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|Molar mass||414.71 g mol−1|
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
β-Sitosterol is one of several phytosterols (plant sterols) with chemical structures similar to that of cholesterol. Sitosterols are white, waxy powders with a characteristic odor. They are hydrophobic and soluble in alcohols.
It is widely distributed in the plant kingdom and found in Nigella sativa, pecans, Serenoa repens (saw palmetto), avocados, Cucurbita pepo (pumpkin seed), Pygeum africanum, cashew fruit, rice bran, wheat germ, corn oils, soybeans, sea-buckthorn, wolfberries, Cannabis sativaand Wrightia tinctoria. It is also found in dandelion coffee.
Alone and in combination with similar phytosterols, β-sitosterol reduces blood levels of cholesterol, and is sometimes used in treating hypercholesterolemia.[medical citation needed] β-Sitosterol inhibits cholesterol absorption in the intestine. When the sterol is absorbed in the intestine, it is transported by lipoproteins and incorporated into the cellular membrane. Phytosterols and phytostanols both inhibit the uptake of dietary and biliary cholesterol, and reduce the levels of LDL and total serum cholesterol. Because the structure of β-sitosterol is similar to that of cholesterol, β-sitosterol takes the place of dietary and biliary cholesterol in micelles produced in the intestinal lumen. This reduces cholesterol absorption in the body.
β-Sitosterol should be avoided during pregnancy and breast-feeding, since not enough is known about its effects on unborn and newborn children.
β-Sitosterol is also not recommended for individuals with sitosterolemia, a rare inherited fat storage disease. Because people with this condition have too much β-sitosterol and related fats in their system, taking β-sitosterol will only worsen this condition.
High levels of β-sitosterol concentrations in blood have been correlated with increased severity of heart disease in men having previously suffered from heart attacks.
While plant sterols are usually beneficial, there is a rare autosomal recessive genetic disorder phytosterolemia which causes over-absorption of phytosterols. Phytosterols accumulate and are over-absorbed in tissue, which causes premature coronary artery disease and tendon xanthoma.
Precursor of anabolic steroid boldenone
Being a steroid, β-sitosterol is precursor of anabolic steroid boldenone. Boldenone undecylenate is commonly used in veterinary medicine to induce growth in cattle but it is also one of the most commonly abused anabolic steroids in sports. This led to suspicion that some athletes testing positive on boldenone undecylenate didn't actually abuse the hormone itself but consumed food rich in β-sitosterol.
The use of sitosterol as a chemical intermediate was for many years limited due to the lack of a chemical point of attack on the side-chain that would permit its removal. Extensive efforts on the part of many laboratories eventually led to the discovery of a pseudomonas microbe that efficiently affected that transformation. Fermentation digests the entire aliphatic side-chain at carbon 17 to afford a mixture of 17-keto products including dehydroepiandrosterone.
Total synthesis of β-sitosterol has not been achieved. However, β-sitosterol has been synthesized from stigmasterol 1, which involves a specific hydrogenation of the side-chain of stigmasterol (See Figure Below). The first step in the synthesis forms stigmasterol tosylate 2 from stigmasterol 1 (95% purity) using p-TsCl, DMAP, and pyridine (90% yield). The tosylate 2 then undergoes solvolysis as it is treated with pyridine and anhydrous MeOH to give a 5:1 ratio of i-stigmasterol methyl ether 3 (74% yield) to stigmasterol methyl ether 4, which is subsequently removed by chromatography. The hydrogenation step of a previously proposed synthesis involved the catalyst Pd/C and the solvent ethyl acetate. However, due to isomerisation during hydrolysis, other catalysts, such as PtO2, and solvents, such as ethanol, were tested. There was little change with the use of a different catalyst. Ethanol, however, prevented isomerisation and the formation of the unidentified impurity to give compound 5. The last step of the synthesis is deprotection of the β-ring double bond of 5 with p-TsOH, aqueous dioxane, and heat (80 °C) to yield β-sitosterol 6. The cumulative yield for the final two steps was 55%, and the total yield for the synthesis was 37%.
The regulation of the biosynthesis of both sterols and some specific lipids occurs during membrane biogenesis. Through 13C-labeling patterns, it has been determined that both the mevalonate and deoxyxylulose pathways are involved in the formation of β-sitosterol. The precise mechanism of β-sitosterol formation varies according to the organism, but is generally found to come from cycloartenol.
The biosynthesis of cycloartenol begins as one molecule of isopentenyl diphosphate (IPP) and two molecules of dimethylallyl diphosphate (DMAPP) form farnesyl diphosphate (FPP). Two molecules of FPP are then joined tail-to-tail to yield squalene, a triterpene. Squalene, through a cyclization reaction with 2,3-oxidosqualene 6 as an intermediate forms cycloartenol.
The biosynthesis of β-sitosterol from cycloartenol is summarized below.
The double bond of cycloartenol (compound 7 in diagram) is methylated by SAM to give a carbocation that undergoes a hydride shift and loses a proton to yield a compound with a methylene side-chain. Both of these steps are catalyzed by sterol C-24 methyltransferase (Step E1 in diagram). Compound 8 is then catalyzed by sterol C-4 demethylase (E2) and loses a methyl group to produce cycloeucalenol. Subsequent to this, the cyclopropane ring is opened with cycloeucalenol cycloisomerase (E3) to form 10. Compound 10 loses a methyl group and undergoes an allylic isomerization to form Gramisterol 11. This step is catalyzed by sterol C-14 demethylase (E4), sterol Δ14-reductase (E5), and sterol Δ8-Δ7-isomerase (E6). The last methyl group is removed by sterol demethylase (E7) to form episterol 12. Episterol 12 is methylated by SAM to produce a second carbocation, which loses a proton to yield 13. This step is catalyzed by 24-methylenesterol C-methyltransferase (E8). Compound 13 now undergoes reduction by NADPH and modifications in the β-ring to form β-sitosterol.
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