174 Comparative Animal Physiology 



starch can also digest a-glucosides. It is likely that the stereochemical specificity 

 of the enzyme which attacks the polysaccharide is similar to that of the enzyme 

 which attacks the sugar. No vertebrate has evolved a cellulase of its own, and 

 none has a /;^-glucosidase. Certainly the discovery of a way to introduce and 

 maintain cellulose digestion in the human digestive tract would go far to 

 circumvent the situation postulated by Malthus. 



Other polysaccharides than starch, glycogen, and cellulose are not much 

 used as animal foods. In its carbohydrases the snail Helix is the most versatile 

 animal of those which have been studied (Table 28). Some early reports of 

 widespread digestion of inulin, pectin, and chitin require authentication and 

 are not included in Tabic 28. It would be interesting to know if all animals 

 which digest inulin also have a /3-fructosidase. Lichenin is digested by many 

 invertebrates— sponges, earthworms, crayfish, tunicates, '*•'' and many insects, 

 1.^0, 164 i^^f probably not by any vertebrates. 



Distribution of Glycosidases. Disaccharidc and trisaccharide sugars must 

 be broken down to their component hexoses before they can be absorbed. The 

 number of animal enzymes is limited, compared with the number present in 

 plants; these enzymes show stereochemical specificity and it is probable that 

 several sugars of one type are digested by the same enzyme. Table 29 shows 

 the distribution of glycosidases, as indicated by the sugars digested or utilized 

 by those animals in which a series has been tested. Maltase (a-glucosidase) 

 is more generally distributed than any other carbohydrase, in combination 

 with amylase or separately. Sometimes a weak maltase occurs with the amylase, 

 as in human saliva or in the crystalline style, and a stronger maltase elsewhere 

 as in the human intestine or in the molluscan diverticula. Maltase, like other 

 hexosidases, has a broad pH optimum, usually weakly acid, pll 6 to 7. The 

 pH optimum for the maltase of several molluscs and crustaceans is lower than 

 for vertebrate maltase. In the silkworm the pf^ optimum for intestinal maltase 

 is 6.8, whereas the pH of the intestinal contents is very alkaline, pH 9.5; hence 

 it is probable that starch is digested to maltose in the lumen (amylase optimum 

 pH 9.5), and that digestion by maltase is completed inside the epithelial 

 cells. ^•''^ Mammals have a strong intestinal and an extremely weak pan- 

 creatic maltase, whereas in fish and frogs there is a strong pancreatic maltase; 

 reptiles have more actixe intestinal maltase. ^^~ 



/Vnimal sucrase (saccharase) is an a-glucosidase; yeast sucrase (invertase) 

 is a /i-fructosidase. Maltase activity and sucrase activity go together, and the 

 pfl optima are identical or closely similar. There may be differences in the 

 rates at which the two substrates are attacked. Separate enzymes attacking 

 maltose and sucrose have not been isolated from any animal, and it is entirely 

 possible that the same enzyme acts on both sugars. ^■''•' Existence of such 

 activity is rendered more probable by the fact that when one a-glucoside is 

 digested by a tissue extract or digestive fluid, other a-glucosides are also at- 

 tacked (Table 29). The few exceptions noted were reported in older obserxa- 

 tions (e.g., on pulmonates,'"'* on Chrysnps,''''^ and on /l/'is^"^), and these 

 should be re-examined. An enzyme of the a-glucosidasc class is probably 

 present in all animals. 



Other sugars are not so unixersallv digested. Lactose is digested by a 

 /i^-galaclosidase in the mammalian intestine but is said not to be digested in 

 the intestine of turtle or carp. "'" Lactose is not digested by some crustaceans 



