VOL. 4 (1950) PURINES AND PYRIMIDINES IN RIBOSIDIC LINKAGE 235 



provided that either phosphate or arsenate is present. The ester formed was isolated 

 and identified as desoxyribose-5-phosphate. The authors have evidence for the presence 

 of an enzyme which catalyses the conversion of a primarily formed 1-ester into the 

 5-ester. The same two authors have also made recent contributions towards our under- 

 standing of the enzymatic splitting of pyrimidine desoxynucleosides, especially thymi- 

 dine^". They have isolated an enzyme from bone marrow and kidney which catalyses 

 a splitting of thymine from thymidine, again provided that either phosphate or arsenate 

 is present. The enzyme preparations contain both purine nucleoside phosphorylase and 

 pyrimidine nucleoside phosphorylase. Manson and Lampen's observations point also 

 towards a formation of a desoxyribose-1-ester from pyrimidine desoxynucleoside. Thus, 

 addition of hypoxanthine enhances the liberation of thymine from thymidine in the 

 presence of mixed phosphorylases. This effect indicates at least that an enzymatic 

 exchange between hypoxanthine and thymine takes place. However, since the incor- 

 poration into ribosidic linkage of hypoxanthine and that of thymine is catalysed by 

 two different enzymes the assumption of a formation of 1-phospho-desoxyribose as a 

 common substrate for both enzymes can explain the above mentioned effect. 



THE BIOLOGICAL PATHWAY OF PURINE AND PYRIMIDINE INCORPORATION 



INTO NUCLEIC ACIDS 



The pathway of purine and pyrimidine incorporation into nucleic acids is a problem 

 of major biological importance. The isotope technique has made it possible to make an 

 account of the most significant steps of such a synthesis in the intact organism. In 1941 

 ScHOENHEiMER and his colleagues initiated some studies on purine incorporation in the 

 intact adult organism. I shall not go into a discussion of the interesting feeding experi- 

 ments using N^^ labelled ammonia and C^^ or C^* labelled carbon dioxide which have 

 shed so much light on the synthesis of the purine bases. This discussion is dealing with 

 results of feeding experiments with labelled purines. These studies were initiated by 

 Plentl and Schoenheimer^^ and brought into a very successful and fruitful develop- 

 ment by the studies performed at the Sloan-Kettering Institute by Brown and 

 coworkers. It will be recalled that Plentl and Schoenheimer found that adult rats 

 fed N^^ labelled guanine excreted the entire amount of this substance as uric acid and 

 allantoin and correspondingly the guanine of the nucleic acids was found to be devoid 

 of any excess N^^. This finding was substantiated 6 to 7 years later by Brown and co- 

 workers. Brown and his colleagues synthesized N^-^ adenine and guanine according to 

 recent methods developed by Todd and Lythgoe. The most remarkable result of their 

 studies, was the fact that N^^ labelled adenine was readily incorporated into the ribo- 

 nucleic acids both as adenine and guanine^^. If a moderate amount of N^^ adenine was 

 administered to adult rats about 50% was incorporated as nucleic acid adenine and 

 guanine and the other 50% appeared as allantoin. Bendich and Brown^^ have recently 

 made the interesting observation that 2-6 diamino purine labelled with N^^ appears 

 in large amounts in the nucleic acid guanine but not in the adenine. H5rpoxanthine 

 seems to be converted exclusively into uric acid and allantoin^*. 



How are the present results of the studies on liver nucleoside phosphorylase to be 

 interpreted in the light of recent findings gained from isotope experiments performed on 

 intact organisms? It will be recalled that the liver nucleoside phosphorylase catalyses 

 the incorporation of only two purine bases, hypoxanthine and guanine — exactly the 

 References p. 257. 



