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Studies have been carried out of effects of 17O substitution on a Mo(V) e.p.r. signal from xanthine oxidase, known as Very Rapid. This transient signal is believed to represent an intermediate in enzymic turnover. When Very Rapid was developed from enzyme equilibrate with 17O-enriched water, strong coupling of Mo(V) to a single oxygen atom was observed, with A(17O)1,2,3 1.34, 1.40, 1.36 mT. The isotropic character of the splittings is interpreted as favouring a structure of the type Mo--O--C. The rate of exchange with water of the oxygen atom detected in the signal was studied. In oxidized enzyme, which contains a terminal oxygen ligand, the exchange rate constant was 2--4 h-1 (pH 5.9--7.8 and about 20 degrees C). However, if the exchange was allowed to take place whilst the enzyme was turning over a substrate, then the process occurred within a few seconds. The present and previous results are interpreted as favouring an enzymic mechanism in which a terminal oxygen ligand reacts, as a nucleophile, with a substrate carbonium ion. To complete the reaction, product liberation, by hydrolysis of the enzyme-bound species, occurs in such a way as to cleave the Mo--O bond, thus explaining the fast oxygen exchange in the presence of the substrate.

Oxygen-17 splitting of the very rapid molybdenum(V) e.p.r. signal from xanthine oxidase. Rate of exchange with water of the coupled oxygen atom