Uridine diphosphate glucose oxidation

Strominger, J. L., Kalckar, H. M., Axelrod, J., and Maxwell, E. S., Enzymatic oxidation of uridine diphosphate glucose to uridine diphosphate glucuronic acid. 

It should be noted, in this connection, that there are pyridine nucleotide dehydrogenases which catalyze redox reactions which must occur in two steps. Hydroxymethylglutaryl CoA reductase (discussed on p. 51) is one example. Another is uridine diphosphate-glucose dehydrogenase, which catalyzes the oxidation of the C—6 of the glucose (i.e., a primary alcohol) to a carboxyl group. In both cases, there are two molecules of pyridine nucleotide required, and the overall reactions are essentially irreversible. The former enzyme, with A stereospecificity for the pyridine nucleotide, catalyzes the reduction of an acyl-CoA group... 

Reactions (2) and (3) would be similar to those leading to the formation and transformation of cytosine diphosphate-choline 106) or uridine diphosphate 107). It is conceivable, of course, that the reduction of the pento l group to ribityl does not occur with the free pentose phosphate as pictured [reaction (1)], but by a mechanism involving the conversion of a cystosine diphosphate-pentose to cytorine diphosphate-ribitol. This type of reaction would be distantly related to reactions such as the oxidation of uridine diphosphate-glucose to uridine diphosphate-glucuronic acid 108,109). 

The galactose 1-phosphate is then converted to its epimer at C-4, glucose 1-phosphate, by a set of reactions in which uridine diphosphate (UDP) functions as a coenzyme-like carrier of hexose groups (Fig. 14-11). The epimerization involves first the oxidation of the C-4 —OH group to a ketone, then reduction of the ketone to an —OH, with inversion of the configuration at C-4. NAD is the cofactor for both the oxidation and the reduction. 

At present, it seems that uridine 5-(D-glucosyluronic acid pyrophosphate) is formed only through the oxidation of uridine diphosphate D-glucose, Attempts to join either a- or /3-n-glucosyluronate phosphate to uridine 5-(trihydrogen pyrophosphate) through the reversal of pyrophosphorolysis,... 

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