Inhibition of Angiotensin-Converting Enzyme Activity by a Partially Purified Fraction of Gynura procumbens in Spontaneously Hypertensive Rats

Objectives: To investigate the hypotensive and angiotensin-converting enzyme (ACE) inhibitory activities of a partially purified fraction (FA-I) of the leaves of Gynura procumbens and to qualitatively analyse the putative compounds present in the fraction. Materials and Methods: The hypotensive effect of FA-I was tested in both spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) by an intravenous administration of 0–10 mg/kg of the FA-I. Administration of captopril (20 μg/kg) served as the control. In vitro 0.0–2.0 mg/ml FA-I was added to a mixture of ACE and hippuryl-L-histidyl-L-leucine and assayed by a modification of the colourimetric method of Hurst and Lovell-Smith. All blood pressure measurements were monitored by the Macintosh MacLab set-up. ACE activity was measured by an in vitro assay in which the enzymatic cleavage of hippuryl-L-histidyl-L-leucine to form histidyl-leucine and hippurate was determined colourimetrically by a cyanuric chloride/dioxane reagent. Results: The FA-I produced a marked dose-dependent reduction in mean arterial pressure (MAP) in SHR and WKY rats, with an ED50 of 1.09 and 1.05 mg/kg, respectively (p 0.01). Furthermore, FA-I at 10 mg/kg strongly inhibited the angiotensin I-induced rise in MAP (p 0.01). This response was comparable to that of captopril at 20 μg/kg. In the in vitro assay, ACE activity was inhibited with an IC50 of 0.8 mg/ml. The qualitative phytochemical analysis of FA-I indicated the presence of glycoconjugates and peptides. Conclusion: These results suggest that the hypotensive effect of G. procumbens may be due, in part, to the glycoconjugated or peptidal substances found in FA-I that exhibit an inhibitory effect on ACE.