1 64 NUTRITION OF FARM ANIMALS 



After abstracting the elements of urea, there remains con- 

 siderably over half the hydrogen and oxygen of the protein and 

 the larger part of its carbon. A substantially similar result is 

 reached in case of the other nitrogenous metabolic products. 

 The splitting off of these products from the proteins leaves a 

 non-nitrogenous residue. 



230. Two stages of protein katabolism. Two general stages 

 in the katabolism of the proteins may be distinguished. The 

 first is a hydrolysis by which the proteins are split up into their 

 constituent amino acids. The second is a deaminization of 

 the amino acids in which the nitrogen of these acids is split off 

 as ammonia. 



231. Protein hydrolysis. The first stage in the katabolism 

 of the body proteins is a hydrolytic cleavage, more or less 

 similar to that effected in digestion and like the latter brought 

 about by enzyms, which in this case are contained in the body 

 cells the intracellular enzyms (209). 



The truth of this view is attested by the facts that the pres- 

 ence of proteases in almost all of the tissues and organs of the 

 body has been demonstrated and that under proper conditions 

 they effect a rapid solution of the tissue proteins the so-called 

 autolysis. Further confirmation is afforded by the known 

 facts regarding the transformation of one protein into another 

 in the body, while finally the production in the organism of 

 some of the cleavage products of the proteins, presumably as 

 products of katabolism, may be indirectly shown. 



232. Is protein hydrolysis a reversible process ? If the 

 katabolism of body proteins is initiated by an enzymatic cleav- 

 age in the body cells, this is precisely the reverse of the syn- 

 thetic action by which it is believed that body proteins are 

 built up out of the products of digestive cleavage (226), and 

 the question at once arises whether we have to do here with a 

 reversible enzym reaction, analogous to that which has been 

 suggested as occurring in the case of the carbohydrates (214), 

 the general nature of which may be represented by the formula 



Protein ^ Amino acids 



It must be freely admitted that proof of the reversibility of 

 the action of proteolytic enzyms is as yet lacking, such phenom- 

 ena as the formation of the plasteins discovered by Okunew 



