Studies on antioxidant activities of mucuna seed ( Mucuna pruriens var utilis ) extract and various non‐protein amino/imino acids through in vitro models

The antioxidant activities of a methanolic extract of mucuna beans ( Mucuna pruriens var utilis ) and several non‐protein amino/imino acids, namely L ‐3,4‐dihydroxyphenylalanine ( L ‐dopa), L ‐3‐carboxy‐6,7‐dihydroxy‐1,2,3,4‐tetrahydroisoquinoline (compound I), (−)‐1‐methyl‐3‐carboxy‐6,7‐dihydroxy‐1,2,3,4‐tetrahydroisoquinoline (compound II) and 5‐hydroxytryptophan (5‐HTP), were evaluated. By virtue of their hydrogen‐donating ability, all the tested compounds and the mucuna seed extract showed excellent reducing power, with the highest values being recorded for L ‐dopa in a dose‐dependent manner. Similarly, as compared with synthetic antioxidants (BHT and BHA) and quercetin, all the tested compounds and the seed extract were found to be more potent in free radical‐scavenging activity ( P < 0.05) against α,α‐diphenyl‐β‐picrylhydrazyl (DPPH • ) radicals. Hydroxyl radicals (OH • ) and superoxide anion radicals (O 2 •− ) were effectively scavenged by the tested compounds, with the exception that no scavenging activity of 5‐HTP was observed on (O 2 •− ) up to a concentration of 2 mg ml −1 , as was also the case for BHA. Among the tested non‐protein amino/imino acids and seed extract the highest peroxidation‐inhibiting activity (95%) was recorded for 5‐HTP. On the other hand, in the linoleic acid/β‐carotene‐bleaching system, L ‐dopa, compound I and compound II acted as pro‐oxidants, whereas the seed extract showed only weak antioxidant activity as in the linoleic acid emulsion system.