152 METABOLISM 



HANSTEEN (p. 144) has drawn certain conclusions as to the formation of 

 proteid from the disappearance of starch on the addition of asparagin ; these 

 researches show, however, that he was only dealing with an acceleration of the 

 diastatic activity by asparagin. 



Whether such an accelerating agent be present or not, a solution of diastase 

 is unable to transform the whole of the starch into maltose usually a certain 

 amount remains in the form of dextrin. Doubtless the reason for this is not 

 that the diastase is used up after the dissolution of a certain quantity of starch, 

 but that its power of transforming starch into sugar is inhibited by the accumu- 

 lation of the products of the reaction. If adequate arrangements be made for the 

 removal of the sugar formed, all the dextrin is finally turned into maltose, 

 and theoretically only a small amount of diastase is necessary to transform an 

 unlimited quantity of starch, without its losing its diastatic power in the process. 



Quite a number of peculiarities, which we have now recognized as belonging 

 to the diastases, are found in other substances formed by the organism, and to 

 these bodies has been given the name of enzymes or ferments. Those with 

 which we are at present concerned induce hydrolytic decompositions ; later on we 

 shall have to study enzymes which bring about decompositions otherwise than 

 by hydrolysis (Lecture XVI). These enzymes act in very small quantities and 

 take no part, or at least no permanent part, in the reaction. The reaction is 

 always incomplete and may be accelerated or retarded by certain substances ; 

 their activity is dependent on temperature in the same way as we have seen 

 that of diastase to be. The enzymes may be extracted from the organism by 

 water or by glycerine, from which media they may be precipitated by alcohol. 



Each individual enzyme apparently attacks only one or at most a few 

 related bodies. We may distinguish at least five classes of enzymes, although 

 apparently their number is much greater and their spheres of operation much 

 more limited : 



1. Amylases, or diastases, which transform starch into sugar. 



2. Cytases, which manufacture sugar from cellulose and the other carbo- 

 hydrates associated with it in forming the cell-wall. 



3. Invertases, which change disaccharides into monosaccharides, e. g. cane 

 sugar into dextrose and levulose, maltose into two molecules of dextrose. 



4. Lipases, which break up fats into glycerine and fatty acids. 



5. Proteases, which act on proteids and produce from them diffusible 

 bodies already enumerated elsewhere (p. 140). 



In addition to these specific (hydrolytic) activities, the enzymes (all?) 

 possess the power of splitting off oxygen from peroxide of hydrogen. In many 

 respects, more especially in their dependence on temperature and many chemi- 

 cals, the enzymes resemble organisms themselves, and it has for long been the 

 custom to regard them as portions of the protoplasm, or, at least, as very highly 

 complex substances. But it is by no means necessary to make such an assump- 

 tion, since enzymes resemble in many ways a series of inorganic bodies which 

 possess peculiar characteristics, and which are known as catalytic agents, and 

 it is now the custom more than ever to regard the activities of enzymes as 

 catalytic in their nature. 



Catalytic agents are those which alter the rate of a reaction without them- 

 selves entering into the final product (OsrwALD, 1902). The catalytes which 

 specially interest us for the moment are those which accelerate reactions. As 

 a type of such catalysis we may take the decomposition of hydrogen-peroxide 

 into oxygen and water in the presence of finely divided metals. Peroxide of 

 hydrogen, it is true, also decomposes spontaneously, but the separation of 

 oxygen takes place much more rapidly in presence of the metal, a very small 

 quantity of the catalyte decomposing a very large amount of the peroxide 

 without suffering any loss of power in the process. But the catalytic value of 

 the metal is entirely dependent on its finely divided condition ; platinum wire 



