Pyrimidine nucleosides, utilization

The chemistry of nucleic acid analogs has received much attention in recent years, and a series of nucleic acid models has been designed and widely prepared, in order to estimate and utilize their functionalities in relation to the specific basepairing properties ( J., i, ). These monomers and polymers, particularly those containing purines, pyrimidines, nucleosides, and nucleotides, are not only of interest to the field of heterocyclic organic chemistry, but also to that of biomimetic macro-molecular chemistry as synthetic analogs of the nucleic acids. 

Pharmacology Zalcitabine, active against HIV, is a synthetic pyrimidine nucleoside analog of the naturally occurring nucleoside deoxycytidine in which the 3 -hydroxyl group is replaced by hydrogen. Within cells, zalcitabine is converted to the active metabolite, dideoxycytidine 5 -triphosphate (ddCTP), by cellular enzymes. ddCTP inhibits the activity of the HIV-reverse transcriptase both by competing for utilization of the natural substrate, deoxycytidine 5 -triphosphate (dCTP), and by its incorporation into viral DMA. 

Few pyrimidine bases are salvaged in human cells. However, the pyrimidine nucleosides uridine and cytidine can be salvaged by uri-dine-cytidine kinase, deoxycytidine can be salvaged by deoxycytidine kinase, and thymidine can be salvaged by the enzyme thymidine kinase. Each of these enzymes catalyzes the phosphorylation of a nucleoside(s) utilizing ATP, and forming UMP, CMP, dCMP, and TMP. 

Irreversible lactim-lactam tautomerization was recognized a long time ago and is generally achieved by either heat or catalysts. One of the synthetic applications was reported by Knorr as early as 1897. Later, this reaction was first applied to the synthesis of pyrimidine nucleoside by Johnson and Hilbert, thus being called the Hilbert-Johnson reaction (HJR). The reaction has been employed as one of protocols for the preparation of pyrimidine nucleosides. The biological and medicinal interest in pyrimidine affords further impetus to prepare new types of derivatives. Because of the synthetic utility of the HJR for synthesis of pyrimidine nucleosides, a more sophisticated version of the HJR has been developed by Vorbriiggen (the silyl HJR VHJR) employ-... 

Two studies of ORD on pyrimidine nucleosides have appeared which assist in the assignment of the anomeric configuration.The anomeric configuration can be assigned utilizing an NMR technique which compares the pyrimidine nucleoside with its 5,6-dihydroderivative. ... 

Johnson and Bergmann, as well as Helferich and Kosche, noted the possibility that the glycosyl ureides might be utilized as intermediates for an improved method of synthesis of pyrimidine nucleosides. Fischer s method was used for obtaining 1-n-glucosylurea as well as 1-n-arabinosyl-urea and 1,3-di-n-xylosylurea. 

When acylations of nucleosides with acid anhydrides in the presence of lipase from Pseudomonas fluorescence (PFL) in DMF or DMSO as the solvent first proceeded, the regioselectivity was unsatisfactory 1252l However, this lipase together with subtilisin can be utilized to effect highly specific deacylations of various pyrimidine nucleosides 145 (Fig. 18-18) 1253l Thus, lipase from Pseudomonas fluorescence (PFL) preferably attacks the hexanoyl group on the secondary hydroxy function of the N-... 

Cytarabine is a pyrimidine nucleoside related to idoxuridine (43). It is used primarily as an anticancer rather than an antiviral agent. Cytarabine acts by blocking the utilization of deoxycytidine, thereby inhibiting the repiication of viral DNA. The drug is first converted to mono-, di-, and triphosphates, which interfere with DNA synthesis by inhibiting both DNA polymerase and the reductase that promotes the conversion of cytidine diphosphate into its deoxy derivatives. 

Although the free bases did not appear to be utilized, pyrimidine nucleosides were anabolized. In 1950, Hammarsten, Reichard and Saluste showed that labeled cytidine and uridine were incorporated into RNA and DNA of rat liver, and in 1951, Rmchard and Estbom demonstrated that thymidine was incorporated into rat liver DNA (for a full account of early developments in this field, see references 1-4) ... 

A study of the mechanism of uracil incorporation into uridine phosphates was carried out in the Ehrlich ascites tumor, a tissue which utilized uracil as well as small molecule precimsors for nucleic acid formation (312). Uridine 5 -phosphate (UMP) was formed from uracil, ATP, and ribose 1-phosphate (R-l-P). Uridine was an intermediate in the formation of the nucleotide and was formed by the reaction of uracil and R-l-P with pyrimidine nucleoside phosphorylase (313, 314). Nucleoside kinase reacted the nucleoside with ATP to form UMP. The sequence is ... 

