Inhibition of thymidylate synthase and cell growth by the phenanthroindolizidine alkaloids pergularinine and tylophorinidine

Abstract

Biological activity of the phenanthroindolizidine alkaloids pergularinine (PGL) and tylophorinidine (TPD) isolated from the Indian medicinal herb Pergularia pallida has been evaluated and assessed for the first time employing thymidylate synthase (TS) (5,10-CH 2H 4 PteGlu: dUMP-C-methyltransferase, EC 2.1.1.45), a key target enzyme in cancer chemotherapy. TS used in the present investigations was purified from Lactobacillus leichmannii. Toxicity studies showed that PGL and TPD were potently toxic and inhibited growth of L.leichmannii cells. Both PGL and TPD significantly inhibited TS activity ( IC 50 = 40 and 45 μM, respectively). PGL concentrations > 80 μM and TPD concentrations 90 μM resulted in a complete loss of the TS activity, thus suggesting that both these phenanthroindolizidine alkaloids are promising potential antitumor agents. Our results show that the alkaloid-binding to TS is irreversibly tight through a probable covalent linkage. Inhibition kinetics reveal that the enzyme has K i values of 10 × 10 −6 and 9 × 10 −6 M for PGL and TPD, respectively and that the inhibition in both the cases is a simple linear ‘noncompetitive’ type.