ANTIOXIDANT POTENTIAL OF PHENOLICS IN TWO LOCAL VARIETIES OF RICE AND THEIR ANTICHOLINESTERASE ACTIVITIES AFTER IN VITRO DIGESTION

Abstract

Rice is an important staple food crop for human health because it provides the bulk of calories for more than half of the world’s population. Rice crops are rich in polyphenols that are responsible for lowering the risk of developing several chronic degenerative diseases. This present investigation was aimed at examining the antioxidant potential of phenolics in two local varieties of rice and their anticholinesterase activities after in vitro digestion. Phytochemical screening revealed the presence of alkaloid, tannin, flavonoid and cardiac glycosides while phlobatannins, steroids, anthraquinones and saponin were absent in the two rice samples. HPLC-DAD identification of phenolic compounds in the samples revealed the presence of gallic acid, catechin, chlorogenic acid, caffeic acid, epicatechin, rutin, quercitrin, quercitin, kaempferol and luteolin in the raw and boiled samples. The quantitative estimation of the phenolic compounds showed a reduction in the level of the identified phenolic compounds with exception of chlorogenic acid in ‘Gboko’ rice after boiling. In vitro digested rice (raw and boiled) showed significantly higher total phenolic content (TPC) than the aqueous-methanolic extracts. Similar results were obtained for total flavonoid content, ferric reducing antioxidant power, ABTS˙+ assay and NO˙ assay. The aqueous-methanolic extracts displayed higher DPPH˙ scavenging activity than the in vitro enzyme digested samples. The aqueous-methanolic and enzyme digested rice extracts inhibited ˙OH radical formation in a dose dependent manner, with the aqueous-methanolic demonstrating a higher ability to inhibit ˙OH radicals. The boiled digested rice samples exhibited higher antioxidant activities than the raw samples. Boiled digested and aqueous-methanolic extracts of ‘Ofada’ rice exhibited a higher ability to inhibit ˙OH radicals compared to raw Ofada’ rice, while the raw digested ‘Gboko’ rice sample exhibited a higher ability to inhibit ˙OH radicals compared to the boiled variety. The ability of the aqueous-methanolic extracts and enzyme digested rice samples to inhibit Fe2+ lipid peroxidation in rat’s liver and brain homogenate decreased as the concentration of the extracts increased. The aqueous-methanolic extracts demonstrated a higher ability to inhibit Fe2+ induced lipid peroxidation in rat’s brain and liver homogenate compared to the in vitro enzyme digested samples. In vitro enzyme digested raw ‘Ofada’ rice as well as undigested control (acid-alkaline treated rice samples) of raw and boiled studied rice samples have the potential of inhibiting AChE activity. Aqueous-methanolic extracts of raw and boiled ‘Ofada’ and ‘Gboko’ rice also exhibited the potential to inhibit butrylcholinesterase activity. The result of this study implies that the selected rice varieties possess moderate antioxidant capacity which are better released after digestion and therefore, can be considered as health supplements. Therefore, these plants could be harnessed in preventive health care as nutraceutical foods