The first step in the utilization of galactose has been found to be a phosphorylation by adenosine triphosphate (ATP) catalyzed by galactokinase (l-3). Evidence has been presented (4) showing that the enzymatic transformations which follow are In the initial studies (5) it was found that a thermostable factor was required in the over-all Reactions II and III. This fact led eventually to the discovery and isolation of glucose diphosphate (6), which acts as a coenzyme in Reaction III. These

more »

... s have been confirmed by Sutherland et al. (7) , and biological (8, 9) as well as chemical (10, 11) methods for the synthesis of glucose diphosphate have been described. Reaction II, which consists in a Walden inversion of C-4, is catalyzed by an enzyme which is currently called "galactowaldenase" in this laboratory. The corresponding coenzyme has been named uridine-diphosphate-glucose (UDPG) for reasons which will become apparent in what follows. Estimation and Isolation of UDPG The method of estimation of UDPG was based on its property of accellerating Reaction II. In practice the rate of Reaction II was measured as the sum of Reactions II and III by the rate of appearance of reducing power. The reaction mixture contained galactose-l-phosphate and a maceration juice of Saccharomyces jragilis plus variable amounts of UDPG. Glucose diphosphate was also added; so that the velocity of the over-all reaction was limited by the rate of Reaction II. The results obtained by varying the amounts of UDPG are shown in Fig.