Antidiabetic activity, metabolomics and nutritional profile of wild and cultivated Amaranthus spp. and isolated compounds

Diabetes mellitus is one of the major health problems in the world, with the incidence and associated mortality increasing. It is associated with high levels of blood sugar and inadequate regulation of blood sugar imposes serious consequences for health. Conventional antidiabetic drugs are effective, however, they have unavoidable side effects. Edible plants can act as an alternative source of antidiabetic agents. Amaranthus cruentus and A. hybridus (commonly known as Amaranth) belong to the Amaranthaceae family. Amaranth leaves are an inexpensive and excellent source of vitamins, including β-carotene, vitamin B6, vitamin C, riboflavin, folate, as well as dietary minerals like calcium, iron, zinc and potassium. Previous studies focus mostly on the wild Amaranth variety and there is a dearth of information about the effect of cultivation on both primary and secondary metabolites which affect the activity of the plant. Thus, the study sought to assess the effect of wild and cultivated A. cruentus and A. hybridus leaf extracts on in vitro α-glucosidase and α-amylase enzymes. Furthermore, hypoglycaemic compounds from A. cruentus were isolated and elucidated and their antidiabetic activity determined. Vitamin A, zinc and iron content were also analysed. The chemical profile of cultivated and wild A. cruentus and A. hybridus were determined by multivariate statistical analysis using Nuclear Magnetic Resonance (NMR) spectroscopy. Differences were observed in primary metabolites namely, sucrose and maltose. These metabolites were dominant in cultivated Amaranthus spp. when compared to their wild counterparts. In addition, a higher content of proline, an amino acid, was found in cultivated A. cruentus and A. hybridus, whilst leucine was only abundant in A. hybridus. Metabolites that were present in both wild and cultivated Amaranth in similar concentrations were: trehalose, trans-4-hydroxy-L-proline, trigonelline, betaine, valine, alanine, fumarate, formate and kynurenine. Trehalose and trigonelline are important contributors and regulators of water deficit stress responses in plants enabling Amaranthus to be resistant to water deficiency. Chlorogenic acid was annotated only on cultivated A. hybridus. Findings on the antidiabetic activity of extracts demonstrate that methanol (MeOH) extracts of wild A. hybridus was a potent α-glucosidase inhibitor at the lowest concentration tested (0.125 mg/mL). Cultivated A. cruentus exhibited close to full inhibitory activity of the α-glucosidase enzyme. Notably, none of the extracts tested were able to inhibit the activity of α-amylase beyond 50%. Wild A. cruentus showed good antidiabetic activity and a rich preliminary phytochemical profile. As a result it was selected for the isolation and identification of antidiabetic compounds. Amaranthus cruentus yielded three compounds; α-spinasterol, a plant sterol, and palmitic acid alongside pheophorbide A-methyl ester which was isolated from A. cruentus for the first time. The structures of isolated compounds were elucidated using proton (1H) and carbon (13C) NMR spectroscopy and comparison of spectral data with literature values. All compounds were potent α-glucosidase enzyme inhibitors. Palmitic acid in particular demonstrated the highest inhibition against α-glucosidase in all the concentration tested. Finally, true to Amaranth’s ability to store micronutrients, cultivated and wild Amaranth crops were rich in zinc. Cultivated A. hybridus and wild grown A. cruentus showed similar zinc concentration, 19.5 mg/100 g and 19.6 mg/100 g respectively. The highest iron amount was observed in cultivated A. hybridus. None of the samples accumulated vitamin A.

D.Phil. (Life Sciences)

Life and Consumer Sciences