METABOLISM AND TRANSrORT IN ANIMALS 



521 



Digestion of protein 



No protein digestion occurs in the mouth. 

 In the stomach, the enzyme pepsin is effec- 

 tively aided by hydrochloric acid to hydro- 

 lyze proteins into proteoses and peptones. 



In the small intestine the enzyme trypsin 

 from the pancreas may reduce proteins and 

 partially digested proteins into amino acids. 

 In the small intestine, erepsin, an enzyme, 

 further acts on the split products of the 

 gastric digestion of proteins to form amino 

 acids. These amino acids are in a condition 

 to be absorbed. 



VITAMINS 



Vitamins are organic food substances, 

 present in very small amounts, and neces- 

 sary for normal metabolism and growth. 

 There is no generally accepted theory as to 

 the way in which they influence nutrition. 

 Some act as coenzymes, or influence enzyme 

 systems within cehs. Minute quantities are 

 effective; for example, only 0.5 mg. of vita- 

 min Bi is required daily by a healthy adult; 

 but the presence of vitamins is necessary in 

 our food, or deficiency diseases develop. 

 Plants and fish-liver oils furnish many vita- 

 mins, but some of them can now be manu- 

 factured in the laboratory. Rats, mice, and 

 birds have been used a great deal for experi- 

 mental work. The usual procedure is to 

 feed an animal on a diet normal in every 

 respect except for the absence of a particular 

 vitamin, and to observe the results, which 

 are usually similar to those that appear in 

 man when he is deprived of the same vita- 

 min. 



Originally vitamins were designated by 

 capital letters, but as their chemical struc- 

 ture became known, they have been given 

 chemical names. 



which in man is changed into vitamin A. 

 Lack of this vitamin disturbs the secreting 

 powers of mucous membranes; for example, 

 the lacrimal glands do not keep the eye 

 moist, which leads to a disease called 

 xerophthalmia, "dry eye." The eyesight is 

 impaired; resistance to certain infections de- 

 creases; and growth is retarded (Fig. 381). 

 Night blindness, inability to see in the dark, 

 has been known for centuries. This condi- 

 tion results from insufficient amounts of a 

 photochemical substance in the eye known 

 as visual purple, which is essential for good 

 vision in dim light. In order to regenerate 

 visual purple, vitamin A is required. Al- 

 though few individuals show the more pro- 

 nounced effects of vitamin A deficiency, 

 such as xerophthalmia, nevertheless, a large 

 number do suffer from some degree of night 

 blindness. The chief sources of vitamin A 

 are vegetables such as spinach, asparagus, 

 carrots, sweet potatoes, butter, cream, eggs, 

 liver, and fish-liver oils, especially halibut- 

 liver oil. 



Vitamin Thiamine 

 (Antineuritic Vitamin, 

 B1-C12H17N4OSCI) 



Vitamin Bi was the first member of the 

 B complex to be differentiated. It prevents a 

 disease known as beriberi, which is prevalent 

 among Oriental people who live on a diet 

 consisting largely of polished rice. Beriberi 

 is characterized by loss of appetite and de- 

 generative changes in the nervous system. 

 Thiamine is abundant in brewer's yeast and 

 whole cereals, that is, in cereals that have 

 not been polished or milled so as to discard 

 the vitamin, as is the case in polished rice 

 and wheat from which white bread is made. 

 It also occurs in peas, beans, nuts, and liver. 



Vitamin A (Antixerophthalmic 

 Vitamin C2uH.joO) 



The precursor of this vitamin occurs in 

 plants in the form of the pigment carotene. 



Vitamin Riboflavin (Vitamin 



G, B2— Cl7H2.)N40(5) 



Deficiency of riboflavin in the food re- 

 sults in soreness at the corner of the mouth 



