The effect of pH on the exchangeability with deuterium of protons coupled to molybdenum(V) in the active and the desulpho forms of xanthine oxidase.

The effect of pH variation on the exchangeability with deuterium of protons strongly coupled to Mo(V) in the active and desulpho forms of xanthine oxidase was studied by e.p.r. and rapid freezing, in extension of the work of Gutteridge, Tanner & Bray [Biochem. J. (1978) 175, 887-897]. Above neutrality, exchange rates increased with increasing pH. Detailed studies were made on the desulpho enzyme under a variety of conditions, and exchange rate constants at 22 degrees C ranged from 0.16s -1 at pH 6.6 to 1.6s -1 at pH 11.3. The mechanism of proton exchange in the enzyme is discussed. The interpretation by the above workers that the strongly coupled proton of the active enzyme is on sulphur and that of the desulpho enzyme is on oxygen remains valid (and is in agreement with other work), as do their proposals for the structures of the protonated and deprotonated species. However, pK values cannot be calculated from the exchange data. It is likely that the relatively low rates of exchange observed are due to the difference of structure between the protonated and the deprotonated forms. In the case of the desulpho enzyme, an exchange mechanism, which involves the proton exchanging both as such and along with oxygen in the form of a hydroxyl ion, is discussed.