[Synthesis of nucleosides labeled with tritium at the 5'-carbon atom of the ribose residue].

Chemical and enzymatic syntheses of [5'-3H]adenosine, [5'-3H]guanosine, and [5'-3H]uridine have been developed. The reduction of beta-D-ribo-pentadialdo-1,4-furanosyl derivatives of corresponding bases is used in the chemical synthesis. The maximum molar activity of the labelled products was 220 TBk/mol in reactions with [3H]NaBH4 and 370-740 TBk/mol in reactions with gaseous tritium. The enzymatic synthesis was performed by the rebosylation of heterocyclic bases with nucleoside phosphorylase and [5'-3H]uridine as a ribosyl donor. Nucleoside phosphorylase is proposed to be used in the immobilized form to avoid the decrease of molar activity. Nucleosides labelled with tritium both in ribosyl and heterocyclic moieties were synthesised enzymatically.