Pneumocystis carinii polyamine Catabolism

DL-alpha-Difluoromethylornithine (DFMO) causes polyamines of the AIDS-assoc iated opportunistic pathogen Pneumocystis carinii to diminish 15 times more rapidly than mammalian host cells. The proposed mechanism was that, unlike mammalian cells, P. carinii is unable to regulate polyamine catabolism whe n synthesis is blocked. To test this, the responses of the polyamine catabo lic enzymes spermidine/spermine acetyltransferase (SSAT) and polyamine oxid ase (PAO) were determined using a new high-performance liquid chromatograph y assay to measure the products of these enzymes. The specific activities i n untreated Pneumocystis carinii were 1.78 +/- 0.5 pmol min-1 mg protein-1 for SSAT, similar to mammalian cells, and 6.42 +/- 0.8 pmol min-1 mg protei n-1 for PAO, 19% of that of mammalian cells. DFMO treatment for 12 h caused reductions of only 11 and 4% in SSAT and PAO, respectively, despite polyam ine reductions of 94, 96, and 90% for putrescine, spermidine, and spermine, respectively. The P. carinii SSAT K-m value of 25 mu M spermidine is 20% o f that of mammalian cells, and the PAO K-m value of 14 nM N-1-acetylspermid ine is 0.01% of that of mammalian cells. Acetylated polyamines continue to be lost from P. carinii even when exposed to DFMO. Collectively, these resu lts support the hypothesis that P. carinii is unable to regulate polyamine catabolism.