Conversion of uracil deoxyriboside to thymidine of deoxyribonucleic acid.

Most of the literature dealing with the participation of citrovorum factor and vitamin Blz in the formation and transfer of l-carbon intermediates stresses the donor compounds serine, glycine, and formate as precursors of the 5-methyl group of thymidine (2). Uracil deoxyriboside, first isolated in crystalline form by Dekker and Todd (3) in 1950, is known to arise as a result of the deamination of cytosine at the structural level of polynucleotide or nucleoside (4-7). Uracil deoxyriboside can also be directly synthesized by the enzymatic attachment of the deoxyribosyl group to uracil (5,8-10). In this paper, evidence is presented that uracil deoxyriboside, which structurally is the simple demethylated form of thymidine, may indeed be the primary acceptor of the methyl group.