Uridine 5 - , enzymic conversion

The primary end product is uridine monophosphate (UMP). In the conversion of UMP to dTMP, three important enzymes are ribonucleotide reductase, thymidylate synthase, and dihy-drofoiate reductase. All three enzymes are targets of antineoplastic drugs and are summarized in Table I-18-1. 

Conversion of the a-D-glucopyranosyl derivative (94a) into the a-D-galactopyranosyl ester (95a) was demonstrated370 in 1951 as the first example of an enzymic reaction of a sugar nucleotide. The enzyme that catalyzes this reaction, namely, uridine 5 -(a-D-glu-copyranosyl pyrophosphate) 4"-epimerase,371 is common in Nature. Purified preparations have been obtained from yeast,372 373 Escherichia coli 374-376 mung-bean seedlings,377 wheat germ,378 and animal tissues.244,379 380... 

The analogous conversion of the uridine derivative 107d has been observed in extracts of tobacco leaves136 and Chlorella cells,137 although the enzyme has not yet been purified. The enzymic reactions described here are frequently used for preparation of the 6-deoxy-hexosyl-4-ulose esters of nucleoside pyrophosphates that are inacces-... 

Hydrolysis of sugar nucleotides with unspecific pyrophosphatases has already been mentioned (Section 11,1, p. 310). A similar reaction is catalyzed by a bacterial enzyme specific for adenosine 5 -(a-D-glucopyranosyl pyrophosphate).459 The specific conversion of uridine 5 -(a-D-glucopyranosyl pyrophosphate) into a-D-glucopyranosyl phosphate, uridine, and inorganic phosphate was observed with an enzyme from Escherichia colt 459,460 a preparation from Bacillus subtilis can act in a similar manner461 on different sugar nucleotides. ... 

Quercetin and other flavonols are subject to metabolic conversion during the absorption process in the small intestine. Phase II enzymes including uridine 5 -diphosphate glucuronosyl transferase (UGT), phenol-sulfotransferase (PST),... 

The thermal decarboxylation of 1,3-dimethylorotic acid 56 was studied as a model for uncatalysed decarboxylation of orotidine 5 -monophosphate to form uridine 5 -monophosphate.97 This reaction catalysed by the enzyme oritidine 5 -monophosphate decarboxylase is the essential step in nucleic acid biosynthesis.98 Samples of 1,3-dimethylorotic acid were heated at 190°C to conversion over 90% and unreacted substrate was recovered after conversion into the methyl ester. 

Orotidine S -monophosphate decarboxylase (ODCase) is a key enzyme in the biosynthesis of nucleic acids, effecting the decarboxylation of orotidine 5 -monophosphate (OMP, 1) to form uridine S -monophosphate (UMP, 2, Scheme l).1,2 The conversion of OMP to UMP is biomechanistically intriguing, because the decarboxylation appears to result, uniquely, in a carbanion (3, mechanism i, Scheme 2) that cannot delocalize into a it orbital.3,4 The uncatalyzed reaction in solution is therefore extremely unfavorable, with a AG of... 

The presence of this enzyme in muscle was first reported in 1958 and has since been investigated by many workers. Villar-Palasi and Lamer demonstrated the conversion of a-D-glucosyl phosphate into glycogen by a uridine-coenzyme-linked reaction in rat skeletal and diaphragm muscle ... 

Starting from ceramide D-glucoside, enzymic synthesis has been studied for producing gangliosides222,223 and for the conversion of Tay-Sachs ganglioside into monosialoganglioside by the uridine 5 -pyrophosphate of brain.224... 

It should therefore be possible to design chemical structures, modeled after known or putative reaction intermediates that resemble postulated transition states. They may exhibit high affinities for the reactive sites of enzymes and therefore function as effective, but reversible, inhibitors. A successful example is the potent and specific cytidine deaminase inhibitor 3,4,5,6-tetrahydrouridine, which effectively blocks the conversion of cytidine to uridine. It was similarly demonstrated that 1,6-dihydro-6-hydroxymethylpurine effectively blocked the deamination of adenine to hypoxanthine by adenine deaminase (Eq. 2.13). 

Figure 4-16 outlines the biosynthesis of pyrimidines and their conversion to the required deoxyribose triphosphates of uridine and cytidine, the necessary building blocks of RNA. The first step involving the condensation of carbamoyl phosphate with aspartic acid is catalyzed by aspartate transcarbamylase. This enzyme is strongly inhibited by the transition-state inhibitor PALA (Chapter 2). Other steps where drug intervention in the scheme can interfere to inhibit DNA synthesis are indicated. 

It has also been demonstrated that expensive substrates such as UDP-Gal can be readily prepared in situ by enzymatic conversion of the relatively inexpensive sugar nucleotide uridine 5 -diphospho-a-D-glucopyranose (UDP-Glc) using a UDP-Gal 4-epimerase enzyme. This system, coupled with an appropriate UDP-Gal transferase, provides more economic access to enzymatically galactosylated compounds. In these multienzyme systems, to increase enzyme efficiency and also avoid multiple fermentations for separate enzyme preparations, fusion proteins have been constructed that contain both the Gal-epimerase and Gal-transferase enzymes. The use of these fused enzyme systems has increased in the recent years as their catalysis of sequential reactions can have a kinetic advantage over the mixture of two separated enzymes since the product of the first enzyme travels a shorter distance before being captured by the next enzyme in the sequence. 

H, = H, H, = H. Configurational analysis at C-5 of [5- H, H]uridine diphospho-D-xylose involved conversion to [ H]glycolic acid (from C-4 and C-5), using chemical and enzymic methods, followed by treatment with glycolic acid oxidase [catalyzing the loss of the pro-(R) hydrogen] (221). 

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