Activity of XDH and XR depends on sources. You report your value and compare it with literatures.

My values are rely between 10- 23 U/L.

I think you value is less.

What is the source of isolation of these enzymes? Normally as a rule, the enzymes can retain their activity for a few days at 4C  and then starts decaying as at this temperature, there will be increase in S-S and S-H bonds depending upon the amino acid residues like cysteine, in their primary structure. This will change their conformation leading to loss of activity. Again as I said these are general rules. You have to give me the details of your source and what were the temperature of assay, to get a fairy precise answer.

My source of isolation is thermo-tolerant yeast. and enzyme isolated can be store in -20o C for long time.