results showed a relationship between the total polyphenol content and antioxidant function in case of ABTS and ORAC. The hydrophilic ABTS values correlated significantly with the hydrophilic DPPH values and the hydrophilic ORAC values correlated reasonably well with the hydrophilic ABTS values. The hydrophilic DPPH values and hydrophilic ORAC values also showed a strong correlation. However, antioxidant activities with the lipophilic extracts were not significantly correlated. Among the methods examined, ABTS proved to be the best method for antioxidant determination in orange- fleshed sweetpotatoes followed by the ORAC method. The sweet potato extracts demonstrated similar activity in the ABTS, DPPH and the Folin assays.  Further, previous studies have shown a relationship between color intensity and antioxidant activity. This relationship is seen here as well with the purple sweetpotatoes having higher total phenols and antioxidant activity than the lighter colored orange varieties. However, there is no direct relationship observed between antioxidant activity and AGH inhibition. The information of this research will facilitate the genetic and chemical breeding study for improvement of the desired quality of orange and purple fleshed sweetpotatoes, as well as other produces.