IN-VITRO INHIBITORY EFFECT OF TURMERIC RHIZOME (Curcuma longa) EXTRACT AND FRACTIONS ON THE BIOACTIVITY OF ANGIOTENSIN-1 CONVERTING ENZYME

Abstract

Angiotensin-1 converting enzyme (ACE-1), is a key component the renin angiotensin aldosterone system (RAAS), which regulates blood pressure. Since the over expression of RAAS is known to be associated with vascular hypertension, ACE-1 inhibition has become a major target of control for hypertension. Research on potential ACE inhibitors is expanding broadly and most are focused on natural product derivatives such as peptides, polyphenolics, and terpenes. Curcuma longa has been used to treat a wide range of diseases due to its medicinal values. This study investigated the inhibitory effect of turmeric rhizome (Curcuma longa) methanolic extract and fractions on the bioactivity of angiotensin-1 converting enzyme (ACE) in vitro. The powdered sample was extracted with 80% methanol. The resulting filtrate was freeze-dried to obtain dry crude extract which partioned between water, n-hexane, dichloromethane, ethylacetate and n-butanol. The methanolic extract and fractions were subjected to phytochemical screening. ACE inhibitory potential of the methanolic extract and fractions were compared with that of Ramipril®, a known ACE inhibitor used as a positive control. Different concentrations of the methanolic extract and fractions of Curcuma longa and Ramipril® were used against a single concentration (0.50 mM) of N- [3-(2-furyl) acryloyl]-Phe-Gly-Gly (FAPGG), the substrate. Also, the kinetic parameters (Km, Vmax, Ki) of the inhibition study were determined for the extract and fractions. Phytochemical screening of the crude extract (80% methanol) of turmeric rhizome showed the presence of alkaloids, terpenoids, anthocyanins, anthraquinones, saponins, tannins, phlobatannins, steroids, cardiac glycosides, and flavonoids. Ethylacetate fraction showed the presence of phlobatannins, terpenoids, alkaloids, flavonoids and anthocyanins, and the dichloromethane fraction showed the presence of tannins, terpenoids and alkaloids while n-butanol fraction showed the presence of tannins, anthraquinones, and saponins. N-hexane fraction was shown to contain steroids only. The inhibitory effects of the methanolic extract, ethylacetate (EAF) fraction and dichloromethane fraction (DCMF) of turmeric rhizome on ACE-1 was dose-dependent with highest concentration having the highest inhibition. The methanolic extract displayed maximum inhibitory activity (78.12%) at 1.0 μg/mL. Also, the highest percentage inhibition of EAF and DCMF were 96% and 67.73% respectively, at 0.1 μg/mL each, using FAPGG as substrate. The 50% inhibition concentration (IC50) of ACE activity of methanolic extract, EAF, DCMF and Ramipril® were 0.59 μg/mL, 0.035 μg/mL, 0.075 μg/mL and 1.57542 μg/mL respectively. N-hexane and n-butanol fractions had no inhibitory effect on ACE-1 as observed in this present study. The kinetic studies shows uncompetitive type inhibition for both methanolic extract and ethylacetate fraction, while the DCMF showed a mixed type inhibition against ACE when FAPGG was used as substrate. When the IC50 of extract and fractions were added, Vmax and Km were 0.00504, 0.00593, 0.00558 ΔA/min and 0.137, 0.136, 0.175 mM respectively. Also, without the extract and fractions, they were 0.006741 ΔA/min and 0.1361 mM. Inhibition constant, Ki values were 1.757, 0.0957, and 0.361μg/mL respectively. The results suggested that the ACE inhibitory capacity of the extract and fractions increased with purity which may be due to the presence of pure bioactive substance in the fractions. The result revealed that the ethylyacetate fraction had the highest inhibition capacity on ACE bioactivity. In conclusion the inhibitory effect of Curcuma longa methanolic extract and fractions on ACE-1 might mean that the consumption of turmeric rhizome could aid in the prevention and management of hypertension and related cardiovascular disorders.