122 BIOLOGICAL CHEMISTRY 



Removal of proteins by trichloracetic acid (or other 

 protein precipitant) and estimation of the nitrogen 

 remaining in the solution. 



Associated with the digestive action of gastric juice is the 

 power of coagulating milk. The coagulation is due to the 

 enzyme rennin acting upon the caseinogen of the milk. The 

 coagulation occurs in two stages. The first is due to the action 

 of rennin in producing a soluble product (paracaseinogen) 

 from the caseinogen and the second is due to the calcium of 

 the milk forming an insoluble substance (casein) from the 

 paracaseinogen. 



There is some doubt as to whether pepsin and rennin are the 

 same or different enzymes. Some investigators state that 

 there are two separate enzymes, whilst others declare that 

 there is only one. A modification of these views is that there 

 is only one substance but that the actions are due to two 

 different side chains in the same molecule. Rennin action 

 is found throughout the animal and vegetable kingdoms 

 wherever a proteoclastic enzyme occurs. The only animals 

 that receive milk are mammals, so the presence of a milk- 

 coagulating enzyme in other animals is difficult to explain. 

 It seems that the coagulation of milk may be the first stage 

 of proteoclastic activity and the coagulation is a purely 

 accidental circumstance.* There does not seem to be any 

 functional advantage of the coagulation as the acid of the 

 gastric juice is capable of precipitating caseinogen from its 

 solution. The fact that it is possible to prepare a rennin 

 solution with little or no peptic action and a pepsin solution 

 with little or no rennin action indicates that they are two 

 separate enzymes. 



In the gastric juice is found a Lipase which acts upon 

 emulsified fats but not upon unemulsified fats.j 



Like the salivary secretion the secretion of gastric juice is 

 caused by a reflex consisting of impulses from the organs of 

 sight, smell, taste, etc., to the central nervous system and 

 from the central nervous system to the stomach by the vagus 

 nerves. The secretion is not so prompt as the secretion of the 

 salivary glands as stimulation of the nerves going to the 

 stomach is followed by a latent period of five minutes before 

 the secretion appears. 



* H. M. Vernon, Intracellular Enzymes (John Murray), 1908, p. 



!57- 



f W. Marcet, Proc. Roy. Soc., 1858, vol. 9, p. 306; F. Volhard, 

 Zeit. f. klin. Med., 1901, vol. 42, p. 414. 



