Sterol esterification

Batch No. Esterification (%) Stanol/Sterol Esterification Ratio Biocatalyst Recovery ... 

The second cholesterol esterifying activity in intestine, is ACAT (see[15]), a microsomal enzyme[67,68] which is not well characterized. Unlike CE ase the ACAT system requires high energy intermediates of the fatty acid, which suggests that overall absorption and esterification of sterols is an energy-dependent transport. This activity is inhibited by bile salts[68] and the lack of specificity for sterol esterification has recently been suggested. 

EFFECT OF INTRACELLULAR STEROL CONCENTRATION ON STEROL ESTERIFICATION IN YEAST... 

J. Zimowski and Z.A.Wojclechowskl, Acyl donors for sterol esterification by cell free preparations from Sinapls alba roots. Phytochemistry 20 1795 (1981)... 

J.V. Torres and F. Garola-Olmedo, Genetic control of sterol esterification in developing wheat endosperm. Biochim. Biophys. Acta 409 367 (1975)... 

Sterols are minor constituents of most fats. Those of animal origin contain cholesterol and traces of other sterols, whereas plants contain phytosterols, of which p-sitosterol is the most common. Sterols occur in the free form or, after esterification to fatty acids through the 3-OH group, as steryl esters. The presence of sterols in archaeological residues can be a useful indicator of a plant or animal origin or an indicator of both if cholesterol and phytosterols are detected in the same sample. That noted, cholesterol is a potential contaminant of all archaeological samples subjected to handling. 

This enzyme [EC 3.1.1.13] (also known as cholesterol esterase, sterol esterase, cholesterol ester synthase, and triterpenol esterase) catalyzes the hydrolysis of a steryl ester to produce a sterol and a fatty acid anion. This class represents a group of enzymes exhibiting broad specificity. They act on esters of sterols and long-chain fatty acids, and may also bring about the esterification of sterols. These enzymes are typically activated by bile salts. See also Esterases D. P. Hajjar (1994) Adv. Enzymol. 69, 45. 

The purity of lanolin and standard tests have been described in the European Pharmacopoeia (EP), in The United States Pharmacopoeia (USP), and according to other national standards.13,14 Lanolin is a semisolid with a melting point of approximately 40 6°C and has a molecular weight in the range of 790 to 880 Da. Lanolin is a complex and variable mixture of mainly esters, diesters, hydroxy esters (87.0-93.5%, w/w),7 8 15 lanolin alcohols (6.0-12.5%, w/w), lanolin acids (<0.5%, w/w), and lanolin hydrocarbons (<1.0%, w/w). The latter are also called paraffins and petrolatum by the EP and USP, respectively.13,14 16-18 Approximately 40% of the esters are a-hydroxy esters. Due to the extremely complex nature of lanolin, the true number of different esters present is unknown. Barnett calculated the theoretical number of monoester combinations from random combinations of 69 aliphatic lanolin alcohols, 6 sterols, and 138 saturated lanolin acids to total 10,350.8 This is most probably an underestimate of the total number of esters, as dibasic acids and dihydric alcohols also occur naturally in lanolin.19 Further combinations of cyclic mono- and di-esters may be formed by dehydration and from inter- and intra-esterification due to heating during the manufacturing process.7 8... 

The esterification of either of the sterols enhances its solubility in a hydrophilic environment. This modification makes the phytosterol a more active ingredient when used to fortify edible products that lower serum cholesterol. The cholesterol-lowering action of (3-sitosterol was first reported more than 40 years ago (Farquhar et al., 1956). The differences between the plant sterols and the cholesterol include the addition of an ethyl group to the aliphatic side-chain of both sterols. Saturation of the double bond in the second ring of (3-sitosterol yields sitostanol. The presence of the ethyl group on the side-chain increases the hydrophobicity of both phytosterols such that it is greater than the hydrophobicity of the base molecule, cholesterol. This increase in hydrophobicity is expected to decrease the absorption of... 

To conduct three reactions (hydrolysis of acylglycerols, esterification of sterols with FFAs, and methyl esterification of FFAs), two lipases from C. 

Hydrolysis of partial acylglycerols Esterification of sterols with FFAs... 

Figure 2.8. Process of purifying tocopherols and sterols from VODD. Reactions including methyl esterification of FFAs are surrounding with shaded box.

Hydrolysis of FAStEs with various lipases was conducted but their reactions reached steady state at low degree of hydrolysis (<50%), owing to the reversible reaction (Fig. 2.10A). Meanwhile, as noted earlier, lipases act more weakly on FAME than FFA. If FFAs, in the hydrolysis of FAStEs, are converted to FAMEs, the reverse reaction (esterification of sterols) will scarcely occur because FAMEs do not participate in esterification of sterols with FFAs. 

Figure 2.10. Lipase-catalyzed reaction on a mixture of FAStE and water in the presence or in the absence of MeOH. A, Hydrolysis of FA steryl ester and esterification of sterol with FFA. B, Lipase-catalyzed reactions in a mixture of FAStE, water and MeOH.

Unesterified sterols modulate the function of eukaryotic membranes. In human cells, sterol is esterified to a storage form by acyl-co-enz)me A (CoA) cholesterol acyltransferase (SGTase). In plants, free sterols are associated mainly with microsomal membranes, whereas the steryl esters are stored in lipid granules. The esterification process may, thus, allow regulation of the... 

Phytosterols are industrially isolated from the distillates, resulting from the deodorization of vegetable oils (1, 34—36). Phytosterols are sometimes hydrogenated to produce phytostanols (37). As the solubility of sterols and stanols is very low (< 1% at 25°C), it limits their application in food products. Esterification of sterol and stanols is, therefore, performed to make them fat-soluble and easy to incorporate in food products (37, 38). Two margarines containing 8-9% sterols (Becel Proactiv of Unilever) or stanols (Benecol of Raisio), in the form of esters, are now available in the markets in Europe and the United States. 

In the DOD, phytosterols are present in both the free and esterified forms with fatty acids. Therefore, the first step in the extraction of phytosterols is conversion of phytosterol fatty esters into free phytosterols. This is achieved either by hydrolysis or trani-esterification. Hydrolysis could be carried out under strong basic conditions (saponification with further acidulation), under strong acidic conditions, or under chemical or enzyme (specific or nonspecific) catalyzation. Re-esterification of phytosterols occurs during methyl ester distillation as a result of the high temperatures involved therefore, a further trani-esterification step for free sterols is required. Esterification of phytosterols or trani-esterification of sterol fatty acid esters is the second step in this process. Methanol is the most commonly used alcohol, and it leads to methyl esters, which are characterized by a higher volatility, however, other Ci to C4 alcohols may also be used. Esterification and trans-esterification of fatty acids or phytosterols can be catalyzed by metal alcoholates, or hydroxide, by organic catalysts, or by enzymes (Table 7). 

Free sterols are slightly less volatile than tocopherols (Figure 2). Deodorization of soybean oil under varying process conditions (temperature 220-260°C, low pressure 1.5 mbar and 1.5% steam) resulted in a 10-35% reduction of the total sterol content (20). This sterol reduction is totally attributed to the free sterol fraction because esterified sterols are not volatile under the conditions prevailing inside the deodorizer. In case of steam refining, an increase of the steryl ester content can sometimes be observed, probably because of a heat-promoted esterification reaction between free sterols and fatty acids (20). This phenomenon will not take place during deodorization of chemically neutralized oil because the initial FFA concentration is much lower in that case. 

---