ANTIOXIDANT ACTIVITIES, PHENOLIC COMPOSITION AND THE ANTICHOLINESTERASE ACTIVITIES OF SOME UNDERUTILIZED LEGUMES AFTER IN VITRO ENZYME DIGESTION

Abstract

Underutilized legumes are indigenous, available, accessible, well-adapted, easy and cheap-to-produce crops that constitute the most abundant and least inexpensive source of protein. In this study, a model mimicking the human digestion process of Sphenostylis Stenocarpa (African yam bean-AYB), Cajanus cajan (Pigeon pea) and Vigna subterranean (Bambara nut) was simulated after which the antioxidant activities and anticholinesterase activities of the ‘digests’ were carried out. The antioxidant and anticholinesterase activities of the methanolic extract and undigested control of the legumes were also carried out. Phytochemical examination of the legumes revealed the presence of alkaloids, terpenoids, flavonoids, cardiac glycosides and saponins. HPLC-DAD qualitative study of phenolic compounds revealed the presence of some flavonoids and phenolic acids in the raw and cooked samples The quantitative estimation of the phenolic compounds revealed that there was a reduction in the level of most of the identified phenolic compounds in Cajanus cajan and Vigna subterranean after cooking while in Sphenostylis Stenocarpa, there was a increase in most of its identified phenolic compounds after cooking. The in vitro enzyme digested samples of the legumes showed higher total phenolic content than that of the methanolic extracts. Similar results were obtained for Ferric reducing antioxidant power (FRAP), ABTS radical scavenging activity, hydroxyl radical (OH) scavenging activity. Conversely, the methanolic extract of the legumes showed higher total flavonoid content and DPPH radical scavenging activity compared to that of the in vitro enzyme digested samples. The ability of the methanolic extract and enzyme digested samples of the legumes to inhibit lipid oxidation using brain and liver homogenate decreases as the concentration of the extract increases, with methanolic extract demonstrating a higher ability compared to the in vitro enzyme digested samples. The result also revealed that methanolic extract of the cooked legumes showed higher Total Phenolic and Total Flavonoid content (TPC, TFC) than that of the raw legumes with exceptions in AYB whereas the FRAP and radical scavenging assays (DPPH*, NO*, OH*, ABTS*) were higher in the raw legumes compared with the corresponding cooked legumes with exceptions of AYB. However, the in vitro enzyme digested samples of the raw legumes (with some few exceptions in Bambara nut) revealed higher TPC, TFC, FRAP, DPPH radical scavenging and NO radical scavenging activities than that of the cooked legumes with exception of ABTS and OH radical scavenging activities. The anticholinesterase activity of the studied legumes revealed that enzyme digest of raw Bambara nut showed some level of AChE inhibitory activity; whereas other studied legumes appeared to enhance AChE activity. Pigeon pea (undigested control), African yam bean (methanolic extract) and Bambara nut (in vitro enzyme digested samples and methanolic extract) exhibited some degrees of BChE inhibitory activity. This study has showed that antioxidant activities after in vitro enzyme digestion of the legumes, is relatively higher than that of organic solvent extract (methanol). The study revealed that Bambara nut with minimal processing could be a potential candidate in the management and treatment of neurodegenerative diseases such as Alzheimer.