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 composition. 

 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 

 nucleic acid of muscle, has it been found possible to prepare crystalline deriva- 

 tives, 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 chief purine bases obtained from the hydrolysis 

 of nucleic acid are guanine and adenine. 



Hypoxanthine and xanthine are often obtained as products of decom- 

 position of nucleic acid, but are generally formed by the deamination and 

 oxidation of guanine and adenine. Fischer has shown that all these bodies 



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