Stereochemical course of DNA hydrolysis by nuclease S1

Nuclease S1 hydrolyzes the Sp-diastereomer of 5'-O-(2'-deoxyadenosyl)-3'-O-thymidyl phosphorothioate in H2(18)O to [18O]deoxyadenosine 5'-O-phosphorothioate which can be phosphorylated enzymatically to the Sp-diastereomer of [alpha-18O]deoxyadenosine 5'-O-(1-thiotriphosphate). 31P nmr spectroscopy shows the oxygen-18 in this compound to be in a nonbridging position at the alpha-phosphorus, indicating that the hydrolysis reaction catalyzed by nuclease S1 proceeds with inversion of configuration at phosphorus. This result is compatible with a direct nucleophilic attack of H2O at phosphorus without the involvement of a covalent enzyme intermediate.