Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Hormones
Thymus hormones

The Role of the Thymus as an endocrine gland has been known for a long time. It is also well established that shortly after birth, the thymus supplies lymphoid Cells to the Lymph Nodes AND Spleen, and synthesizes and secretes specific HORMONES that influence the development and maturation of certain lymphoid tissue cells. However, the Chemical Nature of these hormonally active preparations remained unknown, although animal experiments clearly demonstrated that a Cell-free extract of the Thymus gland affects both overall organismal growth and the development and maintenance of immunological competence, thereby ensuring the normal functioning of cellular and humoral Immunity.

To date, several hormones have been isolated from thymus extracts and characterized; these are primarily low-molecular-weight Polypeptides. They affect various types of lymphoid cells performing specific Functions. Below is the Introduction/19.html">Primary Structure of thymopoietin II, isolated from the calf thymus, which appears to be the primary hormone stimulating T-lymphocyte formation.

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Thymopoietin II consists of 49 amino acid residues*. It is hypothesized that the active center of the hormone is a pentapeptide (highlighted in red, occupying positions 32–36 from the N-terminus). Recently, this short five-member peptide has been chemically synthesized and named "thymopentin-5"; when administered into the body, it enhances nonspecific defense factors.

* A similar 49-residue hormone was recently isolated from the bovine spleen. Named splenine (synonym: thymopoietin III), it differs by containing Glu instead of Asp at position 34.

Another hormone isolated from the calf thymus by A. Goldstein et al. is thymosin a1 (consisting of 28 amino acid residues) with the following structure:

It is hypothesized that thymosin aj performs a regulatory function in the body during the late stages of T-Cell Differentiation. It has also been shown to exhibit pronounced pharmacological activity in the Treatment of leukemias and immune deficiencies.

Recently, a novel thymus hormone (a nonapeptide) that induces T-cell differentiation has been obtained. The manifestation of its biological activity requires the presence of divalent zinc ions. This zinc-containing hormone possesses a distinctive configuration.

In addition to Peptide Hormones, an active nonpolar fraction, biologically similar to Steroid Hormones and designated as thymosterin, has been isolated from the thymus; its chemical nature has not yet been elucidated.

This chapter does not cover all hormonal substances known to date. For instance, in the Pineal Gland (epiphysis), an interesting yet poorly studied hormone, melatonin, is synthesized from The amino acid Tryptophan. More than 20 biologically active hormones have been isolated from the digestive tract. The best studied among them are gastrin I and gastrin II (17 and 14 amino acid residues, respectively), which regulate gastric juice secretion; progastrin (34 amino acid residues), considered to be the circulating prohormone form that is converted into active gastrin I within the target organ cells; as well as Glucagon and secretin (27 amino acid residues) (the latter being the first substance identified as a hormone). Furthermore, Somatostatin is synthesized in the intestinal mucosa. It has been suggested that interstitial somatostatin and glucagon regulate the secretion of hormones synthesized in the Hypothalamus and Pancreas, respectively. Information on Other Hormones, including Plant Hormones, can be found partially in chapters 12 and 17, or in specialized literature.