Anatomy & Physiology

Anatomy & Physiology Study Guide

Amino Acid Derivatives Amino acid derivatives are relatively small molecules that are structurally related to amino acids, the building blocks of proteins. This group of hormones, also known as the biogenic amines, is synthesized from the amino acids tyrosine and tryptophan. Tyrosine derivatives include thyroid hormones, created by the thyroid gland, and the compounds epinephrine (E), norepinephrine (NE), and dopamine, which are sometimes called catecholamines. Peptide Hormones Peptide hormones are chains of amino acids. In general, peptide hormones are synthesized as prohormones—inactive molecules that are converted to active hormones either before or after they are secreted. Peptide hormones can be separated into two groups. One group consists of glycoproteins. These proteins are more than 200 amino acids long and have carbohydrate side chains. The glycoproteins include thyroid-stimulating hormone (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the adenohypophysis (also called the anterior pituitary), as well as several hormones produced in other organs. The second group of peptide hormones is large and diverse; it includes hormones that range from short polypeptide chains, such as antidiuretic hormone (ADH) and oxytocin (9 amino acids apiece), to small proteins, such as growth hormone (GH; 191 amino acids) and prolactin (PRL; 198 amino acids). This group contains all the hormones secreted by the hypothalamus, heart, thymus, digestive tract, pancreas, and neurohypophysis (also called the posterior pituitary), as well as most of the hormones secreted by the adenohypophysis. Lipid Derivatives There are two classes of lipid derivatives: eicosanoids and steroid hormones. Eicosanoids are small molecules with a five-carbon ring at one end. These compounds are important paracrine factors that coordinate cellular activities and affect enzymatic processes (such as blood clotting) in extracellular fluids. Leukotrienes are eicosanoids released by activated white blood cells, also called leukocytes. Leukotrienes are important in coordinating tissue responses to injury or disease. Prostaglandins , a second group of eicosanoids, are produced in most tissues of the body. Within each tissue, the prostaglandins released are involved primarily in coordinating local cellular activities. In some tissues, prostaglandins are converted to thromboxanes and prostacyclins, which also have strong paracrine effects. Steroid hormones are lipids structurally similar to cholesterol. Steroid hormones are released by male and female reproductive organs (androgens by the testes; estrogens and progestins by the ovaries), by the cortex of the suprarenal glands (corticosteroids), and by the kidneys (calcitriol). Hormone release typically occurs where capillaries are abundant, and the hormones quickly enter the bloodstream for distribution throughout the body. Within the blood, hormones may circulate freely or be bound to special carrier proteins. A freely circulating hormone remains functional for less than one hour, and sometimes for as little as two minutes. It is inactivated when it diffuses out of the bloodstream and binds to receptors on target cells, it is absorbed and broken down by cells of the liver or kidneys, or it is broken down by enzymes in the plasma or interstitial fluids. For each hormone, an equilibrium state exists between the free and bound forms. As the free hormones are

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