BODY TISSUES 313 



mucosa, is secreted in response to the presence of fatty foods in the 

 intestine. It functions by slowing down the discharge of the gastric 

 chyme so as to permit more complete digestion of the lipides in the 

 intestine. 



Extracts of intestinal mucosa contain a factor which stimulates the 

 contraction of the gall bladder. This hormone, cholcystokinin, ap- 

 pears to resemble secretin chemically in that it is a basic polypeptide 

 with similar solubility and electrophoretic properties. 



Pancreas. This organ functions not only as a source of pancreatic 

 juice but also as an endocrine tissue. When this gland is removed, 

 animals develop diabetes, a metabolic disorder in which the body 

 loses its power to oxidize sugars and fats. In this disease the blood 

 sugar becomes abnormally high (hyperglycemia) and glucose is dis- 

 charged into the urine. I'he latter condition is called glycosuria. The 

 pancreatic hormone, which prevents diabetes, has been isolated in crys- 

 talline form and is known as insulin. 



Insulin is secreted by special areas of cells in the pancreas, known 

 as the islands of Langerhans. When these cells become diseased and 

 fail to function, insulin is not secreted and diabetes results. 



The complete amino acid sequence in the insulin molecule has been 

 elucidated in a classical study by a group of English biochemists. It is 

 a protein with a molecular weight of 12,000 g./mole. The protein can 

 be split by mild reduction into two pairs of different peptide chains. 

 The four peptide chains are cross linked through disulfide bonds 

 ( — S — S — ). If these linkages are destroyed by reducing agents, such 

 as hydrogen sulfide or cysteine, resynthesis of an active hormone by 

 oxidation does not occur. This has led to the suggestion that folding 

 occurs in the original chains, and that on release the chains shift so 

 that proper orientation for the cross links of disulfide groups is not 

 possible. 



Insulin used for medical purposes is prepared commercially from 

 pancreatic glands of sheep, beef, and hogs. It cannot be administered 

 orally because it is destroyed by proteolytic enzymes. Therefore it is 

 injected in the form of the water-soluble hydrochloride. Such insulin 

 disappears rather rapidly so that control of blood sugar is only tem- 

 porary. However, the disappearance can be delayed by injecting a 

 zinc-protamine-insulin complex or a globin-zinc-insulin complex. 

 These forms are slow to act. Frequently a mixture of the regular and 

 slow-acting types is injected to get both immediate and prolonged 

 control. If injection of excessive quantities lowers the blood sugar to 

 the point where insulin shock occurs, injection or oral consumption 



