Pneumocysterol [(24Z)-ethylidenelanost-8-en-3 beta-ol], a rare sterol detected in the opportunistic pathogen Pneumocystis carinii hominis: Structuralidentity and chemical synthesis

Pneumocystis carinii pneumonia (PcP) remains among the most prevalent oppor tunistic infections among AIDS patients. Currently, drugs used clinically f or deep mycosis act by binding ergosterol or disrupting its biosynthesis. A lthough classified as a fungus, P. carinii lacks ergosterol. Instead, the p athogen synthesizes a number of distinct Delta(7), 24-alkylsterols, despite the abundance of cholesterol, which it can scavenge from the lung alveolus . Thus, the pathogen-specific sterols appear vital for organism survival an d proliferation. In the present study, high concentrations of a C-32 sterol were found in human-derived P. carinii hominis. The definitive structural identities of two C-24 alkylated lanosterol compounds, previously not repor ted for rat-derived P. carinii carinii, were determined by using GLC, MS, a nd NMR spectroscopy together with the chemical syntheses of authentic stand ards. The C-31 and C-32 sterols were identified as euphorbol (24-methylenel anost-8-en3 beta-ol) and pneumocysterol [(24Z)-ethylidenelanost-8-en-3 beta -ol], respectively. The identification of these and other 24-alkylsterols i n P. carinii hominis suggests that (i) sterol C-24 methyltransferase activi ties are extraordinarily high in this organism, (ii) 24-alkylsterols are im portant components of the pathogen's membranes, because the addition of the se side groups onto the sterol side chain requires substantial ATP equivale nts, and (iii) the inefficacy of azole drugs against P. carinii can be expl ained by the ability of this organism to form 24-alkysterols before demethy lation of the lanosterol nucleus. Because mammals cannot form 24-alkylstero ls, their biosyntheses in P. carinii are attractive targets for the develop ment of chemotherapeutic strategies against this opportunistic infection.