254 K- ^- KALCKAR VOL. 12 (1953) 



TABLE I 



INCUBATION OF HYPOXANTHINE DEOXYRIBOSIDE (80 //g) WITH BACTERIAL EXTRACTS 

 (0.5 MG protein) for 2 H AT 35° PH J.O. ANALYSIS OF PROTEIN-FREE FILTRATES FOR URIC ACID 



Enzyme fig Purine liberated 



None 000 



L. casei 3.4 32.0 3.4 



L. delbruckii 4.8 4.0 15.8 



(Kalckar19) 



phosphate, in the presence of the specific phosphorylase, does not exchange witli in- 

 organic phosphate {^^P labelled) except in the presence of the nitrogenous bases. 



The question of a prosthetic group for transglycosidases or phosphorylases, especial- 

 ly of the type of Leloir's uridine diphospho-glycosyl compound, has been in the minds 

 of several investigators, including ourselves^^-*^^-^^. Polysaccharides, disaccharides, 

 nucleosides or phosphoglycosyls might play the role of glycosyl donors with uridine 

 diphosphate as acceptor. The UDP-glycosyl formed could then donate the glycosyl to 

 acceptors including inorganic phosphate. Whether the recent report^^ that uridine di- 

 phospho-glucuronate is the cofactor of glucuronide synthesis can be classified as an 

 example of this t^^pe of reaction remains to be seen. 



The physiological role of mideoside phosphorylases. The demonstration of nucleoside 

 formation from pentosyl phosphates and in particular the demonstration of highly 

 active growth factors for micro-organisms formed from deoxyribosyl phosphate and 

 purine by liver nucleoside phosphorylase, might ver}/ well lead an investigator to believe 

 that this could be the clue to an understanding of nucleotide and nucleic acid synthesis. 

 This may be the case if the discussion is confined to biosyntheses in micro-organism. It 

 must, however, be borne in mind that the enzymes used originated from rat liver, and 

 the logical question is, therefore, how does this enzymic step reaction benefit the nucleo- 

 tide and nucleic acid synthesis in the rat organism ? Since radioactive pentoses so far 

 have not been accessible, the fate of the pentoses of inosine or its deoxyribo analogue has 

 yet to be determined. We can, therefore, only discuss the importance of nucleoside 

 phosphorylase for the incorporation of N-bases into nucleic acids. For an evaluation of 

 this problem the inertness of adenine in this enzymic step is bound to become a crucial 

 ]:)oint. Without adenine as an active partner the role of this group of enzymes of nucleo- 

 tide and nucleic acid synthesis must somehow be more or less indirect. This is particu- 

 larly evident from the studies performed on the intact rat with isotoj)ically labelled 

 purines. Brown and co-workers^^ have found that adenine is incorporated into nucleic 

 acids on a large scale, the hypoxanthine of inosine to a small extent and free hypo- 

 xanthine not at all. The fact that riboside-bound hypoxanthine is definitely incorporated, 

 whereas the free hypoxanthine is not, points towards the existence of important alter- 

 native pathways for nucleotide synthesis from ribose-i-phosphate. The formation of 



* Kalckar and CuTOLo^i have found that achUtion of UDl* or UDPG to inosine nucleoside 

 phosphorylase and xanthine oxidase brought about an increase in the rate of liberation of uric acid as 

 compared with the samples without the uridine nucleotides. The rates were, however, much slower 

 than those obtained by addition of free phosphate. It has not been possible to demonstrate a formation 

 of uridyl phospho-i-ribose. The formation of this compound by the action of uridyl transferase (see 

 later) has not been observed. 



References p. 263I264. 



