Modulation of Eicosanoeds Biosyntheses in Vivo as a Mechanism-Based Evaluation of Putative Anti-Inflammatory Plant Extracts

Abstract

The eicosanoids are a group of oxygenated unsaturated 20-carbon chemical compounds produced from arachidonic acid (AA), that mediate almost every step in the inflammatory process naturally. Some of the eicosanoids are anti-inflammatory while others are proinflammatory. Desmodium adscendens, a medicinal plant used by local herbalists to manage asthma, had been shown to be anti-anaphylactic in vivo, and to modulate AA metabolism in vitro. In this work, the anti-inflammatory properties of D. adscendens in vivo were examined by measuring the effect of its aqueous extract on eicosanoid production. Two other putative anti-inflammatory medicinal plants, Parquetina sp (‘Tina A’) and Cassia sieberiana (‘Kenken’) were also examined. The effects of the extracts on phospholipase A2 (PLA2), the enzyme responsible for the mobilization of arachidonic acid (AA) from membrane glycerophospholipid stores to initiate the de novo biosyntheses of the eicosanoids were also examined. Microsomal enzymes prepared from the lungs of both extract-treated (test) and untreated (control) male guinea pigs were used to catalyze the metabolism of arachidonic acid (AA), the natural substrate for eicosanoid synthesis, via the cyclo- mono- and the lipoxygenase pathways to produce various eicosanoids. Reduced glutathione, GSH, was added to the reaction mixture for the cyclooxygenase pathway while NADPH was added for the monoxygenase pathway. For the lipoxygenase pathway no cofactor was added. The monooxygenase metabolites were not quantified due to limited resources. However, the influence of added NADPH on the production of the metabolites of the other two pathways was evaluated. All synthesized eicosanoids were assayed by ELISA. As sources of secretory phospholipase A2 (sPLA2), blood samples were taken from the experimental animals and used to assess the effects of the plant extracts on PLA2 activity. The cyclooxygenase and lipoxygenase pathways were identified as good bioassay systems for assessing the anti-inflammatory properties of D. adscendens. The plants extract inhibited the pro-inflammatory lipoxygenase pathway in a dose-dependent manner (68 % and 98% reductions in peptidoleukotrienes syntheses for the lower and higher doses respectively). In the cyclooxygenase system, the extract enhanced the syntheses of the anti-inflammatory prostanoids; PGI2 (6437% increase) and PGE2 (581% increase). The effect on the synthesis of the pro-inflammatory prostanoid PGF2α was insignificant (0.5% increase) and that on TXA2 was 49% increase, both at the higher dose of the extract. Using D. adscendens as a model, the anti-inflammatory effects of ‘Tina A’ and ‘Kenken’ were assessed using the cyclooxygenase bioassay system only. Like D. adscendens, the two medicinal plants also increased PGI2 and PGE2 production and hardly showed any effect on PGF2α and TXA2 syntheses. The increases in PGI2 production ranged between 290% and 1417%; those for PGE2 were 57% and 78%, all at the higher doses of the extracts. The effects at the lower doses were not significant except for ‘Tina A’. In all, D. adscendens proved to be the best anti-inflammatory plant with respect to enhancing antiinflammatory eicosanoids syntheses, followed by ‘Tina A’. All three extracts inhibited phospholipase A2 activity with ‘Tina A’ showing a dosedependent inhibitory effect even with a small dose difference of 1: 2.5. ‘Tina A’ was the best PLA2 inhibitor followed by ‘Kenken’ The results indicate that the medicinal plants evaluated provide therapeutic relief to inflammatory disorders by the following mechanisms: (a) directly reducing PLA2 activity and thus release of AA which the rate determining step in eicosanoid production, and/or by (b) enhancing PGI2 and PGE2 production when arachidonic acid has been released. Inhibition of peptido-leukotriene synthesis could also be said to be a good determinant of the anti-inflammatory status of D. adscendens in particular. Increased synthesis of the anti-inflammatory prostanoids PGE2 and PGI2, and/or inhibition of phospholipase A2 activity appear to be good bioassays for evaluating medicinal plants claimed to have anti-inflammatory properties. It was therefore concluded that good bioassay systems have been developed for in vivo evaluation of putative anti-inflammatory drugs with respect to eicosanoid biosynthesis.