INHIBITORY EFFECT OF PHENOLIC-RICH EXTRACTS OF SOME GUAVA (Psidium spp.) LEAVES AND FRUITS ON ANGIOTENSIN – I CONVERTING ENZYME IN VITRO

Abstract

Guava (Psidium spp.) have been employed in folk medicine in the treatment/management of hypertension for centuries with little or no scientific basis. This study was designed to investigate the inhibitory effect of four varieties of guava leaves and fruits extract on the angiotensin – I converting enzyme (ACE); a key enzyme linked to hypertension, in order to provide an insight into the possible mechanism of the antihypertensive action of guava extracts and to know the part and variety with the highest antihypertensive property. The extract was prepared by extracting the dried samples with methanol – 1M HCl solution (20:1 v/v) and filtrate collected and evaporated to dryness. Subsequently, the total phenol, flavonoid and antioxidant properties of the extracts were evaluated. Furthermore, the inhibitory effect of the extracts on angiotensin – I converting enzyme was determined and the physioactive phenolic constituents partially characterised using gas chromatography (GC). The study revealed that pink guava leaf had the highest phenolic (39.44mg/g) and flavonoid (28.13mg/g) content as well as exhibiting the highest antioxidant properties [DPPH (EC50 = 5.91 mg/ml), ABTS˙+ (0.55 mmol. TEAC/g), ˙OH scavenging ability (EC50 = 5.45 mg/ml), NO˙ scavenging ability (76.507%), Fe²+ chelating ability (EC50 = 3.50 μg/ml), reducing activity (41.14 mg AAE/g)] and inhibition of both Fe²+ and Sodium nitroprusside induced lipid peroxidation in rats’ heart. Furthermore, pink guava fruit had the highest inhibitory effect on ACE (IC50 = 0.7 mg/ml). The GC partial characterisation of the guava samples revealed the presence of some phenolic phytochemicals. Hence, inhibition of ACE and antioxidant activity may be the possible underlying mechanism for the antihypertensive property of guava. The pink guava with its leaf having the highest antioxidant properties and the fruit with the highest ACE inhibitory activity appeared to be the most potent.