Ya of course proteinase K will dissolve in PBS. why it happened so and did not dissolved not sure. May be the concentration of proteinase K is more than saturation. this could be a reason behind.

I have tried using proteinase K in PBS but the max concentration what i have used is 45mg/10ml. I alos remember making a solution of higher concentration and realized that there were some particle undissolved in the solution. 

You try a low concentration and sure it will work 

thanks

I found adding the powder little by little while mixing to the solution will help dissolving the mixture to completion. After that, may carry out filter sterilization.

Proteinase K likes to be at pH 8.0 for optimal activity:

A good buffer for it is:

Add 25 mL 1 M TRIS-HCl (pH 8.0) + 25 mL 0.5 M EDTA (pH 8.0) + 200 mL deionized water.  Can store at room temp - and use anywhere from 20 to 60C.

PBS is ~pH 7.4  (somewhat suboptimal for proteinase K - but should still work to some extent).

Hello everyone, thanks for your help!

Maybe the ionic concentration of PBS have effect on the dissolve of proteinase K.

Finally, I add deionized water to dilute the final concentration to 2mg/ml and the protinase was dissloved.

Be careful - the "2 mg/mL" concentration you obtained is 10X different from your original target concentration "20 mg/mL". pH is the crucial consideration in this case to achieve the desired concentration/solubility (and to preserve/engender enzyme activity).  If 2 mg/mL is all you need, great, but realize that enzyme activity is also pH-dependent my friend ;]  Proteinase K likes to be at pH 8.0

A classic Proteinase K Buffer has long been:

For 1 Liter of Pro-K Buffer:

15.76 grams TRIS:HCl

18.61 grams EDTA

Adjust pH to 8.0 with 10N NaOH

Qs to 1000 mL

Can proteinase  K be dissolved in 100mM sodium phosphate buffer pH 8.0 ?