MULTIPLE ENZYME SYSTEMS 



(PRA) is deaminated very rapidly to ribose-5-phosphate and 

 NH^ and none accumulates. However, a study of the 

 balance of the second reaction, the glutamine disappearance, 

 and glutamic acid and pyrophosphate formation called for the 

 presence of phosphoribosylamine. It was apparent that the 

 degradation reaction which might be enzymatic could be 

 removed, and such fractionations are in progress. However, 

 prior to fractionation Goldthwait and co-workers (10) synthe- 

 sized PRA chemically as it fulfilled the requirements for a simple 

 synthesis (ribose-5-phosphate plus liquid ammonia) and it was 

 tested in the system in place of PRPP and glutamine. PRA 

 was found to be at least 3 times as effective as PRPP and glu- 

 tamine but still required ATP. These studies also gave some 

 insight into the chemical nature of PRA. It was found to be a 

 rather unstable compound which would require special han- 

 dling. Hindsight is the simplest branch of science, but it may be 

 useful to backtrack in the development of ideas in a problem. 

 The mechanism as it was visualized after the isolation of glycin- 

 amide ribotide went through various revisions. Ribose-1, 

 5-diphosphate was a logical possibility as the ribosyl donor. It 

 was also possible that ribose-1 -phosphate was involved, forming 

 a riboside which then was phosphorylated. When Kornberg 

 and co-workers (18) first showed the involvement of PRPP in the 

 conversion of orotic acid to uridylic acid, this became a choice 

 possibility. But other problems were involved. Did the ribose 

 derivative react with glutamine or with glycinamide or with a 

 product of glutamine and glycine? Of some importance was 

 the problem of finding a means of activating the amide hydrogen 

 atoms of glutamine which are notably unreactive. Whatever 

 the case, the glutamine does react with PRPP so that funda- 

 mentally the problem resolves itself by virtue of the findings 

 without regard to exact mechanism. In retrospect it is clear 

 that an insight into the mechanism could have been gained by a 

 study of the over-all balance. It also is obvious that the key to 

 the development of this problem was the identification of 

 PRPP (18). 



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