The Mechanism of Pentose Phosphate Isomerization and Epimerization Studied with T,O and H,018*

Since the discovery of uridine diphosphate galactose 4-epimerase by Leloir (I), there has been increased recognition of the importance of epimerization of hgdroryl groups of monosaccharides and related metabolitrs. In addition to the interconversion of uridine diphosphate glucose and uridine diphosphate galactose by yeast (2, 3), mammalian (4, 5), and bacterial preparations (6), examples have been found of the same type of 4-cpimerization between uridine diphosphate derivatives of the corresponding, uranic acids (7, 8), S-acetyl hesosamines (9, 10) and D-XylOSe and Larabinose (11). The discovery of n-ribulose-5-phosphate 3-epimerase in Lactobacillus pentosus (12,13) as an important component in the hesose monophosphate pathway and of L-ribulose-5-phosphate 4-epimerase, which functions in the pathway of L-arabinose metabolism in L. pentosus (14) and Aerobacter aerogenes (15), has extended the proeess to a new type of substrate and point of attack. The existence of additional epimerases can be inferred from reports of the conversion of o-glucose to L-fucose (16), of guanosine diphosphate n-mannose to guanosine diphosphate L-fucose (17)) and of thymidine diphosphate glucose to thymidine diphosphate Lrhamnose (18). Although 2-epimerization of secondary hydroxyl groups has not been reported, the conversion of uridine diphosphate N-acetylglucosamine to N-acctylmannosamine (19) and of nT-acetylglucosamine to N-acetylmannosamine (20) may prove to be examples of a new type of epimerization at carbon atom 2 which resembles the process of racemization for amino acids (21). The demonstration of a diphosphopyridine nuclcotide requirement for uridine diphosphate galactose 4-epimerase of red blood cells and calf liver (2, 22) and the presence of a fluorescence with spectral characteristics corresponding to that of reduced diphosphopyridine nucleotide in yeast uridine diphosphate galactose 4-epimerase (2) have indicated that hydrogen transfer may be an important part of 4-epimerase action. Studies of uridine diphosphate galactose 4-epimerase action in TzO, DzO, and Hz018 have failed to show that the elements of water participate so as to remain stably bound in the substrate molecules (23-25) and, further, it was shown that addition of tritiated diphosphopyridine nucleotide or diphosphopyridine nucleotide reduced with tritium did not lead to incorporation of tritium into the substrates (23). The availability of L-ribulose-5-phosphate 4-epimerase has presented another opportunity to study 4-epimerase action by the