Acylated sterol glycosides

Sterol esters, sterol glycosides and acylated sterol glycosides are also widespread in plants [5],... 

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. 

The 3-hydroxyl group can also form a glycosidic link with the 1-position of a hexose sugar (usually glucose). Sterol glycosides are widespread in plants and algae and the 6-position of the hexose can be esterified with a fatty acid to produce an acylated sterol glycoside. 

Important steroids include the (pro-)vitamins D, bile acids, and various steroid hormones such as testosterone, oestrogens, progesterones and glucocorticoids. Sterols may also be derivitized to give sterol esters, sterol glycosides and acylated sterol glycosides. 

Many changes in the lipid composition of the plasma membrane have been associated with hardening (Lynch Steponkus, 1987). In rye (cv. Puma) an increase of the total lipid content has been measured during hardening (Cloutier, 1987). Free sterols increased while steryl-glycoside and acylated steryl-glycoside decreased. In addition the phospholipid contents of the plasma membrane increased. 

Keywords Plant sterol phytosterol sterol conjugate steryl ferulate gamma-oryzanol steryl glycoside acylated steryl glycoside steryl ester free sterol. 

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 The relationship between total dietary phytosterol content and the fatty acid composition of the diet decreases with increasing saturated fatty acids, whereas the total dietary phytosterol content increases with increasing PUFA (86). Phytosterols consist of a mixture of cell membrane constituents that include free sterols, esterified steryls (e.g., esterified to phenolics), steryl glycosides, and acylated steryl glycosides (Table 2). 

Sterol glycosides are widely distributed in plants and are frequently accompanied by their 6-0-acyl derivatives. As might be expected, preparations have been obtained which glucosylate sterols with UDP-glucose as sugar donor ... 

Glycolipids of this type are synthesized in the chloroplast (Lawlor, 1993). Sulfoquinovosyl diglyceride (12) is widespread in leaves but usually occurs at low concentrations (Stumpf, 1976). This lipid is linked to S04 reduction in the chloroplast (Lawlor, 1993). Sterol esters and acylated steryl glycosides are also found in leaf lipids. 

Sterols are substituted with an isoprenoid side chain at C-17 (Fig. 120). In plants they may be glycosylated (steryl glycosides) or acylated by fatty acids (steryl esters) at the C-3 hydroxy group. Acylated steryl glycosides, e.g., 6-0-palmitoyl-/8-D-glucosyl-sitosterol, also occur in plants. Sterols are grouped according to the pattern of methylation into 4,4-dimethylsterols, e.g., lanosterol and cycloartenol, 4a-methylsterols, e.g., obtusifoliol, and 4-demethylsterols, e.g., cholesterol. 

Steroids are derived from the same squalene precursor and have an oxygen-dependent biosynthetic pathway beginning with the formation of the first intermediate, 2,3-oxidosqualene (for details, see Sect. 5.1). Sterols, also known as steroid alcohols, are a subclass of steroids and may be found either as fi ee sterols, acylated, alkylated, sulfated, or linked to a glycoside moiety which can be itself acylated. Sterol biosynthesis is nearly ubiquitous among eukaryotes but almost completely absent in prokaryotes. As a result, the presence of diverse steranes (saturated four-cycle skeleton) in ancient rocks has been considered as evidence for over 2.7 billion years of eukaryotic evolution. Cholesterol is the most well-known sterol found in animal cell membranes (for establishing proper membrane permeability and fluidity) and red blood cells. 

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]. 

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