316 CHEMISTRY OF THE DIGESTIVE PROCESSES. 



a gland which has been minced up and allowed to stand for a few hours (it 

 may be in nearly all cases made faintly acid with very dilute acetic acid to set 

 free the zymogen as enzyme). Such a minced-up gland is rubbed up in a 

 mortar with some clean sand, taken up with glycerin, shaken up with more 

 glycerin (10-20 parts to 1 part of gland), and allowed to stand so until 

 required ; the process of extraction is very slow, and requires from seven to 

 fourteen days. In the case of gastric mucous membrane, 1 part per 1000 of 

 hydrochloric acid may be added to the glycerin. 



There are many modifications of the process. v. Wittich recommends 

 digesting the minced gland (or mucous membrane) twenty-four hours in alcohol, 

 drying after this in the air, sifting the powder through gauze to remove 

 coarser fragments of tissue, and extracting with glycerin. It is often recom- 

 mended to filter the extract after seven to fourteen days, but this is unnecessary, 

 as the tissue neither decomposes nor becomes digested in the glycerin, and 

 the extract improves on keeping in contact with the tissue. The enzyme 

 accompanied by proteid may be precipitated from a glycerin extract by the 

 addition of absolute alcohol, and so a purer extract be obtained. 



Chemical nature of enzymes. The failure of all attempts to isolate 

 pure enzymes necessarily deprives us of the possession of any certain 

 knowledge of the chemical nature of these substances. Analyses of the 

 purer preparations of the enzymes give figures approximating to those 

 obtained with the various proteids ; but whether or not this is due to 

 admixture with proteid it is at present impossible to say. The behaviour 

 of Briicke's " pure " pepsin solution goes against the supposition that this 

 enzyme is a proteid. This solution did not give the proteid reactions, 

 and was not precipitated by any of the proteid precipitants, save neutral 

 and basic lead acetates and platinic chloride. These results are confirmed 

 by Sundberg, 1 who succeeded in preparing a still more proteid-free solu- 

 tion, which did not even react to these reagents, and was only precipi- 

 tated as a slight, pure white, nocculent precipitate, on adding five to six 

 times its volume of absolute alcohol and allowing to stand, and yet was 

 exceedingly active in digesting fibrin. The amount of this precipitate 

 was much too small for analysis, and it could only be shown that it was 

 nitrogenous, and contained a certain amount of ash. This is not quite 

 conclusive against the proteid nature of the active substance, since, as 

 Sundberg argues, the physiological test by digestion may be much more 

 delicate than any of the purely chemical tests. Still, the fact that it 

 was totally unaffected by tannic acid and precipitated by alcohol has 

 some weight against the substance being proteid in nature ; since tannic 

 acid will show 1 part of ordinary proteid in 100,000, 2 and alcohol is by 

 no means so delicate a proteid test. It is most probable, then, that 

 pepsin is not a proteid ; and it will subsequently be seen in the descrip- 

 tion of the other enzymes that most of these have been obtained in 

 forms which do not yield all the proteid reactions. 



The enzymes are soluble in water, from which they are precipitable 

 by saturation with ammonium sulphate or by adding excess of alcohol. 3 

 Most of them are unalterable, or very slowly alterable in contact with 

 alcohol, but pepsin is an exception, being attacked and rendered inactive 

 if left long in contact. The enzymes are commonly said to be soluble 



1 ZtscJir. f. physiol. Chem., Strassburg, 1885, Bd. ix. S. 319. See also under " Ptyalin." 



2 Hofmeister, ' Ztschr. /. physiol. Chem., Strassburg, 1878-9, Bd. ii. S. 292. 



3 These may only be particular cases of their general mechanical precipitation, whenever 

 a precipitate is caused. 



