Term Paper on Hormones | Endocrine Glands | Biology

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Term Paper on Hormones


Term Paper Contents:

  1. Term Paper on the Introduction to Hormones
  2. Term Paper on the Types of Hormones
  3. Term Paper on the Synthesis of Hormones
  4. Term Paper on the Production of Hormones by Tumors
  5. Term Paper on the Hormone Secretion
  6. Term Paper on the Endothelial Transport of Hormones
  7. Term Paper on the Binding Proteins of Hormones


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Term Paper # 1. Introduction to Hormones:

The hormones are the chemical messengers and are secreted by specific tissues. They travel by way of the blood stream to remote organs, or structures and there exert control over, various body processes: Although ductless glands usually are designated as the principal source of hormones, other tissues—such, as the gut are, known, to be associated with hormone production.

In the field of plant biochemistry, hormone-like, substances are elaborated by non-glandular tissues. Because, the hormones act at sites distant to their origin their function may be likened to that of chemical messengers. Since only minute amounts are usually secreted at any one time the profound physiological changes, elicited indicate the great potency of the hormones.

Because of this high degree of activity, the hormones are considered to act catalytically in helping control various chemical reactions concerned with the maintenance and operation of the, body.

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The hormones, as a class of compounds, have three major functions in the body—namely integrative, regulative and morphological. A particular hormone may be involved in only one function. The integrative action is possible because the hormones travel through the blood from a source to their site of action enabling the body to function as unit in response to stimuli.

The regulatory actions of the hormones are related to almost all the homeokinetic reactions of metabolism, salt and water balance, etc.. The very term “homeokinetic” refers to be dynamic main lance of the constant body environment. Certain of the hormones play important roles in controlling the rate and type of growth of the body.

The trophic hormones of the pituitary and the adrenocortical and thyroid hormones arc examples of this latter type. From another point of view the hormones are considered to be kinetic, metabolic, or morphogenetic. Other definitions and generalizations on the subject of hormone action are given by several authors.

Difficulty is occasionally encountered in differentiating hormone action from the action of vitamins. As pointed out the hormones are formed within the body and there exert their effects. Vitamins to a large extent are supplied in the diet. For many of the vitamins moreover a direct metabolic role has been assigned.

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Their participation in specific enzyme systems has been established. In addition, these enzyme systems are generally widespread, throughout the body. The as yet unknown specific roles of most of the hormones together with the fact that often specific tissues are regulate further differentiate hormones, and vitamins.

In some phases of biochemistry the student is concerned only with one particular family, or group of compounds, such as carbohydrates, fats, or proteins. The chemistry of the hormones, however, knows no such close confines, for the hormones belong to various types of organic compounds.

As pointed out by Li, the hormones can be grouped according to their chemical similarities as (a) phenol derivatives-adrenaline, noradrenaline, thyroxin, (b) proteins-insulin, anterior pituitary hormones, human chorionic gonadotropin, pregnant mare serum gonadotropin, thyroglobulin, and certain (c) peptides—corticotropin (AM), vasopressin, and oxytocin; (d) steroids—estrogens, androgens, progesterone, and adrenal corticoids. The hormones then are studied as a group not because of chemical similarities, but because of their related: physiological role of regulation coordination of vital processes.


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Term Paper # 2. Types of Hormones:

Hormones can be classified into five groups according to structure, synthesis, and mechanisms of action –  (1) Peptides and proteins, (2) Catecholamines, (3) Steroids and vitamin D, (4) Thyronines, and (5) Others.

The peptides and proteins and the catecholamines seem to have their major action at the cell surface. Although more than 100 different hormones exist, less than 20 are of major clinical significance, and only these are emphasized.

The proteins and peptides are the largest category of hormones and consist of oligopeptides, polypeptides, proteins, and glycoproteins. The oligopeptides, hormones with less than 20 amino acids, are cleaved from a larger protein precursor, as is the non-peptide oxytocin. Many of these hormones are closely related to neurotransmitters and are secreted by neurons or other cells of the brain.

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The posterior pituitary hormones— vasopressin, oxytocin, and vasotocin—are directly secreted by neurons of the hypothalamus. The hormones that influence anterior pituitary function—thyroid-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), somatostatin, and others-are secreted by hypothalamic and other brain cells. Other oligopeptide hormones include the gastrointestinal hormones secretin and cholecystokinin.

The polypeptide hormones can be arbitrarily defined as those proteins with 21 to 150 amino acids. This is a large category that comprises most of the proteinaceous hormones made outside of the brain. It includes the hormones of calcium metabolism parathyroid hormone and calcitonin), the hormones of glucose metabolism (insulin, glucagon, and pancreatic polypeptide), the growth factors (insulin-like growth factors I and II, epidermal growth factor. nerve growth factor, relaxin, fibroblast growth factor, and various less well-defined polypeptide growth factors), and the pituitary hormone adrenocorticotropin, or adrenocorticotropic hormone (ACTH).

