Nucleotide derivatives Polynucleotide synthesis

Adenine is inert in the nucleoside phosphorylase systems of both mammalian tissues and microorganisms, but isotopically labeled adenine is effectively incorporated into nucleic acid purines, both in rats " and in yeast.This poses a question as to the possible role of nucleoside phosphorylase in polynucleotide synthesis. It has been suggested that hypoxanthine or guanine nucleosides (or nucleotides) are synthesized first. Then an exchange reaction with free adenine (or a derivative) might occur, For example, adenine might react with inosine to form adenosine and hypoxanthine. Some known exchange reactions are discussed below. 

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

As DNA synthesis proceeds, nucleotides from the solution are added to the growing polynucleotide chain. Chain extension takes place without complication as long as the incorporated nucleotides are derived from dATP, dTTP, dGTP, and dCTP. If, however, the incorporated species is derived from a dideoxy analog, chain extension stops. Because the dideoxy species ddA, ddT, ddG, and ddC lack hydroxyl groups at 3, they cannot engage in the 3 5 phosphodiester linkage necessary for chain extension. Thus,... 

Lohrmann, R., Sole, D., Hayatsu, H., Ohtsuka, E. and Khorana, H. G. (1966) Studies on polynucleotides. Cl. Synthesis of the 64 possible ribotrinucleotides derived from the four major ribomono-nucleotides. J. Am. Chem. Soc. 88, 819. 

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