Abstract
Tritiated uridine-5′-diphosphogalactose (UDP-[ 3H]Gal) has been widely used to study oligosaccharide biosynthesis and structure. It can be synthesized either chemically or enzymatically using galactose oxidase to oxidize the hydroxyl moiety at C-6 to an aldehyde (6-aldo-UDP-Gal), which is then reduced back to the alcohol with tritiated sodium borohydride. Although the enzymatic approach is simple and efficient, there are several problems associated with it. First, incomplete oxidation to the aldehyde reduces the final specific activity. Second, if the galactose oxidase is not removed from the 6-aldo-UDP-Gal prior to reduction, the resulting UDP-[6- 3H]Gal can be reoxidized to 6-aldo-UDP-[6- 3H]Gal. We present evidence for the occurrence of this compound in one commercially obtained preparation of UDP-[6- 3H]Gal. Finally, if an excess of 6-aldo-UDP-Gal is used for good yield, it is necessary to quench the reduction with nonradioactive borohydride, again reducing the final specific activity. We have devised a rapid, inexpensive, and efficient synthesis of UDP-[6- 3H]Gal that circumvents all of these problems. Galactose oxidase is used to produce 6-aldo-UDP-Gal and the completeness of this reaction is confirmed on polyethyleneimine (PEI) cellulose TLC plates. The 6-aldo-UDP-Gal is purified on silica gel 60 TLC plates. This purified compound is then reduced with tritiated sodium borohydride, with the aldehyde present in excess. Unreacted 6-aldo-UDP-Gal is then purified away from the product UDP-[6- 3H]Gal by chromatography on PEI cellulose. Radiochemically pure UDP-[6- 3H]Gal with a specific activity of 10 Ci/mmol was obtained using the above scheme. This same strategy has been extended to the synthesis of UDP- N-[6- 3H]acetylgalactosamine. The tritiated monosaccharides of these sugar nucleotides are also easily obtained by mild acid hydrolysis and passage through cation- and anion-exchange chromatography columns.
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