Usually the instructions for the enzyme give a recommended protocol for resuspension. You probably want to make a concentrated stock solution so that you only need to add a small volume to your final solution to get 0.5U of activity. The Sigma brand chitinase is recommended to be dissolved in 50mM phosphate buffer pH 6.0 at 1mg/ml. Therefore you will want to dissolve your 5U of powder (25mg) in 25 ml and then add 2.5ml to your final solution.

Hi there,

It's up to you to define the actual concentration of the solution taking into account the downstream protocol you intend to run... For instance is the 0.5U the quantity require for an assay? If so you should prepare a concentrated stock first. You also have to take into account the limit of solubility of the enzyme...

I assume that overall in your reaction mixture, you need 0.5 U of the enzyme. In that case, as per your calculation, you add 2.5 mg enzyme in the volume of your reaction mixture. So that your final working enzyme activity is 0.5 U