Spices are used in soup preparation in many homes and serve as ingredients in the preparation of several traditional delicacies endowed with natural antioxidants such as polyphenols. The aim of this study is to evaluate the effect of aqueous extract of garlic, white onion, and purple onion on angiotensin-converting enzyme, α-amylase, and α-glucosidase activity in vitro and determine their antioxidant properties via various antioxidant assays such as OH*, Fe2+ chelation, and 2, 2'-azino-bis 3-ethylbenzothiazoline-6-sulphonic acid (ABTS) radical-scavenging assays. The results reveal that aqueous extract of garlic, purple onion, and white onion inhibited angiotensin-converting enzyme, α-amylase, and α-glucosidase in a concentration-dependent manner (0-4 mg/ml). However, purple onion (IC50 = 0.59 mg/ml) had higher inhibitory effect on angiotensin-converting enzyme than white onion (IC50 = 0.66 mg/ml) and purple onion (IC50 = 0.96 mg/ml). Furthermore, white onion had significantly (p < .05) stronger inhibitory effect on α-amylase (IC50 = 3.93 mg/mL) than garlic (IC50 = 8.19 mg/ml) and purple onion (IC50 = 8.27 mg/ml). Conversely, garlic (IC50 = 4.50 mg/ml) had no significantly higher inhibitory effect on α-glucosidase activity than white onion and purple onion. In addition, the aqueous extracts were able to scavenge 2,2- diphenyl-1-picrylhydrazyl (DPPH*) free radical and reduce Fe3+ to Fe2+ in a dose-dependent manner. Therefore, a possible mechanism by which garlic, white onion, and purple onion exert antidiabetes and antihypertensive properties could be through the inhibitory effect on ACE, α-amylase, and α-glucosidase coupled with their ability to prevent lipid peroxidation in the pancreas and heart, which justify their strong antioxidant properties.
Keywords: diabetes; hypertension; lipid peroxidation; α-amylase; α-glucosidase.