PhD Thesis Citation

Pneumocystis carinii, a microorganism most likely of fungal origin, causes a severe pneumonia in immunocompromised individuals. Although eflornithine, a specific ornithine decarboxylase (ODC) inhibitor, has been successfully used as an anti-Pneumocystis agent, the activity of the enzyme target has been reported to be absent from the parasite. Pneumocystis carinii was re-examined and ODC was found to be present. The therapeutic effect of eflornithine thus results from a direct anti-Pneumocystis action and not from interference with host metabolism as it had been presupposed.

Rats immunosuppressed with dexamethasone and maintained under near-aseptic conditions were inoculated intratracheally with lung homogenates from previously infected rats. Pneumocystis carinii cells were separated from lungs according to a novel isolation procedure consisting of tissue homogenization, host-cell lysis, differential centrifugation and enzymatic digestion. The isolated parasite cells were lyzed and ODC activity measured by capturing and counting ¹⁴CO₂ released from carboxyl carbon-labeled ornithine. Significant and reproducible ODC activity was detectable only after removing low-molecular weight compounds. Unlabeled ornithine, found to be present in high concentrations in Pneumocystis carinii lysates, is most likely the interfering compound which blocked ODC detection by previous workers. The measured activity was linear with respect to protein concentration and was inhibitable by eflornithine.

The possibility that the measured ODC activity was of host origin was considered and rejected for the following four reasons. Mammalian ODC is a cytosolic enzyme and the isolated Pneumocystis carinii contained neither host cells nor intact organelles as judged by electron microscopy. The activity in homogenates of Pneumocystis carinii-infected lungs correlated with the degree of Pneumocystis carinii enrichment. Although pH optima curves for both Pneumocystis carinii and host ODC peaked at 7.5, the two curves were clearly distinct. The eflornithine IC₅₀ values for isolated Pneumocystis carinii of 3.5 μM was 2.7 times that of host ODC.

This demonstration of polyamine biosynthesis, which after folate biosynthesis is only the second biochemical pathway clearly characterized in Pneumocystis carinii, identifies a drug target for Pneumocystis carinii pneumonia.

Muhamed Saric, MD, PhD, FACC, FASE New York University Director, Echocardiography Lab Associate Professor of Medicine

Muhamed Saric, MD, PhD, FACC, FASE
New York University
Director, Echocardiography Lab
Associate Professor of Medicine