In mammalian systems, there are six families of steroid hormones that can be classified on both a structural and a biological (hormonal) basis. They are the estrogens and progestins (female sex steroids), androgens (male sex steroids), mineralocorticoids (aldosterone), glucocorticoids (cortisol), and vitamin D [1α,25(OH)2D3]. Also, the bile acids are structurally related to cholesterol and thus constitute a seventh member of the steroid family. All of these steroids are biologically derived from cholesterol. Table 1 summarizes some fundamental relationships of these principal mammalian classes of steroids.
Table1. Classes of Steroids
The parent ring structure for cholesterol is the fully saturated ring structure cholestane (see top row of Figure 1). Cholestane, which has 27 carbons, differs from sterane by the addition of an eight carbon side chain on carbon-17 of ring D and the presence of two angular methyl groups at the junctures of the A:B (carbon-10) and C:D rings (carbon-13). The cholestane ring structure also gives rise to the parent ring structures for the six classes of mammalian steroids and the bile acids, as summarized in Table 2-1. The parent ring compounds are the completely saturated ring structures pregnane, androstane, estrane, and cholane, which is structurally related to cholestane. The parent ring structures are used as the stem term in constructing the formal nomenclature of any of these steroids.
Fig1. Structural consequences resulting from the reduction of of the 3-keto group of pregnan-3-one to a hydroxyl group (see Row 1). As shown in Row 2, the orientation of the hydroxyl group is not designated (see the green wavy bond of the C3 hydroxyl group). As shown in Row 3, there are two structural options for the orientation of the hydroxyl goup on C-3. The hydroxyl can be either “above” the plane of the page (see the solid blue hydroxyl group) or below the plane of the page (see the dashed red hydroxyl group). Thus, in Row 3, application of steroid nomenclature rules designates that the red hydroxyl below the plane of the page is a 3α-hydroxyl while the blue hydroxy above the plane of the page is a 3β-hydroxyl. Row 4 presents the chair presentation for the A/B rings of the two pregnane isomers; this further emphasizes that the alpha and beta 3- hydroxyl groups each have a distinctly different orientation in space.
The biosynthetic pathway of production of each of these general steroid classes will be presented separately later in this chapter. A discussion of their hormonal and biochemical aspects will appear later in individual chapters.
