Chemistry Of Steroids

A. Basic Ring Structure

Steroids are derived from a phenanthrene ring structure (structure 4) to which a pentano ring has been attached; this yields in the completely hydrogenated form, cyclopentanoperhydrophenanthrene, or the ster-ane ring structure (structure 6, Figure 2-2).

Steroid structures are not normally written with all the carbon and hydrogen atoms as illustrated in 5 of Figure 2-2; instead, the shorthand notation as presented for sterane (6, Figure 2-2) is usually employed. In this representation the hydrogen atoms are not indicated, and unless specified otherwise it is assumed that the cyclohexane (A, B, and C) or cyclopentane (D) ring is fully reduced, that is, each carbon has its full complement of carbon and/or hydrogen bonds. Also, indicated in structure 6 is the standard numbering system for all of the carbon atoms in the four rings. The three six-carbon cyclohexane rings are designated A, B, and C, and the five-carbon cyclopentane ring is denoted as the D ring.

B. Classes of Steroids

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

Sterane (6) FIGURE 2-2 Parent ring structures of steroids.

estrogens (female sex steroids), androgens (male sex steroids), progestins, mineralocorticoids, glucocorticoids, and vitamin D with its daughter metabolites. 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 2-1 summarizes some fundamental relationships of these principal mammalian classes of steroids.

The parent ring structure for cholesterol is the fully saturated ring structure cholestane (7, Figure 2-3). Cholestane, which has 27 carbons, differs from sterane (6, Figure 2-2) 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 (8), androstane (9), estrane (10), and cholane (11), of which is structurally related to cholestane. These relationships are depicted in Figure 2-3.

Hormones, Second Edition TABLE 2-1 Classes of Steroids

Steroid class Principal active steroid in humans" Number of carbon atoms Parent ring structure"

Estrogens

Androgens

Progestins

Glucocorticoids

Mineralocorticoids

Vitamin D steroids

Bile acids

Estradiol

Testosterone

Progesterone

Cortisol

Aldosterone

1,25-Dihydroxyvitamin D3 Cholic acid

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