Screening for the effects of selected Zimbabwean plant extracts on enzymes and processes involved in pain and inflammation.
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Inflammation is a complex process that is mediated by signalling radicals and prostaglandins. Prostaglandins are produced by conversion of arachidonic acid by cyclooxygenase (COX) isoenzymes. Selective inhibition of the inducible cyclooxygenase isoform, COX-2, would probably relieve inflammation without adversely affecting physiological function. Chronic inflammation can be attenuated by the unregulated production and poor elimination of free radicals leading to oxidative stress. One of the major contributors of free radicals is the overproduction of nitric oxide (NO) during inflammation. Chronic inflammation and oxidative stress can create micro-environments that favour development of degenerative diseases such as cancer and rheumatoid arthritis. Herbal remedies are used in folk medicine to treat inflammatory ailments when conventional drugs are unavailable or inaccessible. The plant species used by herbalists to treat pain and signs of inflammation could be potential sources of novel anti- inflammatory agents. Zimbabwean plants that are used to treat pain have compounds that can inhibit enzymes and processes that are involved in inflammation. The objective of this study was to investigate the in vitro anti-inflammatory and antioxidant activities of eight selected Zimbabwean medicinal plant extracts. Amaranthus spinosus , Brachystegia boehmii , Cassia abbreviata , Combretum molle , Combretum platypetalum , Combretum zeyheri , Gymnosporia senegalensis and Parinari curatellifolia were tested for anti-inflammatory activities using the COX enzyme inhibitory activity. Of these, six plants were further tested for membrane stabilisation using the erythrocyte membrane stabilization assay, protein denaturation inhibition using the albumin denaturation inhibition assay and antioxidant activity using the diphenyl picryl hydrazine and tetramethoxy azobismethylene quinone assays. Eight plant extracts were tested for COX-1 and -2 enzyme inhibitory activities. Combretum zeyheri and Combretum molle showed greater COX-2 inhibitory activity of 42 and 68 % respectively while showing the least COX-1 inhibition with percentage inhibition. Combretum platypetalum inhibited COX-2, with a percentage inhibition of 85 %, COX-1 inhibition of 42 %, making it COX-2 selective. IC 50 s of the COX-2 selective C. platypetalum extract and indomethacin against COX-2 were determined to be 571 and 414 µg/ml respectively. The six plant extracts, B. boehmii , C. molle , C. platypetalum , C. zeyheri , G. senegalensis and P. curatellifolia that were active against COX isoforms were evaluated for membrane stabilisation, albumin denaturation inhibition and free radical scavenging activity. C. platypetalum , C. molle and B. boehmii were able to stabilize the erythrocyte membrane and inhibit the precipitation of bovine serum albumin in solution. P. curatellifolia extract showed potent antioxidant activity using both assays with the maximal free radical scavenging at 16 and 18 µg/ml respectively. C. zeyheri and Gymnosporia senegalensis extracts also showed antioxidant activity with values of 21 and 32 µg/ml respectively. P. curatellifolia and C. platypetalum extracts were further evaluated for their effect on NO production in RAW 264.7 cells. C. platypetalum ethanol extract and P. curatellifolia water extract inhibited NO production in RAW 264.7 murine macrophage cells. In conclusion, Zimbabwean plant species evaluated in the study showed anti-inflammatory activity and could be potential sources of novel and potent anti-inflammatory agents.