Xanthine oxidase Slow signals

Fig. 5. Multiple phases in the reduction of xanthine oxidase by xanthine at pH 8.2. Intensities of the Rapid (circles) and Slow (triangles) molybdenum EPR signals expressed as electron/mole enzyme (i-e. per 2 atom Mo) are plotted as a function of time. Note the changes in the time scale. Rapid freezing was used for reaction times (at 22°) up to 1 sec. and manual mixing for longer times (at 25°) enzyme concentrations (immediately after mixing) were 0.09 mM and 0.13 mM respectively. The enzyme had Activity/A45o 125 corresponding to 63% of active enzyme and 20 mole xanthine/mole enzyme was used. (Data from ref. 67.)...

E.s.r. studies at liquid-helium temperatures have given new information relating to the structure and mechanism of action of xanthine oxidase, and further studies have been made on the rapid and slow changes in intensity of the two molybdenum(v) signals during anaerobic reduction of the enzyme. The structure and function of ferritin, an iron storage protein which seems to have close links with xanthine oxidase, have been reviewed. ... 

Formaldehyde is a good substrate for xanthine oxidase so that, here at least, the slow inactivating reaction (Booth, 1938) involved in development of the Inhibited signal (Pick et a/., 1971) must represent a side reaction, rather than a step on the main catalytic pathway. 

Scheme 1. This must represent all the signals from species bearing strongly coupled protons, that is Rapid and Slow from xanthine oxidase, as well as the low-pH signals from nitrate reductase and sulfite oxidase. In representing the complementary deprotonated forms where these are not EPR- silent, as structure (b), we are implicitly assuming that Y is a terminal ligand. We shall return to this point shortly. Note that there is no evidence for the anion site in the mgh-pH form of nitrate reductase, so we have omitted A from structure (b).

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