Does the measurement of xanthine oxidase (XOD)/xanthine dehydrogenase (XDH) ratio reflect the actual tissue content of these enzymes?

The purine oxidation via XOD formed from XDH in ischemia in an oxidative or proteolytic way is regarded as a source of reactive O2- species. Usually the XOD/XDH ratio is measured to determine the capacity of tissues to form these important inducers of cell damage. But we found that it is difficult to fix the in vivo XOD/XDH ratio, because preparation and measurement time and conditions (absence or presence of SH-protecting agent, of cations) influenced this ratio. A supposed additional proteolytic increase of XOD during the killing procedure could not confirmed.