I found some old stocks (2005, 2008) of protease inhibitor cocktails in our -20 C freezer. My instinct is that these are too old (and that I don't want to take chances with my proteins!), but how old have you comfortably gone? 1 year? 2 years? 6 months? Would storing the stock at -80 C help?
I notice that this Sigma PMSF datasheet suggests a 3 year shelf life when properly stored (https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Product_Information_Sheet/1/p7626pis.pdf).
For example, our cocktails have some combination of the following in ethanol: PMSF (Phenylmethylsulfonyl fluoride), Benzamidine, Pepstatin A, Leupeptin, Aprotinin, Antipain, EDTA and EGTA
Thanks!
Despite all commercial companies recommend some limited lifetime for their protease inhibitor cocktails products, they actually try to be on the safe side of any potential troubles from their customers. If your freezer is capable to keep the temperature at -20°C all the time (without thawing cycles) you don't have to worry about the activity of your protease inhibitors stock solutions. If so, you can use your stock solutions even from 2005 if they were prepared and immediately frozen after preparation. The critical point is how long the protease inhibitor cocktails remained in solution before freezing.
don't risk it and throw them away - you're just adding to the list of things that could possibly go wrong with your experiment. PMSF especially - maybe a month or two at most (in isopropanol, storing at -80 helps a bit more). After all, PMSF is a good [irreversible] serine protease inhibitor precisely because it is reactive (and therefore unstable).
Better trust your instinct and use fresh inhibitors. The old preparations might or might not work...
Regards, Christian
It all depends on how soon you want to have data and funding situation. There is an old saying that any doubt - through it out. But if you wish to save money, try once with or without adding the old inhibitor. If it does not show any activity, then buy new one.
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