<b>Background and Objective:</b> Inflammation occurs <i>via</i> several mechanisms, one of which includes the production of Nitric Oxide (NO) catalyzed by inducible nitric oxide synthase (iNOS), which is inhibited selectively by isothioureas. <i>Ageratum conyzoides</i> L. has shown activity in reducing pain and inflammation, although the molecular mechanism had not been undertaken. The objectives of this work were (1) to study the mechanism of anti-inflammatory activity of <i>A. conyzoides</i> through inhibition of iNOS, (2) to correlate the iNOS inhibitory activity of the plant with the total flavonoid content of the plants and (3) to identify the flavonol synthase (FLS), an enzyme that catalyzes the production of quercetin. <b>Materials and Methods:</b> The inhibitory activity against iNOS was assayed by <i>in vitro</i> method. The total flavonoids (calculated as quercetin) of <i>A. conyzoides</i> were determined by fluorometry. The protein extraction of the leaves was carried out by employing Laing and Christeller's (2004) method, followed with SDS-PAGE. <b>Results:</b> The inhibitory activity (IC<sub>50</sub>) of ethanol extract and ethyl acetate fraction of <i>A. conyzoides</i> against iNOS was 92.05 and 4.78 μg mL<sup></sup><sup>1</sup>, respectively. Pearson correlation analysis resulted in 0.548 (ethanol extract) and 0.696 (ethyl acetate fraction). The total flavonoids (calculated as quercetin) contained in the ethanol extract and ethyl acetate fraction of <i>A. conyzoides</i> were 0.71 and 7.65%, respectively. The FLS in <i>A. conyzoides</i> leaves was identified at 31 kDa. <b>Conclusion:</b> <i>A. </i>c<i>onyzoides</i> L. is potential in inhibiting iNOS due to quercetin contained in the leaves. This report will add a scientific insight of <i>A. conyzoides</i> for biological sciences.
Keywords: Ageratum conyzoides L.; L-arginine; NSAIDs; SDS-PAGE; inflammation; nitric oxide synthase.