N DNA cytosine —deoxycytidine DNA thymine -thymidine Fig. 19. Utilization of pyrimidine nucleosides. 

When N -orotic acid was injected into rats, the label was incorporated into the p3nimidines of RNA (346) as well as DNA (347). C -Orotic acid vas utilized similarly for p uimidine biosynthesis in rat liver and spleen (348, 349) and in yeast (28). Since orotic acid and pyrimidine nucleorides were incorporated into nucleic acid p3nrimidines while free pyrimidine bases were essentially inert biosynthetically (37, 303), it appeared that a ribose or phosphorylated ribose derivative of orotic acid might be a key intermediate in the formation of pyrimidine nucleosides and nucleotides. 

Although AdoHcy hydrolase is not found in bacteria, AdoHcy is cleaved irreversibly to adenine and S-rlbosyl-L-homocystelne by AdoHcy nucleosidase ". This enzyme has been partially purified from E, aoliy and found to catalyze also the hydrolysis of 5 -methy1-thioadenosine to adenine and methylthiorlbose. The enzyme is inactive towards AdoMet and a variety of other purine and pyrimidine nucleosides. The specific activity of AdoHcy nucleosidase is 1000 x greater than that of AdoHcy hydrolase emphasizing the need to remove AdoHcy, a potent inhibitor of reactions which utilize AdoMet as a substrate. 

Like purine nucleotides, pyrimidine nucleotides can be synthesized either de novo or by the salvage pathways from nucleobases or nucleosides. However, salvage is less efficient because, except in the case of utilization of uracil by bacteria, and to some extent by mammalian cells, pyrimidine nucleobases are not converted to nucleotides directly but only via nucleosides. 

Deoxynucleoside phosphoramidates of 4-benzoylpyrazolo[3,4-d]pyrimidine-5-amines were prepared and utilized in automatic DNA synthesis yielding DNA fragments having an isosterically modified nucleoside base <91MI 712-02). 

How is the other major pyrimidine ribonucleotide, cytidine, formed It is synthesized from the uracil base of UMP, but UMP is converted into UTP before the synthesis can take place. Recall that the diphosphates and triphosphates are the active forms of nucleotides in biosynthesis and energy conversions. Nucleoside monophosphates are converted into nucleoside triphosphates in stages. First, nucleoside monophosphates are converted into diphosphates by specific nucleoside monophosphate kinases that utilize ATP as the phosphoryl-group donor (Section 9.4). For example, UMP is phosphorylated to UDP by UMP kinase. 

Salvage pathway. A family of reactions that permits, for instance, nucleosides as well as purine and pyrimidine bases resulting from the partial breakdown of nucleic acids to be re-utilized in nucleic acid synthesis. 

Leucine aminopeptidase is interesting in that its active site contains two zinc atoms which together bind and activate the water molecule [74]. Despite this enzyme containing a dinuclear metal center at its active site, its mechanism, and specifically its mode of proton transfers reactions, appear to follow the general theme established by thermolysin and carboxypeptidase Adenosine deaminase and other members of the family of nucleoside and nucleotide deaminases utilize zinc-bound water as the catalytic nucleophile to displace ammonia from the 6-position of purines or the 4-position of pyrimidines and in all cases display inverse solvent deuterium isotope effects ranging from 0.3 to 0.8 on fec/Kni [75-80]. These effects are reminiscent of those observed for metallopro-teases and have their origins, like those of the proteases, in fractionation factors for the protons of the bound water that are less than one. 

Another approach is to utilize the antimetabolite concept. The synthetic strategies include two types of chemical modifications of nucleosides. One is to alter the base. In the case of pyrimidines (Fig. 7-16) this usually involves replacement of the 5-CH3 group of 2-deoxythymidine with a halogen or CF3 function. The resulting 2-deoxythymidine analog... 

Generally, UV absorption spectra can be utilized to determine the family to which an unknown base, nucleoside, or nucleotide belongs. Indeed, the chro-mophore of the nucleosides or nucleotides is typical of the purine or pyrimidine structvu-e, with different maximum absorption wavelengths in the adenosine, guanosine, cytosine, or thymidine series. For example, it is possible to differentiate readily the UV absorption spectra of guanosine (G), G-monophosphate (GMP), deoxy GMP (dGMP), G-diphosphate (GDP), and G-triphosphate (GTP), from those of adenosine (A), AMP, dAMP, ADR, and ATP. 

---