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Eukaryotic Cell, August 2005, p. 1483-1492, Vol. 4, No. 8
1535-9778/05/$08.00+0     doi:10.1128/EC.4.8.1483-1492.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Ubiquinone Synthesis in Mitochondrial and Microsomal Subcellular Fractions of Pneumocystis spp.: Differential Sensitivities to Atovaquone

Mireille Basselin, Shannon M. Hunt, Hiam Abdala-Valencia, and Edna S. Kaneshiro^*

Department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221

Received 12 February 2005/ Accepted 6 June 2005

The lung pathogen Pneumocystis spp. is the causative agent of a type of pneumonia that can be fatal in people with defective immune systems, such as AIDS patients. Atovaquone, an analog of ubiquinone (coenzyme Q [CoQ]), inhibits mitochondrial electron transport and is effective in clearing mild to moderate cases of the infection. Purified rat-derived intact Pneumocystis carinii cells synthesize de novo four CoQ homologs, CoQ₇, CoQ₈, CoQ₉, and CoQ₁₀, as demonstrated by the incorporation of radiolabeled precursors of both the benzoquinone ring and the polyprenyl chain. A central step in CoQ biosynthesis is the condensation of p-hydroxybenzoic acid (PHBA) with a long-chain polyprenyl diphosphate molecule. In the present study, CoQ biosynthesis was evaluated by the incorporation of PHBA into completed CoQ molecules using P. carinii cell-free preparations. CoQ synthesis in whole-cell homogenates was not affected by the respiratory inhibitors antimycin A and dicyclohexylcarbodiimide but was diminished by atovaquone. Thus, atovaquone has inhibitory activity on both electron transport and CoQ synthesis in this pathogen. Furthermore, both the mitochondrial and microsomal fractions were shown to synthesize de novo all four P. carinii CoQ homologs. Interestingly, atovaquone inhibited microsomal CoQ synthesis, whereas it had no effect on mitochondrial CoQ synthesis. This is the first pathogenic eukaryotic microorganism in which biosynthesis of CoQ molecules from the initial PHBA:polyprenyl transferase reaction has been unambiguously shown to occur in two distinct compartments of the same cell.

Present address: Brain Physiology and Metabolism Section, National Institute on Aging, National Institutes of Health, Bethesda, Md.