Inhibition of thymidylate synthase by pergularinine, tylophorinidine and deoxytubulosine

The activity of thymidylate synthase (TS) purified in our laboratory from L actobacillus leichmannii was inhibited by pergularinine (PGL) and tylophori nidine (TPD) and deoxytubulosine (DTB) isolated from the Indian medicinal p lants Pergularia pallida and Alangium lamarckii respectively. Cytotoxicity studies showed that cell growth of L. leichmannii was inhibited (IC50 = 40- 45 mu M) by all the three alkaloids, the concentrations > 80-90 mu M result ing in complete loss of the enzyme activity. K-i values of the enzyme calcu lated from Lineweaver-Burk and Dixon plots for PGL, TPD and DTB were 10x10( -6) M, 9x10(-6) M and 7x10(-6) M respectively. These are typed as 'non-comp etitive' inhibitors of TS. All the three alkaloids inhibited (IC50 = 50 mu M) the elevated TS activity of leukocytes in cancer patients with clinicall y diagnosed chronic myelocytic leukemia (n=10), acute lymphocytic leukemia (n=8) and metastatic solid tumours (n=3).