The protein hormones are a fairly small category, consisting the related proteins—growth hormone (somatotropin, GH), prolactin and human chorionic somatomammotropin (human placental lactogen, HPL or HCS)—and the unrelated protein, renin.

The remaining hormones in this category are glycoproteins- follitropin (follicle-stimulating hormone, FSH), lutropin (luteinizing hormone, LH), human chorionic gonadotropin (HCG), thyrotropin (thyroid-stimulating hormone, TSH), and erythropoietin.

All the polypeptides, protein hormones, and glycoproteins are synthesized by the standard protein synthesizing machinery of the cell. Many have posttranslational modifications, including amidation of the carboxyl terminus, blockage of the amino terminus, and glycosylation. All the hormones in this category bind to receptors at the plasma membrane. Many stimulate adenylate cyclase, some activate tyrosine kinases, and some have unknown mechanisms of action.


Term Paper # 3. Synthesis of Hormones:

Hormones are synthesized in glands, which may be entire organs (e.g., the thyroid gland), parts of other organs (e.g., the cluster of secretory cells in the islets of Langerhans in the pancreas), or just a few cells (e.g., the neuroendocrine secretory cells in the crypts of the microvilli of the small intestine). Hormones may be synthesized by specific enzymes or as proteins by the normal protein synthesizing machinery of the cell. The proteins are often post transnationally modified to form the active hormones.

Enzymatic Synthesis of Hormones:

Most non-protein hormones are synthesized by specific enzymatic pathways. The precursor molecules are common bio-chemicals, such as the amino acids tyrosine and tryptophan or cholesterol. These pathways can be very complex and require many enzymes. Cortisol synthesis, for example, requires seven enzymes that must work in a specific order.

The coordinated regulation of these enzymes is not understood in humans, but it is unlikely that cells other than specialized endocrine secretory cells or their precursors could produce these hormones, even in disease states. Cholesterol is the precursor for the steroids and vitamin D, tyrosine for the catecholamines, and thyronines and tryptophan for melatonin and serotonin. The prostaglandins are produced from highly unsaturated fatty acids.

Ribosomal Synthesis of Hormones:

The protein hormones are synthesized on the ribosomes of the rough endoplasmic reticulum (ER). The pathway may be very complex. Most protein hormones are synthesized as pre-prohormones and must be processed further to produce the finished hormone. Pre-hormones are produced in the ER. The pre-piece, also called the leader sequence, is the first few amino acids of the protein, often 20 to 30 amino acids in size.

These amino acids are very hydrophobic and are thought to be necessary for the movement of the protein across the membrane lipid bilayer of the rough ER. The leader sequence is always rapidly cleaved from the newly synthesized protein. This process is so rapid that these early hormonal precursors are difficult to demonstrate in animal cells.

The existence of a leader sequence was demonstrated when messenger ribonucleic acid (mRNA) for hormones was translated using plant ribosomes. The leader sequence has not been demonstrated to be present in the circulation in any human disease.

After the leader sequence has been cleaved, the remaining protein, called a pre-hormone, is usually considerably larger than the final hormone and often has little or no biologic activity. It is thought that the extra size is necessary to allow the protein to fold properly.

Once the final disulfide linkage and quaternary structure is set, the extra portions are cleaved from the final hormone by proteolysis. This process often starts in the Golgi apparatus and continues in the secretory granule. The cleaved extra peptides remain in the secretory granule and are secreted with the hormone.

Insulin is made through this process. The first peptide synthesized, preproinsulin, is rapidly cleaved to a 9000-dalton peptide, proinsulin. After proper folding and disulfide bond formation, the extra piece in the middle of the polypeptide chain is cleaved out by a trypsin like proteolytic process.

This produces a 5600-dalton, two-chain insulin molecule; the 2500-dalton extra peptide, called the C-peptide; and four basic amino acids. When insulin is secreted, a mixture of substances is released by the islets of Langerhans.

About 6% of proinsulin is unprocessed and is secreted intact. Equimolar ratios of insulin and C-peptide are also released. In some patients whose insulin cannot be measured, one can still measure this C-peptide to determine the amount of insulin secretion.

Many proteins and peptides may also undergo posttranslational modification. The carboxyl terminus may be amidated, as is GnRH; the amino terminus may be acetylated, as is calcitonin, or otherwise blocked; or the proteins may be glycosylated, as is TSH. No hormones are known to have covalent fatty acids attached.


Term Paper # 4. Production of Hormones by Tumors:

Frequently diseases are caused by excesses of hormones. They may be made by the gland that normally makes the hormone, for example, an excess of thyroid hormone produced by the thyroid gland in Grave’s disease or by some other tissue, usually a cancer. If the cell making the excess hormone is normally differentiated to form this hormone, it is called eutopic production.

