There are several steps one should take to dissolve proteinase K properly. The steps below describe how to dissolve a proteinase K stock solution of 20 mg/mL.
Measure out 5 mL of 200 mM Tris Buffer (pH 8.0), RNase free
Add 1.66 mg (3mM) calcium chloride to the Tris Buffer and dissolve
Measure out 500 µl of the Tris-CaCl2 buffer into a centrifuge tube
Add 20 mg proteinase K and mix gentle to dissolve
Sterilize the solution with a 0.22  µl filter
Fill to a final volume of 1 ml with 100% glycerol
Mix completely until dissolved and store in aliquots at -20°C for one year
There are also alternate protocols with different procedures. For example, 20 mg/mL of proteinase K can also be dissolved in 50 mM Tris-HCL (pH 8), 1-5 mM calcium acetate and stored at 2-8 degrees celsius. The ideal buffer for proteinase K will vary from experiment to experiment.