Ho

Dehydroepiandrosterone (77)

4-Androstenedione (25) Estrone (88)

4-Androstenedione (25) Estrone (88)

Testosterone (15)

Estradiol-17p (16)

Testosterone (15)

Estradiol-17p (16)

Estra-3, 16a,17p-triol (89)

Estra-3, 16a,17p-triol (89)

FIGURE 2-23 Pathway of estrogen biosynthesis. The main steps of metabolism are numbered (in circles) according to the reactions described in Figure 2-18 and Table 2-6.

drosterone sulfate (84) (DHEA sulfate), which acts as a precursor for the estriol (88) of both the placenta and the mother, is discussed in Chapters 10 and 14.

17/3-Estradiol (16) has also been found to be hydrox-ylated by brain tissue on either carbon-2 or carbon-4 to yield a family of steroids with vicinal3 phenolic hydroxyl moieties. These steroids are referred to as the catechol estrogens because of their structural similarity in the A ring to the catecholamines, epinephrine, and norepinephrine (see Chapter 11). Figure 2-24 summarizes the known pathways of metabolism for the production and breakdown of the catechol estrogens.

F. Biosynthesis of Vitamin D Metabolites

Vitamin D3 is chemically closely allied to the classical steroid hormones. Technically it is a secosteroid; that is, due to the breakage of the carbon-9, carbon-10

3 Vicinal hydroxyls are hydroxyl groups on adjacent carbons.

2. Steroid Hormones

2. Steroid Hormones

Estradiol (16)

Estradiol (16)

2-Hydroxyestradiol (92)

NAOPH, 02

NAOPH, 02

4-Hydroxyestradiol (90)

catechol o-methyltransfepase catechol o-methyltransfepase

hjco hjco'

2,3-dimethoxyestradiol (91)

Epinephrine

FIGURE 2-24 Metabolic pathways for biosynthesis of the catechol estrogens (top) and their structural similarity to the catecholamines (bottom).

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