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Introduction

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High-Yield Terms

  • Androstanes: any 19-carbon steroid hormone, such as testosterone or androsterone, which controls the development and maintenance of male secondary sex characteristics

  • Estranes: any 18-carbon steroid hormone, such as estradiol and estrone, produced chiefly by the ovaries and responsible for promoting estrus and the development and maintenance of female secondary sex characteristics

  • Pregnanes: any 21-carbon steroid hormone, such as progesterone, responsible for changes associated with luteal phase of the menstrual cycle, differentiation factor for mammary glands

  • Cytochrome P450, CYP: any of a large number of enzymes produced from the cytochrome P450 genes, are involved in the synthesis and metabolism of various molecules and chemicals

  • Glucocorticoids: any of the group of corticosteroids predominantly involved in carbohydrate metabolism

  • Mineralocorticoids: steroid hormones characterized by their influence on salt and water balance

  • Hormone response elements, HREs: specific nucleotide sequences, residing upstream of steroid target genes, that are bound by steroid hormone receptors

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The Steroid Hormones

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The steroid hormones are all derived from cholesterol. The conversion of C27 cholesterol to the 18-, 19-, and 21-carbon steroid hormones (designated by the nomenclature C with a subscript number indicating the number of carbon atoms, eg, C19 for androstanes) involves the rate-limiting, irreversible cleavage of a 6-carbon residue from cholesterol, producing pregnenolone (C21).

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Common names of the steroid hormones are widely recognized, but systematic nomenclature is gaining acceptance and familiarity. Steroids with 21 carbon atoms are known systematically as pregnanes, whereas those containing 18 and 19 carbon atoms are known as estranges and androstanes, respectively. The important mammalian steroid hormones are shown in Table 50-1 along with the structure of the precursor, pregnenolone.

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TABLE 50-1:Structures of the Mammalian Steroid Hormones
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All the steroid hormones exert their action by passing through the plasma membrane and binding to intracellular receptors. The mechanism of action of the thyroid hormones is similar; they interact with intracellular receptors. Both the steroid and thyroid hormone-receptor complexes exert their action by binding to specific nucleotide sequences in the DNA of responsive genes. These DNA sequences are identified as hormone response elements, HREs. The interaction of steroid-receptor complexes with DNA leads to altered rates of transcription of the associated genes.

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Steroid Hormone Biosynthesis

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The particular steroid hormone class synthesized by a given cell type depends upon its complement of peptide hormone receptors, its response to peptide hormone stimulation, and its genetically expressed complement of enzymes. Luteinizing Hormone (LH) stimulates the synthesis of progesterone and testosterone. Follicle-stimulating hormone (FSH) stimulates the synthesis of estradiol. Adrenocorticotropic hormone (ACTH) stimulates the synthesis of the glucocorticoids, principally cortisol. Angiotensin II stimulates the production of aldosterone.

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Many of the enzymes of steroid hormone biosynthesis are members of the cytochrome P450 family of ...

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