ANTIOXIDANT POTENTIAL AND INHIBITORY EFFECT OF IN VITRO DIGESTED SPECIES OF BITTER YAM (Dioscorea dumetorum) ON CHOLINESTERASE AND LIPID PEROXIDATION

Abstract

Ample studies have related the health promoting properties of most plant foods to their phytochemical constituents as well as their antioxidant properties. Bitter yam (Dioscorea dumetorum) has been reported to be rich in polyphenols and can hence lower the risk of developing several chronic degenerative diseases. However, few studies have investigated cooked D. dumentorum in this regard despite the fact that it is always eaten cooked. The biological properties of antioxidants depend on their release from the food matrix during the digestion process. An in vitro digestion model was used to evaluate the antioxidant potential and inhibitory effect of in vitro digested white and yellow bitter yam (Dioscorea dumetorum) on cholinesterase and lipid peroxidation. The phytochemical screening revealed that phlobatamin, steroid, anthraquinones, tannins and saponins were absent in the evaluated bitter yam species while cardiac glycoside, flavonoids, terpenoid and alkaloid were present. The qualitative HPLC analyses revealed the presence of quercitrin, quercetin, gallic acid, chlorogenic acid, epicatechin, kaempferol, rutin, caffeic acid, catechin, ellagic acid in both species of bitter yam evaluated. Some of the quantified phenolic compounds increased after cooking while others reduced after cooking. The total phenolic content (TPC), total flavonoid content (TFC), ferric reducing antioxidant power, DPPH˙ radical scavenging activity, ABTS˙+ radical scavenging activity, nitric oxide (NO˙) scavenging activity of the in vitro enzyme digested white and yellow bitter yam samples are higher compared to the methanolic extracts. In vitro digested white bitter yam showed a higher DPPH˙ radical scavenging activity compared to in vitro digested yellow bitter yam. Methanolic extracts of white and yellow bitter yam have higher ability to inhibit OH˙ radical formation compared to in vitro digested samples. In vitro digested white bitter yam has a higher inhibitory action against Fe2+ lipid oxidation in liver and brain homogenate compared to the methanolic extracts, whereas methanolic extract of yellow bitter yam has a higher activity against lipid oxidation compared to in vitro digested yellow bitter yam extracts. Except the results obtained for the OH˙ inhibition assay, the in vitro digested samples exhibited appreciable higher antioxidant activities than the methanolic extracts. In vitro digested boiled white and yellow had higher TPC than the in vitro digested raw form while in vitro digested raw white and yellow bitter yam have higher NO˙ and ABTS˙+ scavenging abilities compared to the in vitro digested boiled form. Methanolic extracts and in vitro digested yellow and white bitter yam samples showed no acetylcholinesterase inhibitory ability, while the methanolic extracts of the two evaluated bitter yam varieties displayed butrylcholinesterase inhibitory potential. Bitter yams should be boiled in such a way that the phytochemical constituents and its antioxidant potential will still be retained