LEISHMANIA-DONOVANI - CELLULAR CONTROL OF ORNITHINE DECARBOXYLASE IN PROMASTIGOTES

Ornithine decarboxylase, a key enzyme in polyamine biosynthesis, is es sential for normal cell growth and proliferation. Furthermore, the inh ibition of this enzyme is a potential way of controlling such growth. In order to shed light on the role of ornithine decarboxylase in regul ation of Leishmania growth we examined the activity of this enzyme dur ing the life cycle of these organisms. Exponentially growing Leishmani a promastigotes were resuspended at a density of 3 x 10(6) cells/ml. 2 x 10(7) cells were withdrawn 24 hr later at different time intervals for induction studies and ornithine decarboxylase activity was measure d. Ornithine decarboxylase showed a growth related pattern in L. donov ani promastigotes. Induction studies showed that ornithine decarboxyla se activity rapidly increased in late log phase cells when resuspended in fresh medium. A biphasic induction curve was observed similar to t hat observed in mammalian cells. The first peak was observed at 6 hr a nd the second at 16 hr. Cycloheximide and Actinomycin D inhibited indu ction at 16 hr by 65-68%. Polyamines at a level not inhibitory to grow th (10 mu M) inhibited ornithine decarboxylase induction by 30-40% lat e in the induction period. Putrescine and spermidine both inhibited th e first peak of induction. Putrescine suppressed ornithine decarboxyla se activity by 39% at 16 hr whereas spermidine by only 29%. The half L ife of ornithine decarboxylase in promastigote forms grown in the pres ence of cycloheximide was > 6 hr. These studies indicate that although the Leishmanial ornithine decarboxylase follows a similar induction p attern 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 exploit able in the development of various therapeutic agents to inhibit Leish manial growth.