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The natural steroid hormones are generally synthesized from cholesterol in the gonads and adrenal glands. These forms of hormones are lipids. They can enter the cell membrane quite easily and enter right into the nuclei. Steroid hormones are generally carried in the blood bound to specific carrier proteins such as sex hormone binding globulin or corticosteroid binding globulin. Further conversions and catabolism occurs in the liver, other "peripheral" tissues, and in the target tissues.
Because steroids and sterols are lipid soluble, they can diffuse fairly freely from the blood through the cell membrane and into the cytoplasm of target cells. In the cytoplasm the steroid may or may not undergo an enzyme-mediated alteration such as reduction, hydroxylation, or aromatization. In the cytoplasm, the steroid binds to the specific receptor, a large metalloprotein. Upon steroid binding, many kinds of steroid receptor dimerize: two receptor subunits join together to form one functional DNA-binding unit that can enter the cell nucleus. In some of the hormone systems known, the receptor is associated with a heat shock protein which is released on the binding of the ligand, the hormone. Once in the nucleus, the steroid-receptor ligand complex binds to specific DNA sequences and induces transcription of its target genes.
Synthetic steroids and sterols
A variety of synthetic steroids and sterols have also been contrived. Most are steroids but some non-steroidal molecules can interact with the steroid receptors because of a similarity of shape. Some synthetic steroids are weaker, some much stronger, than the natural steroids whose receptors they activate.
Some examples of synthetic steroid hormones:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Steroid_hormone". A list of authors is available in Wikipedia.|