6-ヒドロキシ-5,6-ジヒドリジン中間体のウラシルのウリジンの酸触媒加水分解のメカニズムについて

酸性媒体では、ウリジンの5,6-ダブル結合は急速に水分補給されて、6-ヒドロキシ-5,6-ジヒドリジン（URD-H2O）の少量を与えます。そのメカニズムは、URD-H2Oの酸触媒脱水の研究から知られています（Prior、J。、J。、santi、d。2422-2428）。脱水に加えて、URD-H2Oは酸性溶液中のN-グリコシド結合の直接加水分解も受けます。上記の反応の速度論は、URD-H2OまたはURDからURD-H2Oへの経路の中間体であり、URDのN-グリコシド結合の加水分解を説明するために速度論的に有能であることを示しています。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および製品。重要なことに、[6-3H] URDの加水分解は、25度CでのKT/kH = 1.15の実質的な逆二次同位体効果で進行します。利用可能なデータから、URDのN-グリコシド結合の加水分解における重要な経路には、5炭素でプロトン化されているURD-H2Oでプロトン化されたURDの自発的な切断のいずれかが関与しているようです。ここで説明されている研究は、ヌクレオフィルに対するピリミジンヘテロサイクルの6位置の既知の感受性とともに、ピリミジンのN-グリコシド結合の酵素触媒切断の化学的に合理的なメカニズムの提案を可能にします。

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.