Sterol 3 3 fatty acyl esters

Figure 10.1 Examples of structural formulas of phytosterol and sterol conjugates. FS = free sterol alcohol, SE = sterol fatty acid ester, SF = steryl ferulate, SG = steryl glycoside, ASG = acylated steryl glycoside.

Phytosterols (PS) are plant sterols or stands found in plants. Plant sterols belong to the triterpene family and differ from cholesterol by having a methyl or ethyl group in C24. Plant stanols, on the other hand, are the saturated form of the plant sterols (Fig. 113.1). PS are present in free or conjugated form as fatty-acyl esters, hydroxycinnamate steryl esters, steryl glycosides, or acylated steryl glycosides. The main function of plant sterols/stanols is to stabilize plant membranes and serve as precursors in the synthesis of steroidal saponins, alkaloids, and other steroids [1]. 

A carboxylesterase (EC 3.1.1.1) from T. fusca [8, 86] and a steryl esterase (EC 3.1.1.13) from Melanocarpus albomyces [59] have also shown activity with PET oligomers and fabrics. The enzyme from M. albomyces with high specificity for fatty acid esters of sterols increased the hydrophilicity of PET fabrics. The highly hydrophobic serine hydrolase from T. fusca with a catalytic triad composed of serine, glutamic acid, and histidine hydrolyzed CTR and PET nanoparticles. The esterase showed high specificity towards short and middle chain-length fatty acyl esters of p-nitrophenol. In addition, p-nitrobenzyl esterases from Bacillus subtilis and B. licheniformis that hydrolyzed short chain dialkylphthalates and PET nanoparticles have been reported [74, 87]. 

FIGURE 18-6 Chemical structures of major sterols and cholesterol derivatives. The major sterols in animals (cholesterol), fungi (ergosterol), and plants (stigmasterol) differ slightly in structure, but all serve as key components of cellular membranes. Cholesterol is stored as cholesteryl esters in which a fatty acyl chain (R = hydrocarbon portion of fatty acid) is esterified to the hydroxyl group. Excess cholesterol is converted by liver cells into bile acids (e.g., deoxycholic acid), which are secreted into the bile. Specialized endocrine cells synthesize steroid hormones (e.g., testosterone) from cholesterol, and photochemical and enzymatic reactions in the skin and kidneys produce vitamin D. 

Katayama, M. M. Katoh. Soybean sterols during maturation of seeds i. Accumulation of sterols in the free form, fatty acid esters, acylated glucosides and non-acylated glucosides. Plant Cell Physiol. 1973,14, 681-688. 

Sterol esters and acylated sterol glycosides have been detected in a number of plant tissues and, in some cases, can be quite significant components (Mudd, 1980). For sterol esters any one of palmi-tate, oleate, linoleate or a-linolenate could be the principal fatty acyl component depending on the tissue. In acylated sterol glycosides, palmitate or linoleate are the most abundant fatty acids. 

Figure 1.4 The core structure and representatives of sterol lipids, (a) The core structure of the majority of the sterol species or from which the sterol species are resulted, (b) The representative structures of cholesterol (R = H) and cholesteryl esters (R = a fatty acyl), (c) A representative structure of the steroid subgroup species (i.e., estrogen), (d) A representative structure of the secosteroid subgroup (i.e., vitamin Dj). (e) A representative structure of the species among the bile acid subgroup of sterol (i.e., cholic acid).

Sterols are a class of lipids containing a common steroid core of a fused four-ring structure with a hydrocarbon side chain and an alcohol group. Cholesterol is the primary sterol lipid in mammals and is an important constituent of cellular membranes. Oxidization and/or metabolism of cholesterol yield numerous oxysterols, steroids, bile acids, etc., many of which are important signaling molecules in biological systems. Cholesteryl esters esterified with a variety of fatty acyls are enriched in... 

The sterols present in cocoa butter make up about 0.3 to 0.4% of the oil and, as in other oils, they exist as free sterols, esters with fatty acids, and as glucosides and acylated sterol glucosides. Sterol determinations are normally carried out on the saponified material after isolation of the sterol fraction by TLC, although recovery from the TLC plate is lower than can be achieved by the use of HPLC, which also gives improved separation of the desmethylsterols and triterpene alcohols. Gas chromatography can be performed on the non-derivatized sterols provided the column is in good condition but acetylation (which can precede TLC) or silylation gives more consistently reliable resolution and peak shape. 

A base, such as sodium methoxide (NaOCH3), is useful in esterifying lipids as found in acylglycerols, sterol esters, and phospholipids (138). Free fatty acid and N-acyl lipids in sphingolipids will not be methylated. The NaOCH3 method does not apparently change the cis-, trans-isomer composition or form methoxy artifacts. However, some artifacts that could interfere with shorter chain fatty acids were observed. 

Important elements of sterol metabolism can also be used to elucidate where in the cell a particular precursor has moved [7]. The arrival of cholesteryl esters within lysosomes is revealed by cleavage of the fatty acid to yield free cholesterol. The subsequent transport of cholesterol to the ER can be monitored by the action of acyl-CoA cholesterol acyltrans-ferase (Chapter 14) that results in the formation of new molecular species of cholesteryl esters. In addition, sphingomyelinase treatment of the cell surface induces cholesterol movement from the plasma membrane to the ER where its arrival can likewise be monitored by the action of acyl-CoA cholesterol acyltransferase. Import of cholesterol into mitochondria (usually restricted to steroidogenic cells) can be followed by side-chain cleavage reactions that produce pregnenolone [8]. Movement of pregnenolone out of mitochondria can be followed by oxidations at positions 3, 17, and 21, which occur in the ER. 

The predominant n-alkanes of insects are odd chain lengths in the range C21-C33. Branched alkanes and alkenes are also usually odd-numbered. In primary alcohol wax esters, even-chain fatty adds and alcohols usually predominate but secondary alcohol wax esters are unusual components. Triacylglycerols, when significant, usually have Cie and Cis saturated and unsaturated acyl groups. Sterols are often found in small amounts with cholesterol the most common... 

Abbreviations SE = sterol ester TG = triacylglycerol DG = diacylglycerol FFA = non-esterified fatty acid DGDG = diacyldigalactosylglycerol DGMG = monoacyldigalactosylglycerol A -acyl PE = W-acylphosphatidylethanolamine PE = phosphatidylethanolamine PC = phosphatidylcholine PI = phosphatidylinositol. 

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