I'm working with simulated digestive fluids.

Due to the low concentration of pepsin that I need for my work, it is difficult for me to weigh very small amounts of the reagent each time I need to use it (in fresh). That is why I decided to prepare a stock using 50 mg, from which I can take aliquots whenever I need it. But I don't know if it affects the fact of defrosting it continuously every time I need to use it.

I diluted the powder of Pepsin in HCl 0.01 N. I didn't find info about the appropriate storage temperature.

Also, does anybody know if its correct to store the solution in HCl 0.01N?

I emphasize on this because a found a protocol that says: "A stock solution of Pepsin is dissolved in 10 mM Tris buffer, 150 mM NaCl, pH 6.5. The stock solution has to be stored on ice or refrigerated at 4°C. Just before the assay, the concentrations of pepsin in 10 mM HCl has to be prepared".

From this, I understood 2 points:

A) The pH is adjusted to 6.5 to limit proteolytic activity

2) Storage on ice limits proteolytic degradation.

But, as I mentioned before, weighing small amounts increases the systematic error. That is why it is more practical for me to have a stock and keep it in storage.