448 



ANIMAL BIOCHEMISTRY 



cogen and plant starches are hydrolyzed by higher animals in their 

 digestive tracts. Hydrolysis proceeds to the glucose stage, and this 

 substance is then distributed by the circulatory system and used di- 

 rectly or converted to glycogen and fat for storage. 



Many mammals cannot digest most other polysaccharides, including 

 the pentosans, inulin, cellulose, hemicellulose. Apparently there has 

 been no evolutionary development in higher animals providing for 

 the direct hydrolysis of such widespread plant materials by animal 

 enzymes. However, some species, particularly the ruminants, possess 

 gastrointestinal systems especially adapted for bacterial hydrolysis of 

 polysaccharides. The hydrolytic products are then metabolized by 

 the routes available for monosaccharides. This adaptation in some of 

 the herbivores makes available a large part of the ingesteci cellulose 

 and at least part of the other polysaccharide material. The general 

 operation of ruminant digestion has been discussed beginning on 

 page 429. 



Of the disaccharides, sucrose is a major plant sugar, and its utiliza- 

 tion would be advantageous to plant-eating animals. Enzymes vari- 

 ously called sucrases, invertases, or saccharases occur in mammalian 

 intestinal tracts and split sucrose into glucose and fructose. The sugar 

 maltose, formed during the digestion of starch and glycogen, is 

 cleaved in the intestinal tract by maltases to glucose. A third disac- 

 charide, lactose, is of particular importance to young mammals since 

 it is the principal carbohydrate of milk. Once again the hydrolysis is 

 intestinal and catalyzed by enzymes, in this case called lactases. 



The three hexoses, glucose, fructose, and galactose, thus made avail- 

 able in quantity plus the less common mannose are phosphorylated 

 by means of ATP, Mg++, and specific kinases according to the mecha- 

 nism: 



HO 



H OH 



a-D-glucose 



glucokinase 



ATP 



ADP 



H, 



CH2OPOI 

 —0 



HO 



vOH H 



H OH 



D-gIucose-6-phospha 



This same enzyme also catalyzes the conversion of D-mannose to 

 D-mannose-6-phosphate. In the case of o-galactose the enzyme is galac- 

 tokinase and leads to D-galactose-1 -phosphate rather than the 6 isomer. 

 The situation is a little more complex for D-fructose with two known 

 pathways. First, D-fructose is phosphorylated as above for D-glucose 



