Steroid hormone biosynthesis and actions in the materno-feto-placental unit

Because steroids are lipophilic, they diffuse easily through the cell membranes, and therefore have a very large distribution volume. In their target tissues, steroids are concentrated by an uptake mechanism which relies on their binding to intracellular proteins (or " receptors ", see below). High concentration of steroids are also found in adipose tissue, although this is not a target for hormone action. In the human male, adipose tissue contains aromatase activity, and seems to be the main source of androgen-derived estrogens found in the circulation. But most of the peripheral metabolism occurs in the liver and to some extent in the kidneys, which are the major sites of hormone inactivation and elimination, or catabolism (see below).

Sex hormone-binding globulin (SHBG) is thought to mainly function as a transporter and reservoir for the estradiol and testosterone sex hormones. However it has also been demonstrated that SHBG can bind to a cell surface receptor (SHBG-R). The SHBG-R has not been completely characterized. A subset of steroids are able to bind to the SHBG/SHBG-R complex resulting in an activation of adenylyl cyclase and synthesis of the cAMP second messenger. [19] Hence the SHBG/SHBG-R complex appears to act as a transmembrane steroid receptor that is capable of transmitting signals to the interior of cells.

FIGURE 2. Major Pathways of Steroid Biosynthesis.
The pathways outlined here are common to the adrenals, the gonads and, to some extent, to the fetoplacental unit. The first committed step is the conversion of cholesterol to pregnenolone, catalysed by the P-450scc enzyme, which is under pituitary hormone control (ACTH or LH depending on the tissue). Cholesterol side-chain removal is blocked specifically by aminoglutethimide , a steroid biosynthesis inhibitor. From pregnenolone, steroid biosynthesis can proceed either through the so-called "delta-5" pathway (17α-hydroxypregnenolone, dehydroepiandrosterone, testosterone), or through the "delta-4" pathway (progesterone onwards). Progesterone is the starting point for mineralocorticoid synthesis, whereas glucocorticoids are derived from its metabolite, 17α-hydroxyprogesterone. Estrogens are formed from androgens (androstenedione and/or testosterone). Most reactions are irreversible (as denoted by a single arrow). Reversible reactions (double arrows) depend on cofactor availability (. the NADP/NADPH ratio). [Abbreviations used here for the various enzymes are listed in the figure].

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Steroid hormone biosynthesis and actions in the materno-feto-placental unit

steroid hormone biosynthesis and actions in the materno-feto-placental unit

  1. Encyclopædia Britannica articles are written in a neutral objective tone for a general audience.
  2. You may find it helpful to search within the site to see how similar or related subjects are covered.
  3. Any text you add should be original, not copied from other sources.
  4. At the bottom of the article, feel free to list any sources that support your changes, so that we can fully understand their context. (Internet URLs are the best.)

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