Effect of Tribulus terrestris as antioxidant, vasorelaxant, and inhibitor of α-glucosidase and lipoxygenase

Abstract

Tribulus terrestris L. species (Zygophyllaceae), popularly known as abrolhos, is an annual herb native to India but is currently distributed in warm regions around the world. The fruit is traditionally used in Chinese and Indian medicine as a tonic, aphrodisiac, for the treatment of erectile dysfunction, and cardiovascular diseases. The aim of this research was to evaluate phenolic contents, in vitro antioxidants, lipoxygenase, and α-glucosidase inhibitory activities, as well vasorelaxant in isolated rat aortic rings of extract and fractions of T. terrestris fruits. The crude ethanolic extract (EtOH), hexane (HEX), dichloromethane (DCM), ethyl acetate (EtOAc), n-butanol (BuOH), and methanol/water soluble (AQSol) fractions were used. The HPLC-DAD was used for the analysis of fingerprints. The DCM fraction was the more complex chemical profile.The EtOAc and BuOH fractions revealed the best effect for extracting phenolic compounds and flavonoids from T. terrestris than the HEX and AQSol fraction. On the other hand, the hexanic fraction showed the best metal chelating power and lipoxygenase inhibitory activity. Concerning in vitro antioxidant activities, the EtOAc fraction showed better capacity for scavenging 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS), hydroxyl radical, reductive power, total antioxidant capacity (TAC), and α-glucosidase inhibitory activity than others fractions and the EtOH. These results correlated closely with the levels of phenolic compounds and flavonoids. The cumulative addition of the EtOH or fractions of T. terrestris fruit in a pre-contraction induced by phenylephrine (10 -7 M) induced vasorelaxant effect concentration-dependent manner and independent of the presence of the vascular endothelium in isolated rat aortic rings. The best vasorelaxant activity was observed after incubating the EtOAc fraction. In endotheliumdenuded rings, EtOAc was able to inhibit phenylephrine-induced contractions (10 -9 to 10 -4 M) in the concentration 1000 µg/mL. Likewise, in a nominally Ca 2+ -free depolarizing Krebs Henseleit solution, EtOAC inhibited CaCl2 (10 -6 -3x10 -2 M)-induced contractions in the concentration 1000 µg/mL. There was not significative difference between the vasorelaxant activity on KCl 25 mM-induced contraction, PHE-induced contraction and in the pretreatment with tetraethylammonium (3 mM), glibenclamide (3x10 -6 M) and 4-aminopyridine (3 mM). Thus, this vasorelaxant effect can probably be attributed by a reduction in calcium influx through CaVL channels and inhibition of calcium mobilization from phenylephrine intracellular stores. UPLC-ESI-MS analyses indicated that the EtOAC was rich in cinnamic acid amides. The EtOAc fraction of Tribulus terrestris fruits has in vitro antioxidant and lipoxygenase and α-glucosidase inhibitory activities. Among others, EtOAc fraction also has the highest vasorelaxant effect, probably due to the reduction in calcium influx through CaVL channels. These effects could be involved the cinnamic acid amides, since EtOAc fraction are rich in these chemical constituents.