= Solution
\b[The <oxygen> installed at C6 is derived from solvent <water>.] An attacking zinc-bound <water> or <hydroxide> therefore provides the straightforward explanation of the <oxygen-18> label. The result supports hydrolytic addition rather than direct <oxygen> transfer from a pre-existing protein carboxyl group as the sole <oxygen> source.
It does not by itself establish the identity of the base, the timing of <proton transfers>, or exclusion of a transient covalent <enzyme> intermediate: <water> could introduce the labeled <oxygen> during breakdown of such an intermediate. Product <oxygen> exchange must also be excluded. Incubate preformed unlabeled <inosine> in labeled <water> with and without <enzyme>, and use early-time quenched reactions to compare incorporation with product formation. These controls test whether labeling reports the formation step rather than later exchange.
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