NUTRITION 263 



Methods of estimating the amount of nitrogen combined in the form of NH 9 

 are of value in determining approximately whether a given protein is a mixture 

 of several simpler ones or is one single large molecule. We have also a means of 

 following the degree of hydrolysis in the course of digestion or under the action 

 of acid. There are two methods available for this purpose. 



Sorenseris method depends on the fact that NH 2 groups react with formaldehyde 

 to form methylene-imino groups : 



RNH 2 + CHOH = RNCH 2 + H 2 O. 



When proteins are acted on by formaldehyde, the basic groups are eliminated and 

 the free carboxyl can be titrated with acid in the usual way. This fact was shown 

 by Schiff (see Plimmer's monograph, 1912, 1913). 



Van Slyke's method depends on the fact that primary amino-groups react with 

 nitrous acid thus : 



RNH 2 + HNO 2 = ROH + H 2 O + N 2 . 

 The nitrogen gas evolved is measured (van Slyke, 1912 and 1913). 



By these methods we find that proteins do not contain more than one per cent, 

 of their nitrogen in the NH form. 



There are several classes of proteins, distinguished by their different properties, 

 in addition to those conjugated with other substances, such as nucleins or carbo- 

 hydrates. The nomenclature of these substances has been agreed upon by the 

 Chemical and Physiological Societies of England and America and should be 

 always used, if confusion is to be avoided. It will be found in Plimmer's 

 monograph (1912, pp. 1 and 2). , 



We have next to inquire what becomes of the proteins taken as food before 

 the end products of their metabolism are excreted. A word may first be said as 

 to the meaning of this " metabolism." It was, as far as I can find out, first 

 suggested by Michael Foster in his "Textbook of Physiology." It is used to 

 express the chemical changes which take place in the various food-stuffs after 

 they have passed from the alimentary canal into the blood. The changes produced 

 by the digestive enzymes are usually excluded. 



In the space that is available in such a book as the present one, it is impossible to describe 

 in detail all the numerous facts which are known as to these phenomena, important as they 

 are. For the subject of protein metabolism, the reader should consult the monograph by 

 Cathcart (1912). 



There is strong evidence to show that the proteins of the food are completely 

 hydrolysed into amino-acids, before being absorbed. For some time, however, it 

 was held that re-synthesis to proteins took place in the wall of the intestine. 

 This view was due to the fact that it had not been possible to detect amino-acids, 

 nor even peptones, in the blood. It seems, on a priori grounds alone, that such 

 an immediate resynthesis would be a very inappropriate one. Suppose that a 

 particular tissue protein is to be built up and that this protein, while containing 

 glycine, contains very little glutamic acid, also that another cell protein contains 

 no glycine, but a large amount of glutamic acid. Now, if the synthetic protein 

 supplied by the blood contained the right proportion of amino-acids for the one, 

 a large quantity of it would have to be taken up by the other, in order to satisfy its 

 requirements, and the remaining part of it, containing the excess of the particular 

 amino-acid not wanted, would be wasted. Even if this last were utilised in some 

 other way, it seems a useless process for a protein to be synthesised in the wall 

 of the intestine, merely to be broken up again when it reaches the cells. 



Direct evidence, moreover, is not wanting at the present time showing that 

 amino acids do actually exist in the blood. According to van Slyke and Meyer 

 (1912), in the first place, the blood of a dog which has received no food for twenty- 

 four hours contains amino-acids equivalent to 4 mg. of nitrogen per 100 c.c. If 

 meat is given, the value rises to 10 mg. during digestion. 



The reason why the quantity is so small is that these amino-acids are rapidly taken up by 

 the tissues in some form, as we shall see later ; it was found, for example, that if 12 g. of 

 alanine were injected into a vein of a dog during ten minutes, only 1'5 g. remained in the 

 blood five minutes later, although only To g. had been excreted by the kidney. 