Thus, excess insulin production by the islets of Langerhans or even by a tumor of the Islets (called an islet cell tumor) is eutopic. Tumors of gland tissue that produce these hormones eutopically do not need to be in the normal location. Thus, a choriocarcinoma (tumor of the placenta) producing chorionic gonadotropin would be considered eutopic, even if the tumor was metastatic to the lungs.

Occasionally, cells make hormones that are not part of their normal differentiation path, this is called ectopic production of hormones. These cells are almost always malignant tumors. For example, a small cell carcinoma of the lung may make chorionic gonadotropin or ACTH. Ectopic production of hormones is always of protein or peptide hormones. No well-established cases of ectopic production of steroids, catecholamines or thyronines have been reported.


Term Paper # 5. Hormone Secretion:

The protein hormones made in the rough ER with the help of the leader sequence enter vesicles that bud from the rough ER and move to the Golgi apparatus. Hormones created by enzymatic synthesis are made in the smooth ER and again appear, entering vesicles that bud and go to the Golgi apparatus. Hormones are further modified by the Golgi apparatus and packaged into the secretory vesicles.

These secretory vesicles move toward the cell membrane along microtubules. Overall movement occurs at about 10 µ/minute. Secretion, at least in some cells, is prevented by microfilaments of the cell web that block fusion of the vesicles with the plasma membrane. The secretory stimulus causes clumping of the microfilaments, opening holes and allowing fusion of the vesicles and exocytosis of the hormone.

  The secretory vesicles normally accumulate and may store a large amount of hormone for later release. Some glands have developed specialized methods of storing hormones. The cells of the thyroid gland form a follicle, a sphere like structure in which the cells form the surface of the sphere.

The cells store the thyroid hormone as part of a complex protein called thyroglobulin, which is stored extracellularly in the center of the sphere. When the thyroid hormone is needed, the cells recover the thyroglobulin and degrade it to the thyroid hormones, which are then secreted.

Role of the Blood:

The vascular system is important as the main route of dispersion of the hormones, but its role may be considerably greater. The vascular system may regulate the amount of hormone reaching the tissues, and it may have binding proteins that alter the levels of free hormone.

Circulations may be specific and targeted, such as the portal circulations, which deliver very high levels of hormones to specific organs, or the general circulation, which delivers similar blood concentrations of hormones to all organs.

Portal Circulation:

Two portal circulations exist. The hypothalamic-hypophyseal portal circulation delivers high levels of the hypothalamic-releasing factors to the pituitary gland. The levels of these factors may be 100 times higher in this blood than in the general circulation.

The hepatic portal circulation brings the pancreatic and gastrointestinal hormones to the liver at levels that are about tenfold higher than in the general circulation. These special circulations may allow certain hormones to have greater effects on the pituitary gland and the liver than on other tissues.


Term Paper # 6. Endothelial Transport of Hormones:

The vascular system may not be passive in the distribution of hormones. The entire vascular system is lined with a continuous layer of endothelial cells that prevents simple diffusion of the protein and other hormones. The endothelial cells specifically bind these hormones on their vascular surface and transport them across the cell, releasing them on the tissue side of the cell.

Alterations in the rate at which these endothelial cells transport the hormones may make major differences in the levels of hormone that reach the tissues. Although this is currently an area of active research, there are no known examples of disease caused by problems with endothelial transport.


Term Paper # 7. Binding Proteins of Hormones:

Certain hormones do not circulate free in the plasma but rather are tightly bound to specific binding proteins. The thyroid hormones, thyroxine and triiodothyronine, are more than 99% bound to thyroid-binding globulin and other serum proteins. Likewise, testosterone is more than .90% bound to sex hormone- binding globulin, Cortisol more than 90% bound to cortisol-binding globulin, and the insulin-like growth factors more than 99% bound to specific binding proteins.

The posterior pituitary hormones, vasopressin and oxytocin, are bound to the neurophysins in the hypothalamus and are transported down the pituitary stalk with them. Once released from the posterior pituitary glands, the hormones are freed from the neurophysin and are transported in blood in unbound form.

The binding of hormones to these proteins alters the properties of the hormones significantly. The plasma levels of active hormone is lowered because only the free hormone is active. The hormone often lasts longer in the circulation since the bound hormone is neither degraded nor excreted by the kidney. Finally, the binding to these proteins buffers the organism against extremes of hormone concentration.

Complications with Binding Proteins:

The binding proteins also complicate the evaluation of the thyroid and adrenal glands and androgens. Only the free hormone is active, but most assays measure the total hormone. Since the binding proteins are usually constant, the free hormone is in direct proportion to the total hormone. However, this is not always the case.

Thyroid binding is increased by oral contraceptives and is sometimes missing in patients with an X-linked recessive condition. In these cases patients with normal free hormone levels might have elevated or reduced total hormone levels.


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