Xanthine oxidase and cytochrome c - superoxide assay

[Jenny]

Hi, Happy New Year!
I am trying to get a standard curve for superoxide by using adding xanthine
oxidase to hypoxanthine, which produces superoxide which will reduce
ferricytochrome c.  I am using a spec, at 550nm and so far nothing is
happening... i was wondering if anyone has tried this, or knows if the
enzyme needs special attention when being thawed and resuspended in buffer
(i am using pH7.4 phosphate).
looking forward to hearing from someone!!
jenny mortimer