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

Abstract

Pneumocysterol [(24Z)-ethylidenelanost-8-en-3␤-ol], a rare sterol detected in the opportunistic pathogenCommunicated by William Trager, The Rockefeller University, New York, NY, November 2, 1998 (received for review February 4, 1998 ABSTRACTPneumocystis carinii pneumonia (PcP) remains among the most prevalent opportunistic infections among AIDS patients. Currently, drugs used clinically for deep mycosis act by binding ergosterol or disrupting its biosynthesis. Although classified as a fungus, P. carinii lacks ergosterol. Instead, the pathogen synthesizes a number of distinct ⌬ 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 and proliferation. In the present study, high concentrations of a C 32 sterol were found in humanderived P. carinii hominis. The definitive structural identities of two C-24 alkylated lanosterol compounds, previously not reported for rat-derived P. carinii carinii, were determined by using GLC, MS, and NMR spectroscopy together with the chemical syntheses of authentic standards. The C 31 and C 32 sterols were identified as euphorbol (24-methylenelanost-8-en-3␤-ol) and pneumocysterol [(24Z)-ethylidenelanost-8-en-3␤-ol], respectively. The identification of these and other 24-alkylsterols in P. carinii hominis suggests that (i) sterol C-24 methyltransferase activities are extraordinarily high in this organism, (ii) 24-alkylsterols are important components of the pathogen's membranes, because the addition of these side groups onto the sterol side chain requires substantial ATP equivalents, and (iii) the inefficacy of azole drugs against P. carinii can be explained by the ability of this organism to form 24-alkysterols before demethylation of the lanosterol nucleus. Because mammals cannot form 24-alkylsterols, their biosyntheses in P. carinii are attractive targets for the development of chemotherapeutic strategies against this opportunistic infection.Sterols and their biosyntheses are excellent targets for chemotherapeutic attack against infectious microbes, especially the fungi. Polyene antibiotics such as amphotericin B bind avidly to ergosterol in fungal cell membranes. After the sterol-drug complexes aggregate, large pores in the membranes are formed, dissipating ion gradients. Fluconazole and some other compounds routinely used clinically for systemic mycosis target ergosterol biosynthesis at nuclear demethylation steps. Ergosterol was not detected in Pneumocystis carinii carinii that was isolated and purified from the lungs of corticosteroid-immunosuppressed rats. In this respect, the pathogen appears to be unlike higher fungi. However, the organism synthesizes its own distinct sterols, e.g., fungisterol (24-methylcholest-7-en-3␤-ol and 24-ethylcholest-7-en-3␤-ol; refs. 1-4). Parasites generally scavenge sterols (e.g., cholesterol) from the host and utilize them for membrane formation and other cell functions. If host sterols do not fulfill the precise stereochemi...

Comprehensive and definitive structural identities of Pneumocystis carinii sterols

“…These two compounds were identified in the present study of rat-derived P. carinii sterols. We here correct the trivial name (eburicol) for 24-methylenelanost-8-en-3␤-ol, which was incorrectly referred to as euphorbol in a previous report on P. jiroveci lanosterol derivatives (20). In the present study the definitive structure of 24-methylenelanosterol in P. carinii was confirmed.…”

“…This was later given the trivial name pneumocysterol when it was found to comprise a major component of some humanderived P. jiroveci (P. carinii f. sp. hominis) isolated organism populations (20). Pneumocysterol and 24-methylenelanosterol have now been definitively identified in ratderived P. carinii (P. carinii f. sp.…”

“…2C) and 24-methylenelanost-8-en-3␤-ol (24-methylenelanosterol; Fig. 2B) were previously identified and characterized in human-derived P. jiroveci (15,20,27,28). These two compounds were identified in the present study of rat-derived P. carinii sterols.…”

An Improved Synthesis of 3b-Acetoxy-lanost-8-en-24-one (24-Ketolanosteryl Acetate)

“…The C-24 of lanosterol is a major site of sterol metabolism in plants, fungi, and animals [1,2,3]. As a result of our continuing studies on sterol biosynthesis, we have devised a simplified chemical synthesis of 3β-acetoxy-lanost-8-en-24-one (2, 24-ketolanosteryl acetate), a key intermediate in the synthesis of C-24 alkylated metabolites and potential regulators of sterol biosynthesis.…”

Biochemical research elucidating metabolic pathways inPneumocystis

“…Gas chromatography data are available for characterization of P. jirovecii with low C 31 and C 32 sterols (Kaneshiro et al, 1999a) but definitive structural assignments by mass spectrometry and nuclear magnetic resonance spectroscopy have yet to be reported. in P. jirovecii populations with high C 31 and C 32 sterols, the lanosterol derivative pneumocysterol (Kaneshiro et al, 1999b) (24Z)-ethylidenelanost-8-en-3b-ol can accumulate often comprising about half the Pneumocystisspecific sterols. Definitive structural analyses of sterols in a population with high C 31 and C 32 sterols indicated that those P. jirovecii organisms lacked 14DM activity (14DM -) (Giner et al, 2004).…”