17β-Hydroxysteroid dehydrogenases (17β-HSD, HSD17B) (), also 17-ketosteroid reductases (17-KSR), are a group of alcohol oxidoreductases which catalyze the reduction of 17-ketosteroids and the dehydrogenation of 17β-hydroxysteroids in steroidogenesis and steroid metabolism. This includes interconversion of DHEA and androstenediol, androstenedione and testosterone, and estrone and estradiol.
The major reactions catalyzed by 17β-HSD (e.g., the conversion of androstenedione to testosterone) are in fact hydrogenation (reduction) rather than dehydrogenation (oxidation) reactions.
17β-HSDs have been known to catalyze the following redox reactions of sex steroids:
20α-Hydroxyprogesterone ↔ Progesterone
() ↔ Androstenediol
Androstenedione ↔ Testosterone
Dihydrotestosterone ↔ 5α-Androstanedione / 3α-Androstanediol / 3β-Androstanediol
Estrone ↔ Estradiol
16α-Hydroxyestrone ↔ Estriol
Genes coding for 17β-HSD include:
HSD17B1: Referred to as "estrogenic". Major subtype for activation of estrogens from weaker forms (estrone to estradiol and 16α-hydroxyestrone to estriol). Catalyzes the final step in the biosynthesis of estrogens. Highly selective for estrogens; 100-fold higher affinity for estranes over androstanes. However, also catalyzes the conversion of DHEA into androstenediol. Recently, has been found to inactivate DHT into 3α- and 3β-androstanediol. Expressed primarily in the ovaries and placenta but also at lower levels in the breast epithelium. Major isoform of 17β-HSD in the granulosa cells of the ovaries. Mutations and associated deficiency have not been reported in humans. Knockout mice show altered ovarian sex steroid production, normal puberty, and severe subfertility due to defective luteinization and ovarian progesterone production.
HSD17B2: Describable as "antiestrogenic" and "antiandrogenic". Major subtype for inactivation of estrogens and androgens into weaker forms (estradiol to estrone, testosterone to androstenedione, and androstenediol to DHEA). Also converts inactive 20α-hydroxyprogesterone into active progesterone.
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3α-Androstanediol also known as 5α-androstane-3α,17β-diol and sometimes shortened in the literature to 3α-diol, is an endogenous steroid hormone and neurosteroid and a metabolite of androgens like dihydrotestosterone (DHT). 3α-Androstanediol is an inhibitory androstane neurosteroid and weak androgen and estrogen. As a neurosteroid, it acts as a potent positive allosteric modulator of the GABAA receptor, and has been found to have rewarding, anxiolytic, pro-sexual, and anticonvulsant effects.
3β-Androstanediol, also known as 5α-androstane-3β,17β-diol, and sometimes shortened in the literature to 3β-diol, is an endogenous steroid hormone and a metabolite of androgens like dehydroepiandrosterone (DHEA) and dihydrotestosterone (DHT). 3β-Androstanediol is a selective, high-affinity agonist of the ERβ, and hence, an estrogen. In contrast to ERβ, 3β-androstanediol does not bind to the androgen receptor (AR). 3β-Androstanediol has been reported to also bind to ERα with low nanomolar affinity, with several-fold lower affinity relative to ERβ.
Estriol (E3), also spelled oestriol, is a steroid, a weak estrogen, and a minor female sex hormone. It is one of three major endogenous estrogens, the others being estradiol and estrone. Levels of estriol in women who are not pregnant are almost undetectable. However, during pregnancy, estriol is synthesized in very high quantities by the placenta and is the most produced estrogen in the body by far, although circulating levels of estriol are similar to those of other estrogens due to a relatively high rate of metabolism and excretion.
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