CARBOHYDRATES 



67 



is the chief sugar obtained on hydrolysis. The best known sea weed 

 mucilage is agar, which is widely used in bacterial culture media because 

 of its property of forming gels. 



Hetero polysaccharides of animals 



Several carbohydrates of this type occur in small amounts in the animal 

 body. Hyaluronic acid, a polysaccharide composed of equimolar portions 

 of D-glucosamine acetate and D-glucuronic acid residues, forms a viscous, 

 gel-like material present in connective tissues, eyes (aqueous and vitreous 

 humor), joints (synovial fluid), and various other organs. It functions 

 as a cementing substance between the cells of connective tissue (so-called 

 "ground substance") and, because of its viscosity, resists penetration by 

 foreign matter, i.e., infection by bacteria. Hyaluronic acid is attacked 

 and liquefied by an enzyme, hyaluronidase, which is present in some 

 bacteria, in certain animal tissues, and in the poisonous secretions of 

 many reptiles and other animals. This enzyme, to the extent that it 

 is present, contributes to the rapid spread of toxic agents throughout 

 the body ; it is therefore of great medical interest. 



Chondroitin sulfate, a major component of cartilage, is a heteropoly- 

 saccharide made up of D-glucuronic acid and D-galactosamine acetate 

 residues, with some of the hydroxyl groups esterified by sulfuric acid. 

 In the living animal it is probably attached through its carboxyl and 

 sulfate groups to the amino groups of proteins. Mucoitin sulfate, present 

 in mucosa (e.g., stomach mucosa and gastric juice), is similarly consti- 

 tuted except that it contains D-glucosamine acetate residues. 



Another animal polysaccharide of considerable importance is heparin, 

 a natural anticoagulant (inhibitor of blood coagulation) , which occurs 

 in the liver, muscles, and other organs of the body. The component 

 building blocks of heparin, as shown by their formation on complete 

 hydrolysis, are D-glucuronic acid, D-glucosamine, and sulfuric acid. It 

 is noteworthy that no acetic acid is involved since in the other animal 

 polysaccharides, which contain amino sugars, the amino group is acety- 

 lated {i.e., combined with acetic acid to form the acetylamino group, 

 — NHCOCH3). Furthermore, heparin contains more sulfate residues 

 than most of the other sulfate-containing carbohydrates discussed above. 

 According to Wolfrom and co-workers the repeating unit in heparin is 

 a tetrasaccharide composed of two residues of glucosamine and two of 

 glucuronic acid plus five sulfate radicals. An unusual feature of the 

 heparin structure is the union of the amino group of each glucosamine 

 unit with a sulfate radical to form a sulfamic acid group, — NHSO3H. 

 The sulfuric acid here takes the place of acetic acid in other animal 

 polysaccharides. 



