64 ENZYMES OR SOLUBLE FERMENTS. 



been described both the secretion and the gland-substance contain a third 

 substance which has not as yet been isolated, of which therefore but 

 little is known from a chemical point of view, but which must be 

 regarded as an enzyme in virtue of the typical conditions under which 

 it is able to effect a hydrolytic decomposition of neutral fats into 

 glycerine and free fatty acid. Bernard first drew attention to the 

 existence of this enzyme 1 . It is most actively present in the substance 

 of the fresh gland or in its secretion, and may be extracted from the 

 former by means of glycerine or water. In every case it is essential to 

 ensure that the gland had not acquired an acid reaction before ex- 

 traction and that all acidification in the extract is absent, since the 

 enzyme is peculiarly sensitive to acids other than fatty and is readily 

 destroyed by them 2 . Hence a dilute alkaline solution should be em- 

 ployed, and according to Paschutin sodium bicarbonate mixed with 

 the normal carbonate is the most efficient solvent 3 . 



The presence of the enzyme is tested for by adding the extract to an emulsion of 

 oil of bitter almonds, or other neutral oil or fat, with gum arabic ; the mixture is then 

 most carefully neutralised and digested at 40 together with a minimal amount of 

 neutral sensitive litmus solution. In presence of the enzyme the mass turns more 

 or less rapidly red owing to the liberation of the free fatty acid. 



The enzymic nature of the active agent is shown by the fact that 

 its activity is greatest at about 40, is destroyed by boiling, and is 

 dependent upon the reaction of the digestive mixture, being greatest in 

 presence of a dilute alkali although it will show itself in a neutral 

 solution. It will also be observed that the decomposition which pialyn 

 effects is typically hydrolytic. 



Rennin. 



Extracts of the mucous membrane of the stomach of young animals 

 and more especially of the calf have been known, from time immemorial, 

 to possess a most remarkable power of causing milk to clot, and rennet 

 was commonly employed by the Romans for the manufacture of cheese. 

 The active agent in producing the clot was in more recent times 

 supposed to be either the acidity of the extract itself or the production 

 of lactic acid from milk-sugar (lactose) by means of some active 

 principle in the extract. Heintz and Hammarsten however showed 

 that this view is untenable, and we now know that the substance to 

 which the clotting is due is an enzyme to which the name of rennin 



1 Compt. Rend. T. xxvni. (1849), p. 249. See also his Lemons de pliysiol. 

 exper. T. n. (1856), p. 253. 



EPOf * J. J.J.* lJ.WWff| JJ 0W 



2 Griitzner, Pfliiger's Arch. Bd. xn. (1876), S. 302. 



3 Arch.f. Anat. u. Physiol. Jahrg. 1873, S. 386. 



