15° 



METABOLISM 



take its place (Lintner and Dull, 1893; A. Meyer, 1895). Maltose betrays 

 its presence by the fluid being capable of reducing alkaline copper sulphate 

 (Fehling's solution). 



We do not yet know exactly how the change into sugar is effected in all 

 cases, but we must assume that the decomposition of starch is a gradual one, 

 dextrin being formed first of all, which later is changed into maltose. By treat- 

 ment with iodine it is often possible to distinguish a series of dextrins, but we 

 are quite unable to say in what relation these stand to starch. They appear 

 to have the same chemical composition as starch, differing from it only in the 

 smaller size of their molecules. The dextrin molecule is still a large one in 

 comparison to that of maltose, its molecular weight being about eighteen 

 times as great. The formation of maltose appears to be effected according to 

 the following equation : — 



C,.fiH36„Oi,o+i8H,0 - i8(C„H,Ai). 

 Dextrin Maltose. 



On the absorption of water a hydrolytic decomposition takes place ; but it 

 does not appear impossible that a similar hydrolysis occurs previously in the 

 formation of dextrin itself. 



The transformation of starch into sugar by the agency of malt extract 

 can be demonstrated in a few minutes in a test-tube kept at suitable tem- 

 perature. As in the case of germinating barley, so other germinating seeds yield 

 diastase after digestion with water or glycerine ; moreover, diastase may also 

 be shown to occur in many other amyliferous plant tissues as well as in digestive 

 secretions in the animal body (saliva ; pancreatic secretion). There is no doubt, 

 however, that diastase is by no means the same in character in each case ; 

 indeed, profound differences have been discovered to exist, not merely as re- 

 gards the products of the reaction but also with relation to the influence of 

 external factors. To all appearance there are several kinds of diastase. It is 

 very likely, for example, that the transformation of starch into dextrin is 

 effected by a diastase differing from that which decomposes dextrin into maltose. 

 By heating the malt extract to a temperature of about 80° C. (compare Duclaux, 

 1899, 400), its capacity for forming maltose is destroyed, although the formation 

 of dextrin still continues. Further, the decomposition of dextrin does not al- 

 ways occur in the same way ; often maltose only is produced, at other times 

 glucose appears as well. In the latter case a hydrolytic decomposition of the 

 maltose molecule into two molecules of grape sugar takes place. Those dia- 

 stases which produce maltose only may be distinguished from each other by their 

 intermediate products (compare Beijerinck, 1895). 



If we now compare the effect of diastases with that of a hydrochloric acid 

 solution, it would appear that the former have a more limited activity than 

 the latter. While one acid is sufficient to transform starch into glucose, three 

 different diastases are required, each having a restricted but definite part to 

 play in the total result. The same is true of other kinds of enzymes. The 

 enzymes are thus much more delicate agents than the acids, and to this is due 

 the importance attached to them in modern physiological chemistry. 



Inquiring now into the chemical characters of diastase, we have first of all 

 to note that our malt extract is by no means a pure solution of diastase, for as 

 yet it has not been found possible to isolate it completely from the other con- 

 stituents of the extract. If we add alcohol to the solution a precipitate is 

 obtained which gives the proteid reaction, and which, when dissolved in water, 

 exhibits the same power of dissolving starch that diastase has, though to a 

 rather less degree. If this solution be heated above 80° C, proteid, and with it 

 the diastase, separate out ; the latter no longer, however, has the power of dis- 

 solving starch. It might be thought, therefore, that diastase was a proteid which 

 is coagulable at high temperatures, but this view cannot be taken as proved, 

 for diastase might have, chemically, nothing to do with the proteid. It might 



