Sterols general discussion

In section 9.3, we discussed in general terms the use of microbial metabolism to selectively remove the side chain from sterols to produce steroids. This removal may also be accompanied by some modification to the ring structure. We did not, however, discuss in any detail any specific reactions. In this section we will focus on some specific reactions. 

The non-mevalonate route to terpenoids appears to be localized in the plas-tids. In plant cells, terpenoids are manufactured both in the plastids and the cytosol (Gray, 1987 Kleinig, 1989). As a general rule, the plastids produce monoterpenes, diterpenes, phytol, carotenoids and the side chains of plas-toquinone and a-tocopherol, while the cytosol/ER compartment produces sesquiterpenes, sterols and dolichols. In the studies discussed above, nearly all of the terpenoids labelled by deox30cylulose (Sagner et al, 1998b Eisenreich et al, 2001) and 2-G-methyl erythritol feeding (Duvold et al, 1997) or... 

Other classes of organic materials, such as alkaloids, pigments, resins, sterols, terpenes, terpenoids, and waxes, and many simple organic compounds are often present in various biomass species, but are not discussed here because they are usually present in very small amounts. The peptides present in herbaceous biomass are also not discussed here because, although the nitrogen and sulfur contents of the biomass should be assessed for certain microbiological processes, the amino acids that make up the proteins are generally not important factors in conversion processes. 

Saponification of the extracts is generally desirable to remove unwanted lipid materials. However, this step is omitted in the isolation of carotenol esters, since these are hydrolyzed by this procedure. It is also omitted in the isolation of carotenoids such as fucoxanthin and peridinin, which are alkali-labile. If acetone has been used in the initial extraction, it is essential that all traces be removed before saponification. The general procedure used involves dissolving the total lipid fraction in an alcoholic (ethanol or methanol) solution of potassium hydroxide. The mixture is then either heated for a short period of time while kept in the dark, or left in the dark at room temperature for 12-16 h. There has been considerable discussion of the merits of these two procedures. Which method is used is dependent on the nature of the samples being analyzed and the requirements of the analysis (Davies, 1976 Liaaen-Jensen, 1971). After saponification, water is added, and neutral lipids (the unsaponifiable fraction) are extracted with diethyl ether or hexane. Acidic carotenoids remain in the alkaline phase and are extracted with diethyl ether or hexane after acidification with acetic acid. The unsaponifiable fraction usually contains sterols as well as carotenoids. If desired, sterol contaminants can be removed by precipitation from cold (- 10°C) petroleum ether or by precipitation of these compounds as their digitonides. 

A reaction unique to plant sterols is the opening of the cyclopropane ring generated in the formation of cycloartenol. However as already mentioned, smother reaction specific to plants is alkylation at C-24. As the enzyme which opens the cyclopropane ring in general acts only on alkylated derivatives, it is logical to discuss the alkylation reactions first. However, an enzyme preparation will occasionally act on nonalkylated intermediates, as in the case just discussed in which an Euphorh/u latex isomerizes cycloartenol to lanosterol. 

Many of the common major fragmentations were discussed by Bergstrom, Ryhage, and Stenhagen (5). Their results have been extended, with more comprehensive discussions of bile acid mass spectra (I, 12, 15, 18, 19). The fragmentation patterns of sterol derivatives are in many respects similar to those of the bile acids discussions of sterol mass spectra are given in (1, 11, 20-23). General reviews of steroid mass spectra may be consulted (1, 24,25). 

In this review, we will first describe the general methodology of MS-lipidomics, then present the state-of-the-art of analysis of different phospholipids, neutral lipids, e.g., TAGS, cholesterol esters (CEs), sphingolipids, and sterols, and, finally, we will discuss the potential clinical applications of MS-lipidomics. Unfortunately, due to lack of space we cannot deal with all relevant publications. These can be found in recent reviews on MS-lipidomics [3-10]. 

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