Sterol esters formation

A complex series of reactions including opening of the cyloartenol cyclopropane ring, double bond formation and isomerization, demethylation of ring carbons, and methylation of the side chain result in formation of a number of different plant sterols [24]. Sitosterol is the most common plant sterol (Fig. 7) however, plants normally contain mixtures of sterols whose proportions differ from tissue to tissue. In addition, sterol esters, sterol... 

Beer contains trace amounts of lipids. A Swedish beer (12° Plato) was found to contain (mg/1) triglycerides, 0 1-0 2 diglycerides, 0 1 monoglycerides, 0 1-0 3 sterol esters, 0 01 free sterols, 0 01-0 02 and free fatty acids, C4-C10, 10-15 Ci2-Cx8, 0-0 5 [42]. Similar data were found for American beers. The role that lipids play in head formation is discussed later. The free fatty acids are volatile and more detailed values are given below. Whereas unsubstituted fatty acids are volatile, the addition of a further substituent usually results in loss of volatility. Data for hydroxy-, keto, di-, and tri-basic acids are given in Table 22.7. 

Sgoutas" has examined the formation and hydrolysis of those sterol esters in which saturated acyl groups differ in chain-length and unsaturated acyl groups differ in the position and configuration of the olefinic centre. 

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... 

Plant waxes are concentrated on leaves and leaf sheaths and on fruit skins, or in some exceptional cases in the seeds of plants. Most vegetable waxes contain predominantly wax esters plus a variety of other lipid materials, which affect the degree of saturation and other properties of the wax derived from different sources. Most use has been made of plant waxes in the cosmetic sector, but there is increasing interest in the use of plant-derived sterols as dietary supplements to reduce cholesterol formation. 

Balance between the active and inactive forms of ecdysone and/or other ecdysteroids may be accomplished by the formation of conjugates such as sulfate esters or glucosides (20,27). The sulfation of phenols and a variety of sterols has been 3emonstrated in insect tissues and this, in close association with an appropriate sulfatase, would constitute a readily reversible mechanism whereby the required balance between active and inactive forms of ecdysteroids could be regulated (27). 

When considering the addition of a bioactive to food, it is useful to classify them as oil-soluble (e.g., polyunsaturated fatty acids, carotenes, lycopene), water-soluble (e.g., anthocyanins, proteins and peptides), or water/oil dispersible components (e.g., probiotics). Bioactives may be added directly to food if they are in a compatible format with the food matrix and provided their direct addition does not impact negatively on food quality or the bioavailability of the bioactive. When the solubility in a food matrix is limiting, its hydrophilicity/lipophilicity may be modified to enable improved incorporation. An example is the conversion of free plant sterols to fatty acid esters in order to make them more oil-soluble and readily incorporated into spreads (Deckere de and Verschuren 2000). 

The preparation of high-purity tocopherols and phytosterols involves steps such as molecular distillation, adduct formation, liquid-liquid extraction, supercritical fluid extraction, saponification, and chromatography (175). The extraction of tocopherols from soybean oil deodorizer distillate by urea inclusion and saponification of free fatty acids resulted in good recovery of tocopherols (208). To improve the separation of sterols and tocopherols, Shimada et al. (209) used a lipase to esterify sterols with free fatty acids. Then the steryl esters and tocopherols were separated better by molecular distillation. Chang et al. (210) used supercritical fluid CO2 extraction to recover tocopherols and sterols from soybean oil deodorizer distillate. A patent by Sumner et al. (211) advocated treatment of the distillate with methanol to converted free fatty acids and other fatty acid esters to methyl esters that can then be removed by a stripping operation. Then separation of sterols and tocopherols could be carried out by molecular distillation. 

Available literature solubility data of pure lipids belonging to major (fatty acids, mono-, di- and triacylglycerols, and fatty acid esters) and minor lipid classes (pigments, sterols, vitamins, and hydrocarbons) in SCCO2 were compiled (26, 27). These references (26, 27) contain exhaustive bibliography on lipid + SCCO2 binary systems. Literature data were correlated using Chrastil s equation, which is an empirical model used quite commonly to correlate the solubility of lipid components (30). This model is based on the formation of a solute-solvent complex on association of the solute and solvent molecules and establishes a linear relationship between In(solubility) and In(density) as follows ... 

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. 

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