Periodate-methylamine degradation of ribonucleotides in rat liver extracts in comparison to extracts of cultured cells

Comparison of the Intracellular Metabolism of 3′-Azido-3′-Deoxythymidine and 3′-Fluoro-3′-Deoxythymidine in Lymphocytes in the Presence of 5-Fluoro-2′-Deoxyuridine

The mechanism of action of 2′, 3′-dideoxythymidine analogues such as azidothymidine (AZT) and fluorothymidine (FLT) is thought to be chain termination and inhibition of reverse transcriptase by the triphosphate. However, little triphosphate is formed intracellularly relative to monophosphate and diphosphate. Azidothymidine and fluorothymidine are phosphorylated intracellularly by the thymidine salvage pathway and therefore must compete with thymine nucleotides for phosphorylation, which may limit the rate of phosphorylation. We have investigated the degree to which competition with thymidine limits the phosphorylation of azidothymidine and fluorothymidine. In this paper we show that inhibition of thymidylate synthase by 5-fluoro-2′-deoxyuridine monophosphate increases the phosphorylation of azidothymidine and fluorothymidine by reducing the pools of thymine nucleotides and therefore removing the competition. 5-fluoro-2′-deoxyuridine abolished the pools of TTP and dGTP and caused the appearance of dUTP in treated lymphocytes. Three times more triphosphate was formed in cells treated with 5-fluoro-2′-deoxyuridine. Ten times more fluorothymidine triphosphate than azidothymidine triphosphate was formed. The results suggest that while competition with intracellular thymine nucleotides does hinder the phosphorylation of AZT and FLT, the major limiting factor is their ability to act as substrates for the phosphorylating enzymes.

A "cleanup procedure" involving periodate oxidation in the enzymatic synthesis of chemically pure alpha-32P and alpha-33P labelled deoxyribonucleotides

Enzymatic synthesis of alpha-(32)P and alpha-(33)P labelled deoxyribonucleotides involves the transfer of radiolabelled phosphorus from either gamma-(32)P adenosine triphosphate (gamma-ATP) or gamma-(32)P guanosine triphosphate (gamma-GTP). Subsequent removal of these ribonucleotides is essential for the preparation of chemically pure deoxyribonucleotides. Agarose-phenyl boronate columns, which bind specifically to cis-diol moieties, have been used for the removal of ribonucleotide contaminants. However, this involves column losses and additional radiation exposure. In the present work we describe a chemical method for the improvement of the chemical purity, based on the preferential oxidation of ribose sugars by periodate. The cis-diol moiety of ribose is specifically oxidised to the dialdehyde. The excess periodate ions were destroyed using ethylene glycol. The phosphate group was then cleaved by beta-elimination using alkali. The product was purified using anion exchange chromatography. The efficiency of the process was validated using tracer gamma-(32)P ATP and alpha-(32)P dATP. Samples at various steps were analysed by TLC, autoradiography and HPLC. During the process ATP is oxidised whereas 2'-deoxyadenosine triphosphate (dATP) remains intact. The alpha-(32)P dATP synthesized by this process was assayed for its incorporation in lambda-DNA by the random priming method and was found to be effectively incorporated. The process developed is an efficient and convenient method for the preparation of chemically pure deoxyribonucleotides.

Comparison of extraction procedures for high-performance liquid chromatographic determination of cellular deoxynucleotides

For the determination of cellular deoxynucleoside triphosphates (dNTPs) by high-performance liquid chromatography, the choice of extracting agent and periodate oxidation procedure is important for accurate results. Different extraction methods were compared using either cold methanol or trichloroacetic acid for extraction of dNTPs from lymphoblastoid cells. The recoveries of the dNTPs varied with the different extraction methods. A modification of the periodate oxidation procedure for degrading ribonucleoside triphosphates to their bases was also compared with the original method. Both methods gave accurate results when trichloroacetic acid was used as extracting agent, but when methanol was used interfering peaks were present on the chromatogram when the original method was used. These peaks were absent when the modified periodate procedure was used.