Structural relationships of the principal categories of steroids

Sterols

A large group, the sterols, is composed of the common 3-monohydroxy steroids of the cholestane, ergostane, and stigmastane series and their methyl sterol biogenetic precursors: lanosterol, cycloartenol, and certain derivatives of these sterols, such as lophenol. Most sterols have a 3β-hydroxyl group, and many (though not the 4-methyl sterols) have a double bond between carbon atoms 5 and 6. Various sterols have double bonds at other positions in the nucleus corresponding to stages in the biosynthesis of cholesterol and other steroids that resemble it in the structure of ring A. Animal sterols, especially in embryonic tissues and skin, and phytosterols (e.g., stigmasterol) also may have a double bond in the side chain. Sterols of feces (e.g., coprostanol) have a 3α-hydroxyl group and cis- (5β-) linked rings A and B; they are formed by metabolism of other sterols by intestinal bacteria. Certain sterols are transformed to calciferols (D vitamins) by ultraviolet light; this process occurs naturally in the skin and is used commercially in the manufacture of vitamin D₂ (ergocalciferol) from ergosterol and of vitamin D₃ (cholecalciferol) from synthetic 7-dehydrocholesterol.

Bile acids and alcohols

The molecular structures of metabolites of cholesterol form an evolutionary series from the bile alcohols, such as myxinol and scymnol of the elasmobranch fishes (e.g., sharks and rays) and the related alcohols of some bony fishes and frogs, through the 5β-cholestanoic acids of crocodiles and alligators, to the 5β-cholanoic acids of the birds and mammals. They are not exclusively confined to the species indicated; for example, chenodeoxycholic acid is a major bile acid in humans and many other mammals, and cholic acid is found in many nonmammalian species, together with primitive bile acids or alcohols that are not found in mammals.

Estrogens

The estrogens of the ovary of vertebrates are steroids that are abundant in the urines of pregnant mares and of stallions. The most potent natural estrogen is estradiol; the less-potent estrogens—estrone, estriol, and other oxygenated phenolic steroids—are metabolites of estradiol. Some species, notably the Equidae, secrete the less-active estrogen equilenin. Estrone, synthesized from diosgenin, has been used as a starting material for synthesis of androgenic and progestational steroids lacking a C19 methyl group (19-nor steroids). Synthetic estrogens, such as estranol or mestranol (18), commonly used in oral contraceptives and for other therapeutic purposes, have acetylenic (containing triple bonds between carbon atoms) substituents. Nonsteroidal synthetic estrogens—e.g., diethylstilbestrol (19) and related compounds—are used clinically and also in animal husbandry to promote fattening of livestock and poultry and to improve the quality of their meat.

Progesterones

Three naturally occurring steroids of the corpus luteum and placenta have progestational action; these are progesterone and two of its metabolites. All possess an unsaturated ketonic structure in ring A. Pregnanediol, the main metabolite of progesterone, lacks both this structural feature and progestational activity.

Synthetic progestational steroids that are used in oral contraceptives and for other therapeutic purposes (see above Pharmacological actions of steroids: Steroid contraceptives) are derivatives of progesterone or of 19-nortestosterone. Among the latter is norethandrolone (20).

Androgens

Testosterone and androstenedione are the major testicular androgens. Several other less-active androgens occur naturally. Major metabolites of testosterone are androsterone and etiocholanolone. The latter compound is androgenically inactive, but it is a pyrogen (e.g., a fever-producing agent) that has been associated clinically with some febrile conditions.

Adrenal cortical hormones

The steroids of the adrenal cortex are progesterone derivatives that bear hydroxyl groups at positions 11, 17α, or 21. The potent mineralocortoid aldosterone carries an aldehyde function in place of the more usual C18 methyl group. Glucocorticoid potency is higher in the trihydroxy derivative cortisol of humans, monkeys, and dogs than in the dihydroxy steroid corticosterone of rats and mice. Every adrenal steroid hormone has a ketone group at C3 and a double bond between C4 and C5. In the liver their physiological activities are lost when ring A is reduced, and they are partially inactivated when the 11β-hydroxyl is oxidized to a ketone group (as in cortisone and 11-dehydrocorticosterone). The adrenal corticoids are among the most chemically reactive of the steroid hormones; they are sensitive to strong acids and alkalies and to elevated temperatures. In many synthetic analogs of cortisol and cortisone, physiological activity is modified.

Ecdysones

The molting hormones (zooecdysones) of insects and crustaceans are generally derivatives of cholestane. All possess a ketone group at position 6, a double bond between positions 7 and 8, and 2β-, 3β-, and 14α-hydroxyl groups. The side chain is hydroxylated at C22 and variously at C20, C25, and C26. Some of these compounds occur in plants, many of which also contain potent ecdysone analogs (phytoecdysones) with ergostane and stigmastane side chains.

The ecdysones show cross-species activity; that is, the ecdysone of one species induces molting in another species. The preponderance of ecdysones related to cholestane even in phytophagous insects probably reflects their capacity to convert dietary phytosterols to cholestane derivatives.

Cardiac glycosides and aglycones

Many species of plants contain toxic (specifically, heart-arresting) steroids of the cardanolide type as glycosides (compounds that contain structural groups derived from sugars) of up to four sugar residues, which may include glucose, rhamnose, and 10 other sugars characteristic of this group of natural products. Typically, these compounds are 5β-steroids and have 3β- and 14β-hydroxyl groups, but hydroxyl groups may occur in many other positions. In all cases, the aglycone (the steroid that results when the sugar groups are removed) is less active than its glycosides, but generally activity declines with increasing numbers of sugar residues after the first. The structures of the sugars have important but not predictable effects on activity.

The most important cardiac glycosides, medicinally, are those occurring in foxglove (Digitalis): digitoxin, gitoxin, and digoxin. Each of these contains a specific aglycone (e.g., digitoxigenin [23] is the aglycone of digitoxin) linked to three molecules of the sugar digitoxose and is derived from a more complex glycoside (digilanides A, B, and C, respectively) from which glucose and acetic acid are removed during the isolation procedures.

The squill, or sea onion, Scilla maritima, a seashore plant, contains several toxic glycosides, the aglycones of which are bufadienolides more typical of the toad poisons than of plant products. (In a bufadienolide, two double bonds are present in the bufanolide side chain.)

A white form of the squill produces the glycoside scillaren A, which contains the aglycone scillarenin, whereas a red form produces scilliroside, which is specifically toxic to rodents and has long been used as a rat poison. The contribution of the side chain to cardiac activity differs little between the bufanolides and the cardanolides.