Inhibition of the recombinant cattle tick Rhipicephalus (Boophilus) annulatus glutathione S-transferase.

Abstract Rhipicephalus ( Boophilus ) annulatus is a bloodsucking ectoparasite that causes severe production losses in the cattle industry. This study aims to evaluate the in vitro effects of tannic acid, hematin (GST inhibitors) and different plant extracts (rich in tannic acid) on the activity of the recombinant glutathione S-transferase enzyme of the Egyptian cattle tick R. annulatus (rRaGST), in order to confirm their ability to inhibit the parasitic essential detoxification enzyme glutathione S-transferase. Extraction with 70% ethanol of Hibiscus cannabinus (kenaf flowers), Punica granatum (red and white pomegranate peel), Musa acuminata (banana peel) (Musaceae), Medicago sativa (alfalfa seeds), Tamarindus indicus (seed) and Cuminum cyminum (cumin seed) were used to assess: (i) inhibitory capacities of rRaGST and (ii) their phenolic and flavonoid contents. Ethanol extraction of red pomegranate peel contained the highest content of phenolic compounds (29.95 mg gallic acid/g dry tissue) compared to the other studied plant extracts. The highest inhibition activities of rRaGST were obtained with kenaf and red pomegranate peel ( P. granatum ) extracts with IC 50 values of 0.123 and 0.136 mg dry tissue/ml, respectively. Tannic acid was the more effective inhibitor of rRaGST with an IC 50 value equal to 4.57 μM compared to delphinidine-HCl (IC 50  = 14.9 ± 3.1 μM). Gossypol had a weak inhibitory effect (IC 50  = 43.7 μM), and caffeic acid had almost no effect on tick GST activity. The IC 50 values qualify ethacrynic acid as a potent inhibitor of rRaGST activity (IC 50  = 0.034 μM). Cibacron blue and hematin showed a considerable inhibition effect on rRaGST activity, and their IC 50 values were 0.13 μM and 7.5 μM, respectively. The activity of rRaGST was highest for CDNB (30.2 μmol/min/mg protein). The enzyme had also a peroxidatic activity (the specific activity equals 26.5 μmol/min/mg protein). Both tannic acid and hematin inhibited rRaGST activity non-competitively with respect to GSH and competitively with respect to CDNB. While red pomegranate extracts inhibited rRaGST activity competitively with respect to GSH, uncompetitive inhibition was observed with respect to CDNB.