Part of “CHAPTER 72 – SYNTHESIS AND METABOLISM OF CORTICOSTEROIDS“

Steroids have a common structure with 17-carbon atoms arranged in three six-membered rings and a fourth five-membered ring labeled A, B, C, and D, respectively (Fig. 72-1). Each of the 17 carbons is numbered in a standard way. Two additional carbons, numbered 18 and 19, may be attached at carbons 13 and 10, respectively. Carbon atoms 20 and 21 may be attached at the 17 position. These various additions yield three steroid families: the C₁₈ estranes with an aromatic ring (estrogens); the C₁₉ androstanes (androgens); and the C₂₁ pregnanes (corticoids and progestins) (see Fig. 72-1). The steroid nucleus lies in a plane that can be modified by the addition of substituents either above or below (Fig. 72-2). The α-substituents occur below the plane (indicated by dotted lines in Fig. 72-2) and the β-substituents lie above the plane (indicated by solid lines). The A and B rings may be attached so that the substituents at positions 5 and 10 are in either the cis or trans orientations (see Fig. 72-2).