CHEMICAL BASIS OF THE ANIMAL BODY. 1189 



alysis shews that their composition approximates more nearly to 

 that of a proteid than of any other class of substances, and this 

 is apparently true even when they do not yield to any marked 

 degree the reactions (xanthoproteic, etc.) "which are character- 

 istic of a true proteid. Ordinarily it is almost impossible to 

 obtain an enzyme solution of any considerable activity which 

 is free from proteid reactions, and hence many authors are 

 inclined to regard these bodies as being really of proteid nature. 

 But this is a point which is as yet by no means settled ; the 

 balance of recent opinion appears to be in favour of the view 

 that the enzymes are proteid in nature, but this is still an open 

 question. 



The enzymes are possessed of certain properties more or less 

 common to them all, by means of which they may be separated 

 from the tissues in which they primarily occur, and isolated 

 from the solutions thus obtained. Soluble in water, they may 

 be precipitated unchanged from this solution loy the addition 

 of an excess of absolute alcohol. They may also in many cases 

 be precipitated from their aqueous or other solution by satura- 

 tion with neutral ammonium sulphate. They are conveniently 

 soluble in glycerin, from which they may as before be precipi- 

 tated by an excess of alcohol. None of the enzymes are dif- 

 fusible and hence they may readily be freed from any admixed 

 diffusible substances by means of dialysis. They possess fur- 

 ther the remarkable property of adhering with great tenacity to 

 any finely divided precipitate which is formed in the solutions 

 in which they are present, more particularly if the precipitate is 

 of a viscid or gelatinous nature. In some cases the enzymes do 

 not exist in the free and active conditions in the cells of the 

 respective tissues but in the form of an inactive antecedent to 

 which the name of 'zymogen' is usually applied. Hence to 

 obtain an active extract it is frequently necessary to treat the 

 tissue with some such reagent as shall ensure the conversion of 

 the zymogen into the active enzyme. 



During prolonged digestions it is essential to ensure the 

 absence of any changes due to the development of bacteria or 

 other organisms. The most suitable antiseptics for this pur- 

 pose are salicylic acid (-1 p.c.) and tlrymol (-5 p.c.). These 

 reagents are dissolved in a small quantity of alcohol and added 

 in the above proportions to the digestive mixture. 



It is frequently a matter of the utmost importance to deter- 

 mine whether the hydrolytic power of any given preparation is 

 due to the action of a soluble enzyme or of a ferment (organ- 

 ized). The discrimination is most readily effected by carrying 

 on the digestion in presence of chloroform, which is inert 

 towards the enzymes but inhibits the activity of ferment or- 

 ganisms. 



