The enzymatic hydrolysis of the phosphate ester bond in some thionucleotides

We prepared the 5'- and 3'-O-phosphorothioate esters of the antitumor agent O2 : 2'-anhydro-1-beta-D-arabinosylcytosine. We also included in this study esters of 2'-thio-2'-deoxycytidine, namely, 2'-S-dCyd-2' : 3'-P, 2'-S-dCyd-2'-P, and 2'-S-dCyd-3'-P, along with natural nucleotides. These compounds were subjected to the action of Escherichia coli alkaline phosphatase, potato acid phosphatase, and bovine pancreatic ribonuclease A. The data were analyzed by Lineweaver-Burk plots to obtain Km and KI values. Only 2'-S-dCyd-2'-P was a substrate for alkaline phosphatase; the anhydro-araCyt phosphorothioates were good competitive inhibitors, while 2'-S-dCyd-3'-P did not associate with the enzyme. Acid phosphatase hydrolyzed all four monoesters investigated, including the S-phosphorothioate. The cyclic phosphorothioate, 2'-S-dCyd-2' : 3'-P was neither hydrolyzed by, nor associated with, ribonuclease A. ORD spectroscopy was also used in an attempt to relate the structural features of analogs to the peculiarity of their hydrolysis.