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A steroid is an organic compound with four rings arranged in a specific configuration. Examples include the dietary lipid cholesterol, the sex hormones estradiol and testosterone and the anti-inflammatory drug dexamethasone. Steroids have two principal biological functions: certain steroids (such as cholesterol) are important components of cell membranes which alter membrane fluidity, and many steroids are signaling molecules which activate steroid hormone receptors. The steroid core structure is composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxyl group at position three and a skeleton derived from cholestane.〔 (PDF ) 〕〔 ''Also available with the same authors (and year) at'' Note, the article co-authors, the Working Party of the IUPAC-IUB JCBN, were P. Karlson (chairman), J.R. Bull, K. Engel, J. Fried†, H.W. Kircher, K.L. Loening, G.P. Moss, G. Popják and M.R. Uskokovic. ''Also available online at'' (See also note 4 therein.)〕 〔Also available in print at 〕 They can also vary more markedly by changes to the ring structure (for example, ring scissions which produce secosteroids such as vitamin D3). Hundreds of steroids are found in plants, animals and fungi. All steroids are manufactured in cells from the sterols lanosterol (animals and fungi) or cycloartenol (plants). Lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene.〔(【引用サイトリンク】 Lanosterol biosynthesis )〕 == Nomenclature == A gonane, the simplest steroid, is composed of seventeen carbon atoms in carbon-carbon bonds forming four fused rings in a three-dimensional shape. The three cyclohexane rings (A, B, and C in the first illustration) form the skeleton of a perhydro derivative of phenanthrene. The D ring has a cyclopentane structure. When the two methyl groups and eight carbon side chains (at C-17, as shown for cholesterol) are present, the steroid is said to have a cholestane framework. The two common 5α and 5β stereoisomeric forms of steroids exist because of differences in the side of the largely planar ring system where the hydrogen (H) atom at carbon-5 is attached, which results in a change in steroid A-ring conformation. Examples of steroid structures are: In addition to the ring scissions (cleavages), expansions and contractions (cleavage and reclosing to a larger or smaller rings)—all variations in the carbon-carbon bond framework—steroids can also vary: * in the bond orders within the rings, * in the number of methyl groups attached to the ring (and, when present, on the prominent side chain at C17), * in the functional groups attached to the rings and side chain, and * in the configuration of groups attached to the rings and chain.〔 For instance, sterols such as cholesterol and lanosterol have an hydroxyl group attached at position C-3, while testosterone and progesterone have a carbonyl (oxo substituent) at C-3; of these, lanosterol alone has two methyl groups at C-4 and cholesterol (with a C-5 to C-6 double bond) differs from testosterone and progesterone (which have a C-4 to C-5 double bond). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Steroid」の詳細全文を読む スポンサード リンク
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