6533b7d8fe1ef96bd126a1b4

RESEARCH PRODUCT

Sterols and Steroids

subject

description

Sterols and steroids, like the terpenes and carotinoids, are isoprene derivatives. The sterols are components of the membranes of all eukaryotic cells and they bind and condense the phospholipid bilayer. The outer cell membrane is particularly rich in sterols, with a molar ratio of sterols to phospholipids of 0.8–0.1, compared with the usual value of 0.1–0.3 for intracellular membranes. The parent substance of all sterols is cholesterol (Fig. 16.1). The sterols are classified with the steroid hormones and bile salts as steroids because they also have the gonan (formerly steran) four-ring system and have their biosynthetic origin in cholesterol. Steroids with hormonal functions are known from the vertebrates (sex hormones and corticosteroids) and arthropods (ecdysteroids). However, the same molecular species are also found in other metazoans, protozoans, plants, and even in primitive eukaryotes such as yeast and lower fungi; they are clearly phylogenetically very old. The critical event for the evolution of hormone systems based on steroids in the vertebrates and arthropods was not the occurrence of new steroids but the “invention” of steroid receptors [23]. The calciferols or D vitamins (Fig. 16.18) are usually discussed in the context of the steroids, although, strictly speaking, they do not belong together because the B ring of the gonan is in this case open; they arise by UV effects on sterols. Finally, several different animal groups possess pharmacologically active glycosides, saponins and alkaloids derived from cholesterol.