Leishmania donovani: cellular control of ornithine decarboxylase in promastigotes

Abstract

Ornithine decarboxylase, a key enzyme in polyamine biosynthesis, is essential for normal cell growth and proliferation. Furthermore, the inhibition of this enzyme is a potential way of controlling such growth. In order to shed light on the role of ornithine decarboxylase in regulation of Leishmania growth we examined the activity of this enzyme during the life cycle of these organisms. Exponentially growing Leishmania promastigotes were resuspended at a density of 3 × 10⁶ cells/ml. 2 × 10⁷ cells were withdrawn 24 hr later at different time intervals for induction studies and ornithine decarboxylase activity was measured. Ornithine decarboxylase showed a growth related pattern in L. donovani promastigotes. Induction studies showed that ornithine decarboxylase activity rapidly increased in late log phase cells when resuspended in fresh medium. A biphasic induction curve was observed similar to that observed in mammalian cells. The first peak was observed at 6 hr and the second at 16 hr. Cycloheximide and Actinomycin D inhibited induction at 16 hr by 65-68%. Polyamines at a level not inhibitory to growth (10 µM) inhibited ornithine decarboxylase induction by 30-40% late in the induction period. Putrescine and spermidine both inhibited the first peak of induction. Putrescine suppressed ornithine decarboxylase activity by 39% at 16 hr whereas spermidine by only 29%. The half life of ornithine decarboxylase in promastigote forms grown in the presence of cycloheximide was > 6 hr. These studies indicate that although the Leishmanial ornithine decarboxylase follows a similar induction pattern to that previously reported in the mammalian cells, it is less susceptible to exogenous polyamines and is comparatively stable. This lack of ornithine decarboxylase regulation and turnover may be exploitable in the development of various therapeutic agents to inhibit Leishmanial growth.