J 70 Comparative Animal Physioloo^ 



appear able to synthesize cellulose; the test (coat) of several tunicates CPhallu- 

 sta and Molgida') contains a structural carbohydrate which appears to be 

 identical with plant cellulose. *'*'• ^'''^ 



Another polysaccharide which is synthesized by some animals and plants is 

 chitin. This is a nitrogen-containing compound which is split by chitinase to 

 acetyl-glucosamine. Very few animals possess chitinase. Several other plant 

 polysaccharides are occasionally digested by animals. These polysaccharides 

 include lichenin, pectin, xylan, inulin, and others. 



Carbohydrascs illustrate well the various types of specificity of enzymes. ^'^■' 

 (1) Absolute or type specificity is shown by enzymes attacking different 

 members of a class. Amylase attacks starch, dextrin, or glycogen. Disacchari- 

 dases are effective only on small carbohydrate polymers. Glucosidases are 

 probably distinct from galactosidases. (2) Stereochemical specificity is shown 

 by the fact that an enzyme such as maltase attacks many a-glucosides, whereas 

 ^S-glucosides are attacked only by /^-glucosidases. (3) Relative specificity or 

 specificity within a class is shown by the different rates at which a given a- or 

 /J-glucosidase attacks various substrates within the class specified. 



The best known carbohydrascs are from plants and especially yeasts, in 

 which these enzymes occur in great variety. Animal biochemists have been 

 concerned more with the enzymes of carbohydrate utilization (intermediary 

 metabolism) than with those of carbohydrate hydrolysis (digestive metab- 

 olism). In general, carbohydrascs are not restricted to a very narrow pH range, 

 but for the most part they function near neutrality. 



Distribution of Polysaccharidases. The majority of mammals have a pan- 

 creatic amylase which attacks starch and glycogen, and intestinal enzymes 

 acting on maltose, sucrose, and lactose. Probably the same enzyme acts on 

 maltose and sucrose, but at different rates. Most investigations of carbohydrascs 

 in digestive fluids and tissue extracts of other animals have been limited to the 

 few substrates which are used by man. However, the number of carbohydrascs 

 of many invertebrates, although not so great as the number of carbohydrascs 

 of some bacteria, yeasts, and fungi, greatly exceeds the number of enzymes 

 found in mammals. Data on those animals which have been studied with 

 respect to various carbohydrate substrates are given in Tables 28 and 29. 



Starch Digestion. Animal amylases are a-glucosidases. The amylolytic 

 activity of three protozoans diminishes in the following order: Amoeba > 

 Paramecium > Frontonia. *"' An extract of Pelomyxa yields an active amy- 

 lase. •^•* However, Mast states that Amoeba proteus cannot digest starch if it 

 has not ingested live food recently, and that the amoeba takes over the amylase 

 from such organisms as Chilomonas, on which it preys. In coelenterates 

 amylase acts intraccllularly in mesenteric filaments and may be optimal in a 

 slightly acid medium. '^•'' In annelids an extracellular amylase occurs in 

 stomach and intestine. Sahella digests starch and glycogen best at pH 6.8. *-'' 

 Starch is said not to be digested by rhabdococl flatworms. '^■"' In madrepore 

 corals which contain symbiotic zooxanthcllae in the mesenteric filaments, 

 there is in the symbionts a weak amylase \\'ith pl 1 optimum at 4.75; the coral 

 may in addition have a glycogcnase with a pi I optimum at 6.5. ^**** 



Herbivorous molluscs have an active amylase, in either the hepatopancreas 

 or style, or in both. Amylase is found in salivary glands of Aplysia ''' and 

 Limnaea. •*'-' Also the amoebocytcs of the digestive tract of the oyster can 



