4a-Methylsterols

The most common sterols are A5-unsaturated desmethylsterols, but 4a-methylsterols and4,4-dimethylsterols occur at lower concentrations in vegetable oils. For example, sesame seed oil sterols are 85-89% des-, 9-11% mono- and 2-4% dimethylsterols (Kamal-Eldin and Appelqvist, 1994). The composition of monomethylsterol and dimethylsterol fractions of some oils is given in Table 6.2. 

Yano K., Akihisa T., Tamura T., and Matsumoto T. (1992) Four 4a-methylsterols and triterpene alcohols from Neolit-sea aciculata. Phytochemistry 31, 2093-2098. 

Sterols include 4a-methylsterols, intermediates of sterols and triterpens biosynthesis. Analysis of the sterol fraction demostrated the constant presence of P-sitosterol, campesterol and stigmasterol, A -avenasterol, while A" -avenasterol may be absent or present in very low amount. P-sitosterol makes up the 75-90% of the total sterol fraction, the rest of sterols occur in minute quantities. 

Figure 9.5 Structures of the main 4a-methylsterols present in olive oil.

A -sterols are major intermediateT in tne . elegans pathway for sitosterol dealkylation. In addition, the predominance of A -4o-methylsterols revealed that the azasterold inhibits the isomerase that converts A - to A8(14)-4a-methylsterols. 

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. 

Fig. 30.1. The pathway of cholesterol biosynthesis. 30. i, Mevalonate kinase 30.2, 3 -hydroxysteroid dehydrogenase of the 4a-methylsterol-4-demethylase complex 30.3, 3 -hydroxysteroid-A, A -isomerase (sterol-A -isomerase) 30.4, 3/ -hydroxysteroid-A - reductase (desmosterol reductase) 30.5, 3 -hydroxysteroid-A -reductase (7-dehydrocholesterol reductase)...

Membrane-bound enzymatic systems obtained from maize embryos catalyze the oxidative C4-monodemethylation of 4,4-dimethyl- and 4a-methylsterols. The... 

Teshima, S., and G.W. Patterson Identification of 4a-Methylsterols in the Oyster Crassostrea virginica. Comp. Biochem. Physiol. 69B, 175 (1981). 

A new 3,20-dihydroxy-4a-methylsterol was isolated from a species of the genus Diacronema. Its biosynthetic origin remains unknown (Rauter et al, 2005). 

The sterol composition is one of the most important aspects of the chemistry of dinoflageUates because it emphasizes the mesokaryotic character of these organisms. The sterols of dinoflageUates are mainly 4a-methylsterols and thus seem to be intermediate between triterpenic alcohols (4,4-dimethyl) and sterols (which do not have a methyl on carbon 4). For a complete study of this question, we direct the reader to a pubUcation dedicated to the sterols of dinoflageUates (Withers, 1983), of which we give here only a brief summary supplemented with some recent data, and to a more general article by Patterson (1991), as well as a comparative study of the... 

Figure n.l The main 4a-methylsterols and sterols of dinofiagellates (Bohlin eta., 1981 Withers, 1983 Patterson, 1991). 

Sterol fractions of holothurians are complex and show the presence of AO sterols (stanols), A5, A7 and A9(ll) as well as 4a-methylsterols and triterpenes of the lanostane series. In studies carried out by Djerassi s group since the mid-1970s using a method based on the incorporation of tritium-labeled tracers," it was shown that the species Stichopus califomicus ... 

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