Sterols and Other Isoprenoid Lipids

Hemmerlin, A., Gerber, E., Feldtrauer, J.R, Wentzinger, L., Hartmann, M.A., Tritsch, D., Hoeffler, J.F., Rohmer, M. and Bach, T.J. (2004) A review of tobacco BY-2 cells as an excellent system to study the synthesis and function of sterols and other isoprenoids. Lipids, 39, 723-35. 

Lipids are a complex group of substances, which include the long-chain fatty acids and their derivatives, sterols and steroids, carotenoids, and other related isoprenoids. It is evident that the term lipid denotes a wide range of compounds that appear to have little obvious interrelation. However, although these compounds possess widely different structures, they are derived in part from similar biological precursors and exhibit similar physical and chemical characteristics. Furthermore, most lipids occur naturally in close association with protein, either in membranes as insoluble lipid-protein complexes or as soluble lipoproteins of the plasma. 

Figure 28. Cyclic isoprenoid lipids and their relationships to squalene. (a) Squalene folded so as to demonstrate its relationship to a sterol carbon skeleton (b). In sterols that occur commonly, some of the methyl groups resulting from the cyclization of squalene move from one carbon to another and others are lost. The substituents marked R can be either methyl or H and the substituent marked R can be H, methyl, ethyl, or propyl.

The term lipids is defined as those organic compounds that are insoluble in water, soluble in organic solvents (e.g., chloroform, ether), contain hydrocarbon groups as primary parts of the molecule, and are present in or derived from living organisms. Compound classes covered in this definition include fatty acids (FA), acylglycerols, FA esters (e.g., waxes), and isoprenoid hydrocarbons. Other compounds also included are often considered as belonging to different classes, such as carotenoids, sterols, and the vitamins A, D, E, and K. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

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