- Steroid hormone
Steroid hormones are
steroids which act as hormones. Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestagens. Vitamin Dderivatives are a sixth closely related hormone system with homologous receptors, though technically sterols rather than steroids.
The natural steroid hormones are generally synthesized from
cholesterolin 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.
ynthetic 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:
prednisone, dexamethasone, triamcinolone
* Vitamin D:
oxandrolone, nandrolone(also known as anabolic steroids)
norethindrone, medroxyprogesterone acetate
Steroids exert a wide variety of effects mediated by slow genomic as well as by rapid nongenomic mechanisms. They bind to
nuclear receptorin the cell nucleusfor genomic actions. Membrane-associated steroid receptors activate intracellular signaling cascades involved in nongenomic actions.
Because steroids and sterols are
lipidsoluble, they can diffuse fairly freely from the blood through the cell membraneand into the cytoplasmof 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 proteinwhich is released on the binding of the ligand, the hormone. Once in the nucleus, the steroid-receptor ligand complex binds to specific DNAsequences and induces transcription of its target genes.
* Steroidogenic enzymes: [http://science.co.il/hi/pub/1992-JSBMB-43-779.asp Review on structure, function, and role in regulation of steroid hormone biosynthesis]
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