The enzymatic synthesis of thymidine diphosphate glucose and its conversion to thymidine diphosphate rhamnose.

. Studies from several laboratories have shown that labeled glucose is converted to B-deoxy hexoses without randomization of the label when microorganisms are grown on n-glucose-l-C14 or n-glucose-MY4 as the sole source of carbohydrate (l-3). Evidence of this type has been interpreted to indicate a conversion of the intact hexose moiety to the deoxy compound without scission of the hexose carbon skeleton. Results of isotope experiments in our laboratory substantiate this pathway for the biosynthesis of L-rhamnose’ (6-deoxy-L-mannose) in Streptococcus juecalis. In order that a synthesis of the deoxy compounds be effected by this route, epimerizations of several carbon atoms and reduction of a primary alcoholic group of the hexose unit must occur. In general, epimerization reactions which have been studied in biological materials have been found to proceed via nucleotide diphosphate hexoses as, for example, the epimerization of the glucosyl unit of uridine diphosphate glucose to a galactosyl unit (4) and the epimerization and reduction of the mannosyl unit of guanosine diphosphate mannose to a fucosyl unit (5). On the basis of the occurrence of thymidine diphosphate rhamnose in several bacteria (6-9), it seemed logical to expect that a thymidine nucleotide, rather than a uridine or guanosine nucleotide, might serve as the carrier of the hexose moiety in the synthesis of n-rhamnose. Experimental verification of this suggestion has now been achieved in our laboratory. Thymidine diphosphate glucose has been synthesized from thymidine triphosphate and cr-n-glucose l-phosphate with a cell-free enzyme preparation from S. jaecalis. The dTDP-glucose has been converted enzymatically to thymidine diphosphate rhamnose with the cell-free extracts. A preliminary note on these experiments has been published (10). A simultaneous report on the occurrence of this series of reactions in another organism, Pseudomonas aeruginosa, has also appeared (11). On the basis of these findings, and the known reactions of carbohydrate metabolism, the sequence of reactions suggested for the conversion of n-glucose to L-rhamnose in X. jaecalis is shown in the accompanying equations: