CHEMICAL COMPOSITION, ANTIOXIDANT, ANGIOTENSIN -1-CONVERTING AND α-AMYLASE, α -GLUCOSIDASE ENZYMES INHIBITORY ACTIVITIES OF WILD LETTUCE (Launaea taraxacifolia Willd) LEAF BIOACTIVE COMPOUNDS

Abstract

Wild Lettuce (Launaea taraxacifolia Willd.), an underutilised indigenous leafy vegetable, contains essential nutrients and medicinal properties. Side-effects from antihypertensive and antidiabetic drugs have motivated researchers to find new medicines in metabolites or extracts from medicinal plants to control hypertension and diabetes that cause fewer side effects. . The proximate and mineral contents of wild lettuce leaves were more than other common vegetables. Hence, this study aimed to determine the chemical composition, antioxidant activities, α-amylase, α-glucosidase and angiotensin converting enzyme inhibitory potentials of wild lettuce leaf powder samples. Freshly harvested wild lettuce leaves were processed into powder to obtain whole leafy powder (WLF), extracted powder (WLE), residue powder (WLR) and leaf protein isolate (WLPI). Chemical composition, antioxidant activities, α- amylase-glucosidase and angiotensin-converting enzyme inhibitory potentials of the powder samples were determined. The crude protein content of wild lettuce samples ranged from 23.27% in WLF to 46.57% in WLPI, crude fibre ranges from 4.17 % in WLR to 37.37% in WLR, and energy values ranges from 264.04 kcal/100g in WLR to 356.24 kcal/100g in WLPI. The leaf powder samples had appreciable amount of minerals in composition with phosphorous being the most abundant, while zinc had the lowest concentration. Total amino acid composition, total essential amino acids and non-essential amino acids of wild lettuce leafy vegetable samples ranged as 96.43% in WLE - 99.06 mg/100g in WLPI, 39.83 - 50.65 mg/100g protein and 47.66 - 58.76 mg/100g protein, respectively. The protein efficiency ratio (PER), essential amino acid index and biological values of raw (WLF), residue (WLR), extract (WLE) and protein isolate (WLPI) of wild lettuce leaves ranged from 2.79 - 3.51, 77.03 – 92.36% and 72.26 – 88.97%, respectively, while that of argine/lysine ratios and branched chain amino acids ranged from 0.94 – 1.95 and 17.42 - 19.21, respectively. The phytochemicals (saponin, tannin, oxalate, phytate, terpennoids, flavonoid and phenol) in wild lettuce powders were within the tolerable levels. The free radical scavenging ability of the wild lettuce powder samples in 2, 2-diphenyl-1-picrylhydrazyl (DPPH) ranged from 18.28 % in WLF to 91.88 % in WLPI, and in ABTS, WLPI (383.92 mg/g) had the most activities, while WLR (169.64 mg/g) had the least free radical scavenging activity. The OH-free radical scavenging ability of WLR, WLF, WLE, WLPI were 60, 74, 74.57 and 80.25%, respectively, while Fe2+chelation antioxidant power of the wild lettuce powder samples had WLPI (36.64%) as the most active; WLR (5.84%) had the least reducing ability. The wild lettuce protein isolate (WLPI) (64.7%) ; exhibited highest inhibition of α-glucosidase followed by wild lettuce extract (WLE) (62.04%), and wild lettuce residue (WLR) had the least activity (50.82 %). Similarly, WLPI had highest percentage of apha-amylase inhibitory activity, while WLR had the lowest percentage inhibition. African wild lettuce samples, except WLF, had higher percentage of inhibition on angiotensin converting enzyme than Captopril (a synthetic antihypertensive agent). However, WLPI, a protein isolate sample, (97.5%) had higher percentage of angiotensin converting enzyme inhibitory activities when compared with WLE (93.3%), WLR (85.3%) and WLF (67.6%) samples. The protein isolate of wild lettuce leaf exhibited high antioxidant activity, percentage inhibition on apha amylase-glucosidase and angiotensin converting enzymes than other powder samples. Therefore, the protein isolate of wild lettuce powder may be suitable as a good source of antioxidant supplement, antidiabetic and antihypertensive agent, which could be developed as a nutraceutical toward prevention, treatment, and management of oxidative stress and degenerative diseases like diabetes and hyprtension.