Evidence for multiple sterol methyl transferase pathways in Pneumocystis carinii

Wenxu Zhou, Thi Thuy Minh Nguyen, Margaret S. Collins, Melanie T. Cushion, W. David Nes

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


The sterol composition of Pneumocystis carinii, an opportunistic pathogen responsible for life-threatening pneumonia in immunocompromised patients, was determined. Our purpose was to identify pathway-specific enzymes to impair using sterol biosynthesis inhibitors. Prior to this study, cholesterol 15 (ca. 80% of total sterols), lanosterol 1, and several phytosterols common to plants (sitosterol 31, 24α-ethyl and campesterol, 24α-methyl 30) were demonstrated in the fungus. In this investigation, we isolated all the previous sterols and many new compounds from P. carinii by culturing the microorganism in steroid-immunosuppressed rats. Thirty-one sterols were identified from the fungus (total sterol = 100 fg/cell), and seven sterols were identified from rat chow. Unusual sterols in the fungus not present in the diet included, 24(28)-methylenelanosterol 2; 24(28)E-ethylidene lanosterol 3; 24(28)Z-ethylidene lanosterol 4; 24β-ethyllanosta-25(27)-dienol 5; 24β-ethylcholest-7-enol 6; 24β-ethylcholesterol 7; 24β-ethylcholesta5,25(27)-dienol 8; 24-methyllanosta-7-enol 9; 24-methyldesmosterol 10; 24(28)-methylenecholest-7-enol 11; 24βmethylcholest-7-enol 12; and 24β-methylcholesterol 13. The structural relationships of the 24-alkyl groups in the sterol side chain were demonstrated chromatographically relative to authentic specimens, by MS and high-resolution 1H NMR. The hypothetical order of these compounds poses multiple phytosterol pathways that diverge from a common intermediate to generate 24β-methyl sterols: route 1, 1 → 2 → 11 → 12 → 13; route 2, 1 → 2 → 9 → 10 → 13; or 24β-ethyl sterols: route 3, 1 → 2 → 4 → 6 → 7; route 4, 1 → 2 → 5 → 8 → 7. Formation of 3 is considered to form an interrupted sterol pathway. Taken together, operation of distinct sterol methyl transferase (SMT) pathways that generate 24β-alkyl sterols in P. carinii with no counterpart in human biochemistry suggests a close taxonomic affinity with fungi and provides a basis for mechanism-based inactivation of SMT enzyme to treat Pneumocystis pneumonia.

Original languageEnglish
Pages (from-to)1177-1186
Number of pages10
Issue number12
StatePublished - Dec 1 2003


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