“…This argument is supported by the finding that at equilibrium, in neutral solution, 2 0 z of 5 'thiouridine is in the adduct form (Reist ef a/., 1964), as a consequence of the intramolecular nature of the reaction. Furthermore, the equilibrium of the addition of the 5 '-SH group of the 2 ',3 '-isopropylidene derivative of 5 '-thiouridine to the 5,6-double bond is shifted further toward the formation of, the adduct, yielding a stable, crystallizable compound (Chambers and Kurkov, 1963), showing the strong influence of the molecular conformation on this equilibrium. Thus, it can be concluded that the nucleophilic addition of an enzymic SH group to the 5,6-double bond of pyrimidine nucleotides could be ~ j A direct 1,2-trans addition followed by exchange through ionization of H-5 and subsequent elimination is also compatible with the results, as was pointcd out by one of the reviewers.…”