470 DIGESTION 



Three rather distinct pectic enzymes are generally recognized: proto- 

 pecttnase, pectase, and pectinase. Protopectinase catalyzes the reaction by 

 which protopectin is converted into pectin. Pectase, which can properly be 

 classed among the esterases, results in the hydrolysis of pectin to pectic acid 

 and methyl alcohol, while pectinase results in the hydrolysis of pectin, pec- 

 tates or pectic acid to arabinose, galactose, and galacturonic acid. The chem- 

 ical aspects of these various transformations among the pectic compounds have 

 already been discussed in Chap. XXII. 



Protopectinase is found both in the higher plants and in certain parasitic 

 fungi. Pectase has been found in a large number of species of plants, and 

 seems to be especially abundant in the leaves. Pectinase, like protopectinase, 

 is found principally in the fungi, although it is not entirely unknown in 

 the higher plants. Certain fungi are apparently able to penetrate the middle 

 lamella of plant cells by secreting this enzyme and digesting the constituent 

 pectic compounds. 



Hetniccllulases (cytases) are enzymes which accomplish the hydrolysis 

 of the various hemicelluloses into hexoses and pentoses. The presence of 

 hemicellulase has been demonstrated in the endosperm of the seeds of various 

 species, especially those in which hemicelluloses are relatively abundant. Hemi- 

 cellulases are also found in some species of bacteria and fungi, and in some 

 invertebrate animals, including certain insects. 



Cellulose, the cellulose digesting enzyme, results in the hydrolj'sis of 

 cellulose into cellobiose (Chap. XXII), a reaction analogous to that oc- 

 curring in the digestion of starch to maltose. Cellobiose is further hydrolyzed 

 by the enzyme cellobiase to glucose. These reactions are as follows: 



_ , , „ Cellulase „ 



(CeHioOs)^ + n H2O > n CisH.sOn 



Cellulose Cellobiose 



C12H22OU + H2O > 2 CeHisOe 



Cellobiose Glucose 



Cellulase has not been isolated from the higher plants, but is found in certain 

 bacteria and fungi, particularly those species responsible for the decay of 

 wood, straw, leaves and other cellulose-containing plant residues. Some of 

 the higher animals, particularly those of the herbivorous group, apparently 

 can also digest cellulose, but this has been found to be due to the presence in 

 the digestive tract of certain species of cellulose-decomposing bacteria. 



Emulsin or yS-glycosidase catalyzes the hydrolysis of any of the j8-glyco- 

 sides such as amygdalin, salicin, phloridzin and arbutin (Chap. XXII). It is 

 not a single enzyme, but a mixture of at least two — amygdalase and prunase. 

 Emulsin is widely distributed in the plant kingdom. It is especially abundant 



