Steroid hormone
Steroid hormones are steroids which act as hormones. They can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestins. Vitamin D derivatives are a sixth closely related hormone system with homologous receptors, though technically sterols rather than steroids.
The natural steroid hormones are generally synthesized from cholesterol in the gonads and adrenal glands. 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 assembled from two or more separately synthesized proteins. In most of the hormone systems known, binding results in cleavage of a smaller protein from the receptor. Cleavage of this "heat shock protein" may occur in either the cytoplasm or the nucleus. Once in the nucleus, the steroid-receptor ligand binds to DNA in chromatin and induces transcription of its target genes.
The principal natural human steroid hormones:
The principal sterol hormone:
