NITROGENOUS METABOLISM 153 



der some conditions so as to be inactive. It is even con- 

 ceivable that it may facilitate the combining of the build- 

 ing-stones. Some enzymes, like other catalysts, may favor 

 the progress of reactions either in one direction or the 

 reverse, according to the proportions of the substances 

 present at the moment. 



It is not easy to estimate the extent of protein synthesis 

 which normally takes place. Of course, it is more promi- 

 nent during growth than during adult life. The present 

 impression is to the effect that only a very small fraction 

 of the usual protein income is thus used. Most people 

 can materially reduce the quantity of protein in the diet 

 and still remain in nitrogenous equilibrium. When the 

 lowest level at which this is possible has been attained it 

 is still true, as we have just pointed out, that the amount 

 of new protein constructed is but a fraction of that sup- 

 plied. It is, therefore, certain that under average con- 

 ditions by far the larger part of the nitrogenous food eaten 

 never exists in the form of proteins after absorption. We 

 must now consider the destiny and value of the uncombined 

 building-stones. 



The surplus amino-acids, either free or in simple com- 

 binations, are borne away from the intestine in the portal 

 blood. Accordingly, these digestive products, like the 

 sugars, are brought under the influence of the liver-cells. 

 They undergo a transformation in this organ and very 

 probably elsewhere which has important consequences. 

 This is the process described as "deaminization." To 

 deaminize an amino-acid is to remove from it the group 

 to which it owes its name, the radicle NH 2 . One of the 

 products of this reaction will be non-nitrogenous, the other 

 will contain nitrogen in a greatly increased percentage. 

 We cannot claim to know all the steps which are gone 

 through in this connection, and we shall not discuss those 

 which are known. We shall emphasize simply the final re- 

 sults. 



The chief nitrogenous compound which issues from the 

 series of reactions occurring in the liver is urea. This is 



