Employing thymidylate synthase (TS) (5, 10-CH2-H4PteGlu: dUMP C-methyltransferase, EC 2.1.1.45), a key target enzyme in chemotherapy, the biological activity of the beta-carboline-benzoquinolizidine alkaloid deoxytubulosine (DTB) isolated from the Indian medicinal plant Alangium lamarckii has been evaluated and assessed for the first time. The TS employed in the present studies was purified from Lactobacillus leichmannii. The DTB was demonstrated to exhibit potent cytotoxicity and inhibited the cell growth of L. leichmannii, and DTB potently inhibited TS activity (IC50 = 40 microM). The DTB concentrations>> 80 microM resulted in a total loss of the TS activity, thus suggesting that the beta-carboline-benzoquinolizidine alkaloid is a promising potential antitumor agent. The DTB binding to TS appears to be irreversible and tight through a possible covalent linkage. Although DTB strongly binds to DNA, it is not known whether DTB binds to RNA associated with TS. Inhibition kinetics showed that TS has a Ki value of 7 x 10(-6) M for DTB and that the inhibition is a simple linear "noncompetitive" type.