Insulin

Insulin, hormone, produced in the islets of Langerhans of the pancreas, that regulates the metabolism of carbohydrates, fats, and starches in the body. Like other proteins, insulin is partially digested if administered orally and hence must be injected into a muscle when used clinically. In the treatment of diabetes mellitus, which is caused by a deficiency of insulin production or by inhibition of its action on cells, insulin is often combined with protamine, which prolongs the period of absorption of the hormone. Insulin crystallized from the pancreas contains zinc, which also lengthens absorption. A preparation called protamine zinc insulin extends the hormone's action still further.

Insulin was first extracted from the pancreatic tissue of dogs in 1921 by the Canadian physiologists Sir Frederick Grant Banting and Charles Herbert Best and the British physiologist John James Rickard Macleod. The Canadian biochemist James Bertram Collip then produced it in sufficiently pure form to be injected into humans. The molecular structure of insulin was determined in 1955 by the British biochemist Frederick Sanger; it was the first protein to be deciphered. Human insulin, the first human protein to be synthesized, was made in 1965. In 1981 insulin made in bacteria by genetic engineering became the first human hormone obtained in this way to be used to treat human disease. For the biochemistry of insulin, see Sugar Metabolism.

Hormone

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.

Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.

Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones are steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action.