 Solution

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.