1194 ENZYMES OR SOLUBLE FERMENTS. 



stance contain a third substance which has not as } T et 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 glycerin and free 

 fatty acid. 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 glycerin or water. In every case it is es- 

 sential to ensure that the gland had not acquired an acid reac- 

 tion before extraction 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. Hence a dilute 

 alkaline solution should be employed, and sodium bicarbonate 

 mixed with the normal carbonate is the most efficient solvent. 



The enzymic nature of the active agent is shewn by the fact 

 that its lipolytic activity is greatest at about 40°, is destined 

 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 ob- 

 served that the decomposition which lipolyn 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 substance to which the 

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

 be conveniently given. 1 The enzymic nature of the active agent 

 in rennet is clearly shown by the typical relationship which it 

 exhibits in its activity to the reaction of the solution in which 

 it is present, to the temperature at which its activity is great- 

 est, to the fact that the briefest exposure to 100° or the more 

 prolonged exposure to lower temperatures (70° or above) suffices 

 to destroy its active properties and to the fact that a minute 

 trace suffices to clot a relatively enormous amount of casein. 



Aqueous and glycerin extracts of the gastric mucous mem- 

 brane are usually found to be active in clotting milk, but the 

 activity of a faintly acid extract is in all cases greater. This 

 is due to the existence of a rennin zymogen (renninogen) which 

 is readily converted into the enzyme by the action of acids. 

 The preparation of highly active and permanent solutions of 

 rennin is of considerable commercial importance in connection 

 with the cheese-making industry. The most efficient extrac- 



1 This name seems more convenient than the more commonly used expres- 

 sions 'the rennet ferment' or ' the milk-curdling ferment.' 



