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In this section plant steroids are described. Steroids also occur in humans and animals and play there an important role in the maintenance of body functions. One example is cholesterol.
Steroids with an hydroxy group at C-3 position are called sterols. Sterols from plants also called phytosterols.
Steroids have the same four-ring hydrocarbon framework because of their biosynthesis starting from squalene, a tetraterpene. The tetracyclic system is mostly non-polar and lipophilic, but can also be functionalized or glycosylated (example therefore are the cardiac glycosides). Due to the lipophilic properties they often occur in the plant parts with a high fat level.
The four rings of steroids are indicated by capital letters. Thus, there are the 6-carbon A ring, B ring, C ring and the 5-carbon D ring. The carbon atoms are counted one by one. First of all ring atoms are counted, then the side chains.
Squalene is oxidized to 2,3-oxidosqualene by squalene monooxygenase. The epoxide is then opened and a positive charge is formed at C-4 position. In a tandem reaction squalene cyclized in one step to the tetracyclic system, the sterane. After a rearrangement reaction cycloartenol forms. From cycloartenol the plant steroids are formed in further steps.
In humans, however, lanosterol is formed, and finally converted to the steroids. Some plant steroids are also formed by the precrusor lanosterol.
Due to their properties, the plant steroids occur in plants and in parts of plants with a high fat level. These include the seeds and grains of sunflower, of wheat, pumpkin, and in soybean and sesame. This allows to isolate the steroids from these plant parts and the steroids are often by-product in the production of vegetable fats.
Bishop, Gerard J., and Takao Yokota. "Plants steroid hormones, brassinosteroids: current highlights of molecular aspects on their synthesis/metabolism, transport, perception and response." Plant and Cell Physiology 42.2 (2001): 114-120.