The early steps in the biosynthesis of steroids of both plants and animals are the same, except that in plants lanosterol is replaced by the related compound cycloartenol, which contains a three-membered ring (C9, C10, C19) in lieu of the nuclear double bond of lanosterol. The side chains of the phytosterols, such as stigmasterol, and of the sterol ergosterol of yeasts and other fungi contain extra carbon atoms that are incorporated in reactions involving S -adenosylmethionine, which donates methyl groups in numerous biological processes. Although most plant tissues contain only traces of cholesterol, this sterol is the biogenetic precursor of such important plant steroids as the sapogenins , glycosides , and alkaloids . Because pregnane derivatives are intermediates in some of these transformations, plants and animals appear to have important features of steroid metabolism in common.
Depending on the number and character of their functional groups, steroid molecules may show diverse reactivities. Moreover, the reactivity of a functional group varies according to its location within the molecule (for example, esters are formed readily by 3-OH groups but only with difficulty by the 11β-OH group). An important property of steroids is polarity —., their solubility in oxygen-containing solvents (., water and alcohols ) rather than hydrocarbon solvents (., hexane and benzene ). Hydroxyl, ketonic, or ionizable (capable of dissociating to form electrically charged particles) groups in a steroid molecule increase its polarity to an extent that is strongly influenced by the spatial arrangement of the atoms within the molecule.
Anabolic steroids are synthetic substances related to male sex hormones (androgens). Although it is illegal in the United States to possess or distribute anabolic steroids for nonmedical use, a "black market" for them exists, and many amateur and professional athletes take them to enhance performance. In many cases, the athletes take doses that are extremely high—perhaps 100 times the doses that might be prescribed for medical use. As a result, they put themselves in real danger of short-term and long-term health problems. Blood testing, as has been used in the Olympic Games, can detect, identify, and quantify the presence of anabolic steroids in the blood of athletes, which can lead to the disqualification of an athlete.