258 H. M. KALCKAR VOL. 12 (1953) 



and perhaps it can, in general, serve as a"ribotyl" (5-phosphoribosyl) unit for precursors 

 of purines and pyrimidines. 



Uridyl phosphoglycosyl compounds. Our interest in these novel types of nucleotide 

 which were discovered by Louis Leloir and his group"*^, started with the idea that they 

 could be considered as glycosyl compounds with a substituted phosphate instead of free 

 phosphate (see formula p. 253). 



In other words, could the uridine diphosphate be considered as a carrier of glyco- 

 syls ? This aspect has already been discussed briefly, but since no direct experimental 

 evidence is available at the present time to indicate that the UDPG (a or /S ?) can partici- 

 pate in glycosyl phosphorylation or transglycosidation, the idea must so far be classified 

 as possible fruitful speculation. 



From the start, however, Leloir and his group put the spotlight on the potentiali- 

 ties of another bond. They found that UDPG is a coenzyme for the inversion of a-galac- 

 tose-i-phosphate to a-glucose-i-phosphate. Two step reactions were proposed^^. 



1. Uridyl-phospho-i-glucose + phospho-i -galactose ^ 



Uridyl-phospho-i-galactose -f phospho-i-glucose 



2. Uridyl-phospho-i-galactose ^ Uridyl phospho-i-glucose. 



The enzyme for reaction i has not been isolated but can be demonstrated separately 

 after the galacto-inversion enzyme has been inactivated (see later). Leloir**^ demon- 

 strated reaction 2 separately by subsequent analysis of the sugars by paper chromato- 

 graphy. In equilibrium he found that 75% is in the form of uridylphospho-i-glucose 

 (UDPG) and 25% in the form of uridylphospho-i-galactose. This type of reaction which 

 so far is a real novelty in enzyme chemistry has been discussed in detail from a chemical 

 point of view by Leloir^^. Apparently it does not conform with any of the known 

 chemical mechanisms of inversions either. It was predicted by Leloir in 1950**^ that 

 UDPG probably would turn out to play a role in other enzymic reactions than the galac- 

 tose inversion. Phosphopyruvate is able in the presence of the proper phosphokinase to 

 phosphorylate uridine diphosphate (UDP) to uridine triphosphate (UTP)^^. It was 

 anticipated that the latter may play a role in the synthesis of UDPG through pyrophos- 

 phorolysis^^, a type of fission discovered a few years ago by Kornberg.^" It was later 

 found^^ that ATP, UDP, and a-glucose-i-phosphate in the presence of mercaption juice 

 from bakers yeast can give a distinct increase intheamount of UDPG present in the juice. 



Uridyl transferases. We have found^^-^^ that brewery yeast as well as galactose 

 adopted yeast {Saccharomyces fragilis) contains an enzyme which catalyzes a p3-rophos- 

 phorolytic fission of UDPG. The specific enzyme UDPG pyrophosphorylase is abundant 

 in the so-called "Zwischenfermcnt" preparation (Warburg and Chkistian 1932^*) 

 which is a protein precipitated with carbon dio.xide at high aqueous dilutions (100 fold) 

 of yeast maceration juice. This fraction can be further purified; the protein fractions 

 precipitated in the range 0.6 to 0.7 saturated ammonium sulphate being the most 

 active. For our investigations we have used the Warburg-Christian-Le Page "Zwi- 

 schenfermcnt" extensively because it turned out, by a stroke of fortune, to contain most 

 of the enzymes needed for good micro assays and practicall3'none which were undesirable. 



The reaction is: UDPG -f- pyrophosphate ?=^ UTP -j- a-G-i-P and the fission of 

 UDPG occurs as shown in Fig. i. This fission will release Cori-ester (a-glucosc-i -phos- 

 phate) which upon addition of phosphoglucomutase (absent in our Zwischcnfcrment 

 preparation) v/ould yield glucose-6-phosphate. Since Zwischcnfcrment together with 



References p. 26 3 j 264. 



