Hormone molecules that affect the metabolism of target cells are often derived from less active "parent," or precursor, molecules. In the case of polypeptide hormones, the precursor may be a longer chained prohormone that is cut and spliced together to make the hormone. Insulin, for example, is produced from proinsulin within the beta cells of the islets of Langerhans of the pancreas (see fig. 3.25). In some cases, the prohormone itself is derived from an even larger precursor molecule; in the case of insulin, this molecule is called preproinsulin. The term prehormone is sometimes used to indicate such precursors of prohormones.
In some cases, the molecule secreted by the endocrine gland (and considered to be the hormone of that gland) is actually inactive in the target cells. In order to become active, the target cells must modify the chemical structure of the secreted hormone. Thy-roxine (T4), for example, must be changed into T3 within the target cells in order to affect the metabolism of these cells. Similarly, testosterone (secreted by the testes) and vitamin D3 (secreted by the skin) are converted into more active molecules within their target cells (table 11.3). In this text, the term prehormone will be used to designate those molecules secreted by endocrine glands that are inactive until changed by their target cells.
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