Tithonia diversifolia, Cyperus rotundus and Hyptis suaveloensis ethanol extracts combinatorially and competitively inhibit affinity purified cowpea storage bruchid (Callosobrochus maculatus) glutathione S-transferase

Abstract

Abstracts: Ethanol extracts of Tithonia diversifolia, Cyperus rotundus and Hyptis suaveloensis have earlier been reported to have insecticidal activity against storage bruchid (Callosbrochus maculatus) reared on cowpea (Vigna unguiculata). Here, antioxidant capacities of the ethanol extracts of these plants were evaluated by determining their effects on DPPH radical scavenging, lipid peroxidation and their total phenolic contents. These extracts were further investigated for their influence on the activity of uncharacterized but purified C. maculatus glutathione S-transferases. The antioxidant properties, as a factor of total phenols and reducing power, are 0.026, 0.043; 0.21, 0.040; 0.15, 0.034 mg/ml for T. diversifolia, C. rotundus and H. suaveloensis, respectively. Using 1-chloro-2,4-dintrobenzene (CDNB) as substrate, the GST activity was inhibited in a concentration depedent manner with 50% inhibition concentration (IC50) of 87.8, 95.4 and 115.8 μg/ml for T. diversifolia, C. rotundus and H. suaveloensis, respectively. When compared with standard GSTs inhibitors, the order of inhibition was cibracon blue > triphenyltinchloride > hematin > T. diversifolia > tributyltin acetate > C. rotundus > H. suaveloensis > ethacrynic acid. These extracts bind competitively to both the active site and the hydrophobic binding site of the enzyme as presented by the Dixon and Scatchard plots. Displacement of the fluorescent probe, l-anilino-8-naphthalenesulfonate, tryptophan quenching and spectral changes induced by the plant extracts binding demonstrated a common high affinity site for which the plant secondary metabolites bind. The combinatorial competitive inhibitory approach of these plant polyphenols might contribute to the insecticidal activity against C. maculatus. © 2011 The Author(s).