SECTION II 

 NUCLEIN OR PURINE METABOLISM 



IN an undifferentiated cell the proteins, as such, form but a small 

 part, the mass of the cell being composed of conjugated proteins. 

 The nucleo-proteins are especially abundant constituents of nuclei, 

 and therefore occur to a greater or less extent in all the ordinary 

 animal foods, eggs and milk excepted. Just as the metabolism of 

 proteins is the metabolism of the amino-acids, so the metabolism 

 of the nucleo-proteins and nucleins is essentially comprised in the 

 history of its main constituents, i.e. the purines. 



The nucleo-proteins themselves are bodies of very varying composi- 

 tion. If any cellular tissue such as thymus or liver be extracted with 

 water or salt solution, a fluid is obtained from which a precipitate 

 can be thrown down by the addition of acid. This precipitate as a 

 rule is soluble in excess of acid or in alkalies. If subjected to gastric 

 digestion it undergoes solution, leaving behind a residue of nuclein 

 which is rich in phosphorus. The amount of this residue varies 

 with the strength of the artificial gastric juice employed, so 

 that the method cannot be looked upon as in any way quantitative, 

 and the question arises whether the original nucleo-protein is to be 

 regarded as an association or a combination of nuclein with ordinary 

 protein. The most convenient source for the preparation of nucleins 

 is the heads of fish spermatozoa. All nucleins are associations or 

 compounds of nucleic acids with proteins belonging to the class of 

 protamines or histones. The nucleins of fish spermatozoa contain 

 protamine as one of their constituents. On separating off the 

 protamine, nucleic acids can be isolated. These acids have been 

 named either according to their source or according to the purine base 

 which is their most prominent constituent. Only from inosinic acid, 

 the nuclein acid of muscle, has it been found possible to prepare 

 crystalline derivatives, so that in all other cases it is difficult to 

 decide whether we are dealing with chemical individuals or with 

 mixtures. 



On hydrolysing any of the nucleic acids by heating with strong 

 mineral acid, they are broken down into a series of bodies belonging 

 to the following four groups : (1) phosphoric acid, (2) purine bases, 

 (3) pyrimidine bases, (4) a carbohydrate. The purine bases obtained 

 from the hydrolysis of nucleic acid are guanine, adenine, hypoxanthine, 

 and possibly xanthine. The last two bodies are probably secondary 



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