NATURAL ESTROGENS AND PROGESTINS
Biologically important natural estrogens and progestins include estradiol, estrone, estriol, and progesterone. Estradiol-17 is the most potent estrogen that is found naturally in women. Estrone is one-tenth as biologically active as estradiol, and estriol is the weakest of the three. Estriol is synthesized by the placenta and is ex-creted at high levels in the urine of pregnant women. Progesterone is the most important naturally occurring progestin.
The ovary is the major site of estrogen and progestin biosynthesis in nonpregnant premenopausal women. In pregnant women, the fetoplacental unit is the major source of estrogens and progestins. Peripheral sites of estrogen synthesis include the liver, kidney, brain, adi-pose tissue, skeletal muscle, and testes. Progesterone is secreted in small amounts by the testes and adrenal gland. The combined estrogen and progestin production by all of these peripheral sites amounts to 10% or less of ovarian synthesis in normal premenopausal women. In postmenopausal women, ovarian steroid synthesis declines and peripheral estrogen biosynthesis accounts for all estrogen produced, both in postmenopausal women and in males.
The naturally occurring estrogens and progestins are not orally active because they are rapidly metaboli-cally inactivated. The major site of estrogen and prog-estin metabolism is the liver. Both are subject to first-pass metabolism. Metabolites are also formed in the gastrointestinal tract, brain, skin, and other steroid tar-get tissues. Estrogens and progestins are primarily ex-creted in the urine. Estrone, estradiol, 2-methoxye-strone, and their respective glucuronide or sulfate conjugates are the most abundant estrogen urinary metabolites. Progesterone is excreted as pregnanediol or as a pregnanediol conjugate. A small fraction (10% or less) of the estrogen metabolites enter the bile, where they may undergo enterohepatic recirculation before elimination.
Plasma proteins bind estrogens and modulate estro-genic activity. More than 90% of estradiol in the blood-stream is protein bound, with sex hormone–binding globulin (SHBG) being the major serum estrogen-binding moiety. Estrogens that are bound to SHBG are biologically inactive because of their high binding affin-ity, while estrogens that are bound loosely to serum al-bumin are available for entry into tissues and are there-fore biologically active. Progesterone in plasma is 89% protein bound. Progesterone binds with a relatively high affinity to the serum protein corticosteroid-binding globulin and also to albumin.