Oxanosine Is a Substrate of Adenosine Deaminase. Implications for the Quest for a Toxicological Marker for Nitrosation Activity

Oxanosine 3r, 5-amino-3-β-(D-ribofuranosyl)-3 H -imidazo[4,5-d][1,3] oxazine-7-one, was isolated as a novel nucleoside antibiotic in 1981 from Streptomyces capreolus MG265-CF3. Oxanosine became relevant in toxicology in 1996 with the discovery that it is formed in nitrosative guanosine deamination. As part of studies of the mechanism of oxanosine formation, the synthesis was attempted of [7- ¹⁸ O] oxanosine by enzymatic ¹⁶ O/ ¹⁸ O-exchange with adenosine deaminase (ADA) in analogy to the synthesis of [6- ¹⁸ O]guanosine from 2-amino-6-chloropurine. Unexpectedly, it was discovered that the incubation of oxanosine 3r with ADA in sodium phosphate buffer (pH = 7.4) results in 1-β-(D-ribofuranosyl)-5- ureido-1 H -imidazole-4-carboxylic acid 4r. The reaction of the 2'-deoxyribose derivative 3d forms 4d in analogy. The reaction products were separated by preparative RP-HPLC and characterized by LC/MS and MS/MS analyses and UV/vis and NMR spectroscopy, and NMR assignments were corroborated by GIAO and GIAO-PCM calculations. Reaction in H ₂ ¹⁸ O leads to ¹⁸ O-incorporation at C7. The hydrolysis of 3 to 4 can be rationalized on the basis of the known mode of action of ADA, and an explanation is provided for ADA's accomplishment of the "usual" substitution at C6 of adenosine (addition to the exocyclic bond) and the "lactone hydrolysis" of oxanosine (addition to the endocyclic double bond). The Michaelis-Men ten constant of K _m = 1.0 (±0.2) mM was measured for oxanosine. Implications are discussed for studies of nitrosative deamination of nucleosides, nucleotides, and oligonucleotides.