In acidic media, the 5,6-double bond of
uridine is rapidly hydrated to give a small amount of
6-hydroxy-5,6-dihydrouridine (Urd-H2O), the mechanism of which is known from studies of the
acid-catalyzed
dehydration of Urd-H2O (Prior, J. J., Maley, J., and Santi, D. V. (1984) J. Biol. Chem. 258, 2422-2428). In addition to
dehydration, Urd-H2O also undergoes direct hydrolysis of the N-glycosidic bond in acidic
solution. The kinetics of the above reaction demonstrates that Urd-H2O, or an intermediate in the pathway leading from Urd to Urd-H2O, is kinetically competent to account for the hydrolysis of the N-glycosidic bond of Urd. The hydrolysis of (1'-2H)Urd proceeds with an alpha-secondary
deuterium isotope effect of kH/kD of 1.11 at 25 degrees C. This
isotope effect is sufficiently large to implicate carbonium ion character at the 1'-carbon during hydrolysis but, since it is not the maximal value expected, suggests that N-
glycoside cleavage is rate-determining with a transition state intermediate between reactant and products. Importantly, the hydrolysis of [6-3H]Urd proceeds with a substantial inverse secondary
isotope effect of kT/kH = 1.15 at 25 degrees C which indicates some degree of sp2 to sp3 rehybridization of C-6 of the
pyrimidine moiety during hydrolysis. From the data available, it appears that an important pathway in the hydrolysis of the N-
glycoside bond of Urd involves either spontaneous cleavage of Urd which is protonated at the 5-carbon or a protonated species of Urd-H2O. The studies described here, together with the known susceptibility of the 6-position of
pyrimidine heterocycles toward nucleophiles, permits the proposal of chemically reasonable mechanisms for
enzyme-catalyzed cleavage of N-glycosidic bonds of
pyrimidines.