770 PHYSIOLOGY 



chain. Three pyrimidine bases have been isolated from the decomposition 

 products of miclein, namely, thymine, cytosine, and uracil. 



NH_ C O NH CO N == C.NH 2 



CO CH CO C.CH 3 CO CH 



I II I II I II 



NH CH NH CH NH CH 



Uracil 2-6-dioxy- Thymine 5-methyl- Cytosine 6-amino- 



pyrimidine uracil 2 -oxy pyrimidine 



Cytosine is easily converted by oxidation into uracil. 

 After separation of the purine and pyrimidine bases and phosphoric acid 

 a substance is left over which gives the reactions of a carbohydrate. 



This carbohydrate differs in different nucleic acids. In plant nucleic acid, as well 

 as in guanylic acid from the pancreas and inosinic acid from muscle, the carbohydrate 

 is a pentose, d-ribose. Most nucleic acids of animal origin yield Isevulinic acid en 

 hydrolysis and must therefore contain a hexose. The researches of Levene, Jones, 

 and others have shown that the nucleic acids vary in complexity and consist of one 

 or more so-called nucleotides linked together. The simplest nucleic acids are mono- 

 nucleotides. Examples of these are inosinic and guanylic acid. Each consists of 

 phosphoric acid and a purine or pyrimidine base linked together by carbohydrate. 

 Upon hydrolysis with boiling mineral acid they are decomposed into their three 

 constituents : 



0=PO - C 6 H 8 Os - C 5 H 4 N 5 + 2H 2 = H 3 PO 4 + C 5 H 10 O 5 + C 5 H 5 N 5 O 

 /guanylic acid d-ribose guanine 



H0 \ 



0=PO - CgHgOs - C 5 H 3 N 4 + H 2 = H 3 PO 4 + C 6 H 10 O 5 + C 5 H 4 N 4 

 / inosinic acid d-ribose hypoxanthine 



When submitted to neutral hydrolysis at 175 C under pressure, decomposition 

 occurs in a different way, the phosphoric acid is split off and a glucoside-like body 

 is left which is called a nucleoside. Thus guanylic acid yields guanosine, inosinic acid 

 yields a body known as inosine or hypoxanthosine. Under the action of ferments, 

 known as nucleases, which may occur in animal tissues, the mono-nucleotides may 

 be split in one of two ways. The phosphonuclease removes the phosphoric acid, leav- 

 ing a compound of purine and carbohydrate, while the purine nucleate sets free the 

 purine base and leaves a hexose- or pentose-phosphoric acid. Of the more complex 

 nucleic acids which occur in cell nuclei, yeast nucleic acid has been the most carefully 

 studied. According to Levene this nucleic acid is a tetranucleotide, having a structure 

 represented by the following formula : 



Ov 



O - PO.C 5 H 8 3 .C 5 H 4 N 6 



/ guanine group 



\ 

 0= PO.C 5 H 8 3 .C 5 H 4 N 5 



/ adenine group 



\ 



0= PO.C S H 8 Q3.C 4 H ? X 2 2 



uracil group 



\ 

 0= PO.C 5 H 8 3 .C 4 H 4 N 3 



cytosine group 



