The trans-2-deoxyribosylation of 4-thiouracil (4SUra) and 2-thiouracil (2SUra), as well as 6-azauracil, 6-azathymine and 6-aza-2-thiothymine was studied using dG and E. coli purine nucleoside phosphorylase (PNP) for the in situ generation of 2-deoxy-α-D-ribofuranose-1-phosphate (dRib-1P) followed by its coupling with the bases catalyzed by either E. coli thymidine (TP) or uridine (UP) phosphorylases. 4SUra revealed satisfactory substrate activity for UP and, unexpectedly, complete inertness for TP; no formation of 2’-deoxy-2-thiouridine (2SUd) was observed under analogous reaction conditions in the presence of UP and TP. On the contrary, 2SU, 2SUd, 4STd and 2STd are good substrates for both UP and TP; moreover, 2SU, 4STd and 2’-deoxy-5-azacytidine (Decitabine) are substrates for PNP and the phosphorolysis of the latter is reversible. Condensation of 2SUra and 5-azacytosine with dRib-1P (Ba salt) catalyzed by the accordant UP and PNP in Tris∙HCl buffer gave 2SUd and 2’-deoxy-5-azacytidine in 27% and 15% yields, respectively. 6-Azauracil and 6-azathymine showed good substrate properties for both TP and UP, whereas only TP recognizes 2-thio-6-azathymine as a substrate. 5-Phenyl and 5-tert-butyl derivatives of 6-azauracil and its 2-thioxo derivative were tested as substrates for UP and TP, and only 5-phenyl- and 5-tert-butyl-6-azauracils displayed very low substrate activity. The role of structural peculiarities and electronic properties in the substrate recognition by E. coli nucleoside phosphorylases is discussed.
对4-硫代尿嘧啶(4S-Ura)和2-硫代尿嘧啶(2S-Ura),以及6-氮杂尿嘧啶、6-氮杂胸腺嘧啶和6-氮杂-2-硫代胸腺嘧啶进行了研究,使用dG和E. coli嘌呤核苷酸磷酸化酶(PNP)进行原位生成2-脱氧-α-D-核糖呋喃糖-1-磷酸(dRib-1P),然后通过E. coli胸腺嘧啶(TP)或尿嘧啶(UP)磷酸化酶催化其与碱基的结合。4S-Ura对UP显示了令人满意的底物活性,而对TP意外地完全不活化;在UP和TP存在的类似反应条件下未观察到2'-脱氧-2-硫代尿苷(2S-Ud)的形成。相反,2S-U、2S-Ud、4S-Td和2S-Td对UP和TP都是良好的底物;此外,2S-U、4S-Td和2'-脱氧-5-氮杂胞嘧啶(Decitabine)是PNP的底物,后者的磷酸解反应是可逆的。在Tris∙HCl缓冲液中,由相应的UP和PNP催化的2S-Ura和5-氮杂胞嘧啶与dRib-1P(Ba盐)的缩合反应分别产生了27%和15%的2S-Ud和2'-脱氧-5-氮杂胞嘧啶。6-氮杂尿嘧啶和6-氮杂胸腺嘧啶对TP和UP都表现出良好的底物特性,而只有TP能识别2-硫代-6-氮杂胸腺嘧啶作为底物。对6-氮杂尿嘧啶及其2-硫代衍生物的5-苯基和5-叔丁基衍生物进行了UP和TP底物的测试,只有5-苯基和5-叔丁基-6-氮杂尿嘧啶显示出非常低的底物活性。讨论了结构特异性和电子性质在E. coli核苷酸磷酸化酶底物识别中的作用。