Phosphorothioate analogues of 2',5'-oligoadenylate. Activation of 2',5'-oligoadenylate-dependent endoribonuclease by 2',5'-phosphorothioate cores and 5'-monophosphates

The preceding paper in this issue described the synthesis and structural elucidation of the phosphorothioate analogues of 2',5'-oligoadenylate (2-5A) dimer and trimer cores. In this report, the binding and activation processes of 2-5A-dependent endoribonuclease (RNase L) have been examined by using four diastereomeric 2',5'-phosphorothioate trimer core analogues and their 5'-monophosphates. These 2',5'-phosphorothioates have revealed a distinct separation of the structural parameters that govern binding vs activation of RNase L. Radiobinding assays have demonstrated that extensive stereochemical modification of the internucleotide linkages of 2-5A is possible without adversely affecting its ability to bind to RNase L. However, a marked difference was observed in the activation of RNase L by the stereochemically modified 2-5A molecules as determined in core-cellulose and rRNA cleavage assays. Three of the four 2',5'-phosphorothioate trimer cores (with R/sub P/R/sub P/, S/sub P/R/sub P/, and R/sub P/S/sub P/ internucleotide linkages) are the first 2-5 A core molecules of able to activate RNase L. The order of RNase L activation was the same for the core analogues and their 5'-monophosphates. The binding/activation ability of the 2',5'-phosphorothioate cores could result from the combined effect of stereoconfigurational and electronic alterations brought about by the replacement of oxygen by sulfur in the internucleotide linkages. The S/sub P/S/sub P/ and pS/sub P/S/sub P/ analogues inhibit the activation process as shown in competition assays. This new 2-5A analogue (pS/sub P/S/sub P/) is the most effective inhibitor of RNase L reported to data and will be useful in in vivo studies. The results reported here are consistent with the hypothesis that RNase L is a functionally stereoselective enzyme and that the binding process is independent of the activation process