Prospecting bioassays and enzyme inhibitory activities of Alysicarpus vaginalis, Biophytum reinwardtii, Mikania cordata, and Plumeria obtusa

Natural products have long been recognized as vital in medicine, agriculture, cosmetics, and food industry. A variety of structurally numerous new chemical compounds can be extracted from crude extracts of microorganisms, plants, or animals. Alysicarpus vaginalis (AV), Biophytum reinwardtii (BR), Mikania cordata (MC) and Plumeria obtusa (PO) are plants used in traditional medicine in Sri Lanka. This study was designed to determine the bioactivities of the whole plants of AV and BR, leaves of MC, and flowers of PO. The plant parts were collected from the central province of Sri Lanka, and processed by washing, air drying, and finally powdering. Extracts were obtained using n-hexane, ethyl acetate (EtOAc), and methanol (MeOH) and they were subjected to testing antifungal activity against Cladosporium cladosporioides (CC), cytotoxicity against brine shrimp, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging antioxidant activity, phytotoxic activity against lettuce seeds, α-amylase inhibitory activity, and lipase enzyme inhibitory activity. The EtOAc extract of MC (MCE) showed inhibitory properties against root and shoot elongation of lettuce seeds. EtOAc extract of PO (POE) and MeOH extract of PO (POM) showed considerable toxicity, and hexane extract of MC caused lethal effects in brine shrimp (LD₅₀: 1.54 mg L^-1). The POM displayed an inhibition zone around a spot of a separated compound from PO on a TLC plate against CC. The MeOH extract of B. reinwardtii (BRM) exhibited the highest antioxidant activity (IC₅₀: 43.7 mg L^-1) compared to ascorbic acid (IC₅₀: 2.21 mg L^-1). The MeOH extracts of BR, AV, and PO showed α-amylase enzyme inhibitory activity, where POM (IC50: 582 mg L^-1), BRM (IC₅₀: 743.4 mg/L^-1), and MeOH extract of A. vaginalis (AVM) (IC₅₀: 1015.9 mg L^-1) showed inhibitory activity. The EtOAc extract of A. vaginalis (AVE) (IC₅₀: 332 mg L^-1) exhibited lipase enzyme inhibition. Since positive results were obtained for almost all the assays, it is worthwhile directing future studies towards the isolation of the responsible bioactive pure compounds in the plant extract.