PubMed · 7194986
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P Paoletti. 1981. Desmosterol in CSF.. https://doi.org/10.1212/wnl.31.5.639
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Campesterol (3) and dihydrobrassicasterol (4), typical C28-sterols in higher plants, are biosynthesized from a steroidal 24-ene precursor (desmosterol 1) via 24-methylenecholesterol (2) and 24-methyldesmosterol (5). A typical plant C29-sterol, sitosterol (6), is produced from 24-methylenecholesterol via isofucosterol (7) and 24-ethyldesmosterol (8). The biosynthetic mechanism, focussing stereochemical features, of these side-chain transformations has been studied in detail by feeding regio- and stereoselectively 13C- or 2H-labeled steroidal substrates to cell cultures of higher plants such as Oryza sativa, Catharanthus roseus and Morus alba. These studies allowed to correlate the metabolic origin of C-26 and C-27 of the intermediate sterols. It has been established that the 1st methylation leading to 24-methylenecholesterol from desmosterol involves a Re-face hydrogen migration from C-24 to C-25 based on unambiguous assignment of the isopropyl pro-R-Me and pro-S-Me of 24-methylenecholesterol. The 2nd methylation leading to isofucosterol was revealed to proceed in a trans-mechanism in which addition of the methyl group and elimination of the C-28 hydrogen occur on opposite faces of the original delta 24(28) plane. The double bond isomerization from delta 24(28) to delta 24(25) was found to proceed in a syn-SE2' mechanism with the pro-S-methyl group of isofucosterol becoming the (E)-methyl of 24-ethyldesmosterol. Finally, feeding studies of [E-Me-13C]- and [Z-Me-13C]-24-methyldesmosterols established that an anti-mode of hydrogen addition is operating in the conversion of 24-methyldesmosterol to campesterol and dihydrobrassicasterol. Similar studies established that 24-ethyldesmosterol is converted to sitosterol in an anti-mode of hydrogen addition. In addition, the mechanism of sterol side-chain formation in hairy roots of Ajuga reptans var. atropurpurea is briefly described.
Mycoplasma gallisepticum was adapted to grow with delta 5-sterols modified in the aliphatic side chain, and stopped-flow kinetic measurements of filipin association were made to estimate the sterol distribution between the two leaflets of the membrane. Cholesterol derivatives with unsaturated side chains (desmosterol, cis- and trans-22-dehydrocholesterol, and cholesta-5,22E,24-trien-3 beta-ol) or an alkyl substituent (beta-sitosterol) were predominantly (86-94%) localized in the outer leaflet of the bilayer. However, cholesterol, 20-isocholesterol, and sterols with side chains of varying lengths (in the 20(R)-n-alkylpregn-5-en-3 beta-ol series where the alkyl group ranged from ethyl to undecyl) were distributed nearly symmetrically between the two halves of the bilayer. Kinetic measurements of beta-[14C]sitosterol and [14C]desmosterol exchange between M. gallisepticum cells and an excess of sonicated sterol/phosphatidylcholine vesicles confirmed the filipin-binding studies. More than 90% of these radiolabeled sterols underwent exchange at 37 degrees C with unlabeled sterols in vesicles over a period of 12-14 h in the presence of 2% (w/v) albumin. beta-[14C]Sitosterol exchange was characterized by biphasic exchange kinetics, indicative of two pools of sitosterol molecules in the cell membrane. Only a single kinetic pool was detected for [14C]desmosterol exchange. Stopped flow measurements of filipin binding to beta-sitosterol and stigmasterol also revealed an asymmetrical localization of these sterols in membranes of growing Mycoplasma. capricolum cells. When an early exponential culture of beta-sitosterol- or stigmasterol-adapted M. capricolum was transferred to a sterol-rich medium at 37 degrees C, approximately three-quarters of the beta-sitosterol or stigmasterol was localized in the outer leaflet after growth was continued for 6 h; in contrast, cholesterol was distributed symmetrically after about 1 h. The asymmetric localization of sterols with alkylated or unsaturated side chains suggests that growth-supporting sterols need not be translocated extensively into the inner leaflet of the bilayers of M. gallisepticum and M. capricolum.
Milk samples were collected from mothers at 2, 6, 12 and 16 weeks postpartum. Desmosterol was found to be present in all the milk samples. Identification of desmosterol was based on retention times with two gas liquid chromatography (GLC) columns and verified by GC-mass spectrometry. The concentration of desmosterol in breast milk increased significantly (P less than .05) from 0.6 mg/100 ml at 2 weeks to 1.3 mg/100 ml at 16 weeks postpartum. Desmosterol was not significantly correlated with total lipid, total cholesterol or free cholesterol in the milk.