Dimethyl sterols

Until recently it seemed that prokaryotic organisms (blue-green algae and bacteria) were unable to synthesize steroids or triterpenoids. Several reports, however, now indicate that this is not the case. Hop-22(29)-ene (128), squalene, and several 4,4-dimethyl-sterols have been identified in the bacterium Methylococcus capsulatus. Hop-22(29)-ene has also been isolated from an... 

Vegetable Oil Desmethyl Sterols 4-Methyl Sterols 4,4-Dimethyl Sterols... 

Detoxification. A ten minute incubation of both trladlmenol resistant and sensitive U. avenae strains In C1 acetate, labelled 4,4 dimethyl sterols equally In both strains, when trladlmenol was present. After further incubation in the absence of radio-isotope, methyl sterols were no longer labelled In the resistant strain, suggesting that rapid turnover... 

Previous investigations from several laboratories have demonstrated that both microsomal membranes and the cytosolic fraction from rat hver are required for the biological synthesis of cholesterol [1-4]. Specifically, the following conversions have been reported to require both microsomes and cytosol acetate to cholesterol [4] squalene to cholesterol [1] squalene-2,3-oxide to lanosterol [3] lanosterol to cholesterol [1,5] A -cholestenol to cholesterol [6] lanosterol to dihydrolanosterol [7] various 4,4-dimethyl sterols to cholesterol [8] and 7-dehydrocholesterol to cholesterol [9,10]. 

Although the major pathway of demethylation is 4,4,14a-trimethyl - 4a,14a-dimethyl - 4a-methyl, occasional deviations from the pattern can exist as for example 4,4-dimethyl sterols are major components in Methylococcus capsulatus and macdougallin (I), a 14 -methyl sterol, occurs in Peniocerus macdougallii. 

Strocchi, A. (1987) Vegetable oils and corresponding hydrogenated fats comparison of sterols, 4-alpha-methyl sterol and 4,4/-dimethyl sterol composition. Riv. Ital. Sostanze Grasse, 64, 401 109. 

Leatherhead Food RA (1989, p. 41) reported the sterol level (mean and range) of 23 specimens of CNO from all major producing countries and Codex (2001) has adopted the same range limits (Table 6.5). The mean values were 836 mg/kg for desmethyl sterols, 39 mg/kg for monomethyl sterols and 146 mg/kg for dimethyl sterols. 

The unsaponifiable material (1-3%) consists mainly of sterols, tocopherols and sesame lignans. For four cultivated species of S. indicum, the level of unsaponifiable material has been reported to be 1.4—1.8% (Kamal-Eldin and Appelqvist 1994b). The sesame oils from these species contained total sterols (0.51-0.76%), including desmethyl sterols (85-89%), monomethyl sterols (9-11%) and triterpene alcohols (dimethyl sterols) (2-4%) respectively. (1-Sitosterol (62-67%), campesterol (15-20%), stigmasterol (5-8%) and A5-avenasterol (7-10%) are the major sterols present in both free and esterified forms. The monomethyl sterols, namely gramisterol, citrostadienol and obtusi-foliol, were present mainly as esters. The total sterols contained 65% in free and 35% in esterified form. Composition (% wt) data of the desmethyl sterols, monomethyl sterols and dimethyl sterols (tritepene alcohols) of four Sesamum species are listed in Table 11.4. Tocopherols of crude sesame oils are generally... 

Dimethyl sterols (triterpene alcohols) Sample (j-Amyrin A8-Sterol a- Amyrin Cycloartenol A7-Sterol 24-Methylene cycloartanol Others... 

Sterols are also present in free and esterified forms. We will take the review of Folstar (1985) as the reference. Free sterols constitute on average 2.2% of the total lipids and the esters with fatty acids 3.2 Mi. In the sterolic fraction, 92.7 % are 4-desmethylsterols, 2 % are 4-methylsterols and 5.1% are 4,4-dimethyl-sterols. The three major components belong to the first category sitosterol (53-54% of the sterolic fraction), stigmasterol (20—22 %) and campesterol (18-19%), The qualitative composition has been reported to be independent of the geographical origin. 

Fig. 4. Correlation of fecal bile acid excretion with the following 1, fecal fat, 2, 3, and 4, jejunal micellar fat and bile acids, and jejunal total cholesterol, respectively, calculated for amounts found in the intestinal pool in which 136 mg of )5-sitosterol administered in the test meal was dispersed. 5, Serum methyl sterol (diunsaturated dimethyl sterol), used as an indicator of augmented cholesterol synthesis (see reference 11). 6, Serum cholesterol., Patients with incomplete ileal resection (25-50 cm of the very terminal ileum intact, 1-2 m of the more proximal gut removed) , complete ileal resection 4, extensive intestinal resections with severe malnutrition (the values omitted from calculations in 5).

A first method simplified the classical analysis of the sum of free and esterified sterol instead of saponification and tedious extraction from a soap solution, the sample is transesterified at ambient temperature and the mixture (containing the fatty acids as methyl esters) is preseparated by NPLC. The LC-UV chromatogram enables one to select the type of sterols (A5, A7, methyl-, dimethyl sterols) to be analyzed by GC (after online transfer through the loop-type interface). 

The unsaponifiable part of edible fats contains a series of cyclic triterpenes which have structures related to that of steroids. Quantitatively, the 3 P-hydroxysteroids are the sterols which are to the fore. Especially diverse is the sterol spectrum of plant fats which contain not only desmethyl, but also 4-methyl and 4,4-dimethyl sterols. 

Plant sterols can be divided, on both stmctural and biosynthetic grounds, into 4-desmethyl, 4-monomethyl, and 4,4-dimethyl sterols based on the number of methyl groups at in the tetracyclic ring system (Figure 2). Plant sterols are not endogenously synthesized in the human body and are therefore derived solely from dietary sources. 

Cholesterol is also a 4-desmethyl sterol, and the similarity between its ring structure and the substitution at C-24 to those of some plant sterols (Figure 3) seem to be critical elements for the important effects of plant sterols on cholesterol absorption. Only 4-desmethyl sterols (not 4-methyl or 4,4-dimethyl sterols) exert this effect. 

Methyl sterols occur naturally in only minor amounts as they are precursors of the desmethyl sterols. In commercial soybean sterols, 98.1% are 4-desmethyl sterols, 1.3% are 4-methyl sterols and 0.6% are 4,4-dimethyl sterols (Sierksma et al, 1999). Compared with 4-desmethyl sterols, the number of different 4-methyl sterol compounds is small. 

Dimethyl sterols are also intermediates in plant sterol biosynthesis and can be found in only trace amounts in most plants. Some plant materials such as rice bran oil (see Chapter 4) have elevated levels of 4,4-dimethyl sterols such as cycloartenol and 24-methyl cycloartenol. The content of 4,4-dimethyl sterols in rice bran oil sterols varies, probably with the rice variety and processing conditions. According to de Deckere and Korver (1996), crude rice bran oil contains 1.8,0.4 and 1.2 g/100 g oil of 4-desmethyl, 4-methyl and 4,4-dimethyl sterols, respectively, mainly as esters of ferulic andp-coumaric acids. 

Figure 2. Moleeular struetures of examples from the three main sterol sub-elasses sitosterol, a 4-desmethyl sterol with no methyl group at C-4 gramisterol, a 4-monomethyl sterol and eyeloartenol, a 4,4-dimethyl sterol.

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