582 METABOLISM, NUTRITION AND DIETETICS 



from the proteins may be assumed to be that small fraction which 

 has already been spoken of as synthetically formed. 



Metabolism of the Nucleic Acids and Purin Bases. Our know- 

 ledge of the metabolism of the nucleo-proteins and nucleins has 

 been greatly augmented in recent years. When nucleo- protein is 

 digested by gastric juice, a certain amount of protein is easily split 

 off and hydrolysed to peptone and the other ordinary products 

 of proteolysis. An insoluble residue of nuclein remains. This is 

 acted upon with difficulty by gastric juice, although eventually an 

 active juice will split it up also. By the action of pancreatic juice, 

 or by heating with dilute acids, it is more easily hydrolysed, yielding 

 a further quantity of protein along with nucleic, acid. This second 

 fraction of protein, which is split off with so much more difficulty 

 than the first, undergoes proteolysis in the usual way. The result- 

 ing amino-acids no doubt take their place in the general metabolism 

 precisely like the amino-acids derived from ordinary proteins, and 

 yield the same end-products. As regards the nucleic acid (or rather 

 acids, since different nucleo-proteins contain different nucleic acids), 

 pancreatic juice is practically inert, although succus entericus can 

 effect a partial hydrolysis. For their complete decomposition more 

 drastic treatment is required namely, heating with hydrochloric 

 acid in a sealed tube. Thus treated, nucleic acids yield a number 

 of components, out of which they may be assumed to be built up, 

 as the proteins are built up out of amino-acids, etc. The charac- 

 teristic components are purin bases (adenin, C 5 H 3 N4.NH 2 ; guanin, 

 C 5 H 3 N 4 O.NH 2 ; hypoxanthin, C 5 H 4 N 4 O; and xanthin, C 5 H 4 N 4 O 2 ); 

 pyrimidin bases (uracil, C 4 H 4 N 2 O 2 ; cytosin, C 4 H 3 N 2 O.NH 2 ; thymin, 

 C 4 H 3 N 2 O 2 .CH 3 ) ; phosphoric acid and a carbo-hydrate group. 



Some of the nucleic acids contain all these components; they are 

 sometimes spoken of as the true nucleic acids. In others certain of the 

 components are absent, and to these nucleic acids the name nucleotids 

 has been applied. The purin bases are always present. The carbo- 

 hydrate group varies in different nucleic acids, being in some a hexose 

 (p. 529), in others a pentose (p. 482). The pentose d-ribose is especially 

 often met with. It is probable that the nucleotids are merely simpler 

 decomposition products of the true nticleic acids. . Thus, inosinic acid, 

 a nucleotid first isolated from meat extract, yields phosphoric acid, 

 d-ribose, and the purin base hypoxanthin. The nucleotid guanylic acid 

 found in the pancreas yields phosphoric acid, ^-ribose, and the purin 

 base guanin. There is evidence that nucleic acids may be built up out 

 of a number of nucleotid groups, and for this reason they have been 

 termed polynucleotids (Levene). The purin bases have a very close 

 chemical relationship to uric acid, which, like them, is characterized by 

 the possession of a group called the purin nucleus. For convenience 

 of reference the atoms composing the purin nucleus are numbered, 

 and the purin bodies are named with reference to the position of the 

 carbon atom or atoms at which oxygen or the amino-group (NH 2 ) is 

 introduced. Purin consists of the nucleus with H atoms introduced at 

 the points shown in the constitutional formula. Adenin is a 6-: mino- 

 purin i.e., purin in which NH 2 replaces the H attached to Q 6 ). Guanin 



