17beta-estradiol is the 17beta-isomer of estradiol. It has a role as an estrogen, a human metabolite, an EC 1.2.3.1 (aldehyde oxidase) inhibitor, a Daphnia magna metabolite, a mouse metabolite and a geroprotector. It is a 17beta-hydroxy steroid and an estradiol.

Biological function

Estradiol (E2), also spelled oestradiol, is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development of female secondary sexual characteristics such as the breasts, widening of the hips and a female-associated pattern of fat distribution. It is also important in the development and maintenance of female reproductive tissues such as the mammary glands, uterus and vagina during puberty, adulthood and pregnancy.It also has important effects in many other tissues including bone, fat, skin, liver, and the brain.

Though estradiol levels in males are much lower than in females, estradiol has important roles in males as well. Apart from humans and other mammals, estradiol is also found in most vertebrates and crustaceans, insects, fish, and other animal species.

Estradiol is produced especially within the follicles of the ovaries, but also in other tissues including the testicles, the adrenal glands, fat, liver, the breasts, and the brain. Estradiol is produced in the body from cholesterol through a series of reactions and intermediates.The major pathway involves the formation of androstenedione, which is then converted by aromatase into estrone and is subsequently converted into estradiol. Alternatively, androstenedione can be converted into testosterone, which can then be converted into estradiol. Upon menopause in females, production of estrogens by the ovaries stops and estradiol levels decrease to very low levels.

The development of secondary sex characteristics in women is driven by estrogens, to be specific, estradiol.These changes are initiated at the time of puberty, most are enhanced during the reproductive years, and become less pronounced with declining estradiol support after menopause. Thus, estradiol produces breast development, and is responsible for changes in the body shape, affecting bones, joints, and fat deposition.In females, estradiol induces breast development, widening of the hips, a feminine fat distribution (with fat deposited particularly in the breasts, hips, thighs, and buttocks), and maturation of the vagina and vulva, whereas it mediates the pubertal growth spurt (indirectly via increased growth hormone secretion)and epiphyseal closure (thereby limiting final height) in both sexes.

Metabolism

Inactivation of estradiol includes conversion to less-active estrogens, such as estrone and estriol. Estriol is the major urinary metabolite.[citation needed] Estradiol is conjugated in the liver to form estrogen conjugates like estradiol sulfate, estradiol glucuronide and, as such, excreted via the kidneys. Some of the water-soluble conjugates are excreted via the bile duct, and partly reabsorbed after hydrolysis from the intestinal tract. This enterohepatic circulation contributes to maintaining estradiol levels.

Estradiol is also metabolized via hydroxylation into catechol estrogens. In the liver, it is non-specifically metabolized by CYP1A2, CYP3A4, and CYP2C9 via 2-hydroxylation into 2-hydroxyestradiol, and by CYP2C9, CYP2C19, and CYP2C8 via 17β-hydroxy dehydrogenation into estrone,[54] with various other cytochrome P450 (CYP) enzymes and metabolic transformations also being involved.

Estradiol is additionally conjugated with an ester into lipoidal estradiol forms like estradiol palmitate and estradiol stearate to a certain extent; these esters are stored in adipose tissue and may act as a very long-lasting reservoir of estradiol.