Km values in enzymology - (Oct/16/2010 )

hello everyone

I would like to know whether it is possible to compare Km values of a alkaline phosphatase enzyme which i have purified (recombinant, expressed and purified from E. coli BL21 using afiinity chromatography) with other Km values in published references.Does Km change with the purification method?
Any suggestions please.

comparing enzymatic activity of native enzyme and of bacterial, overexpressed cloned enzymes may be different; comparison is only preliminary