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), ((-)-1methyl-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 (less than) 0.05) against (alpha),(alpha)-diphenyl,(beta)-picrylhydrazyl (DPPH) radicals. Hydroxyl radicals (OH) and superoxide anion radicals (O2^-) were effectively scavenged by the tested compounds, with the exception that no scavenging activity of 5-HTP was observed on (O2^-) 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/(beta)-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