Hi there,

kcat or enzyme specific activity is what you need. But as such data are highly  dependent on the reaction condition I don't think any database has been thought... 

Yeah, that's a tough one. As Dominique says turnover rates and specificity constants are so dependent on conditions it is incorrect to talk about a single value for these for an enzyme. Try BRENDA  http://www.brenda-enzymes.info/ or one of the other big enzyme databases for kinetic constant's. The best advice is to think of the physiologically relevant conditions under which your enzyme works and compare the database/literature conditions to see if they vary a lot and dependent on what parameters i.e. pH, osmolarity etc

An experimental approach under your conditions with your enzyme sample is advisable.

Thanks Dominique Liger, Stephen Fairweather, and Adam B Shapiro.

So, the best way to find out using i guess by  Michaelis–Menten equation

It is true, there is no specific data base. 

But I would recommend BRENDA-ENZYMES as a reference. For your xanthine oxidase look for :

http://www.brenda-enzymes.org/enzyme.php?ecno=1.17.3.2

You can found a lof of KM values.