POLYAMINE CONTENT OF PNEUMOCYSTIS-CARINII AND RESPONSE TO THE ORNITHINE DECARBOXYLASE INHIBITOR DL-ALPHA-DIFLUOROMETHYLORNITHINE

Difluoromethylornithine (DFMO; eflornithine hydrochloride [Ornidyl]), a suicide inhibitor of the key polyamine biosynthesis enzyme ornithine decarboxylase (ODC), is effective in treating Pneumocystis carinii pn eumonia, a common opportunistic infection associated with AIDS, Despit e DFMO's specificity for ODC, the reason for its selective toxicity ag ainst P. carinii is unknown since both host and parasite are dependent on the same enzyme for polyamine biosynthesis, A new high-performance liquid chromatography method was used with P. carinii cells isolated from infected rat lungs to measure polyamine content, to confirm the p resence of ODC, and to examine the effect of DFMO on polyamine concent rations, Putrescine, spermidine, and spermine were found to be present at 2.00 +/- 0.54, 1.26 +/- 0.51, and 1.59 +/- 0.91 nmol (mg of protei n)(-1), respectively, neither unusually high nor low values, ODC's spe cific activity was 79 +/- 11 pmol (mg of protein)(-1) h(-1), again not a remarkable value, However, the rates of both DFMO-induced polyamine depletion and subsequent repletion upon DFMO removal were unusually h igh, A 3-h exposure to 1 mM DFMO in vitro caused the depletion of putr escine, spermidine, and spermine to levels 12, 29, and 16%, respective ly, of that of control cells, After DFMO removal and incubation for 1 h in serum-free media, polyamine levels returned to 78, 88, and 64%, r espectively, of that of the control cells not exposed to DFMO, Since s uch depletions and repletions usually occur over periods of days rathe r than hours, these rapid changes may provide a clue to the selective action of DFMO against P. carinii and may guide the development of new compounds and an optimal drug administration schedule for DFMO.