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When still lyophilized the recombinant proteins remain stable at room temperature, unless otherwise stated on the Certificate of Analysis. That said, we do recommend storing the lyophilized proteins at -20°C to -80°C.
Once reconstituted per the instructions provided in the Certificate of Analysis, the recombinant proteins can be stored short-term (up to 1 week) at 4°C. For longer periods, aliquot and store the reconstituted protein in a solution containing a carrier protein (refer to Certificate of Analysis for specific suggestions). The aliquots can be stored at -20°C to -80°C, avoiding multiple freeze/thaw cycles which can accelerate denaturation.
Please refer to the Certificate of Analysis (C of A) provided with your recombinant for specific instructions. Here are some additional suggestions:
- Make sure to do the primary dilution in the solution/liquid described in the Certificate of Analysis. The reconstituted protein may precipitate out of solution or become less stable more quickly if a different liquid is use (ie: PBS when water is called for, etc.)
- Subsequent dilutions, for example to dilute into a buffer containing a carrier protein for longer term storage, can be be done with PBS or water with carrier-protein added.
- For these subsequent dilutions, do not dilute beyond 1 µg/ml, and aliquots should not be less than a volume of 10 µl (independent of concentration).
- Aliquot into working dilutions.
Because the recombinant proteins are lyophilized, there is no need for sodium azide or other preservative to be added at bottling. All of our recombinant proteins are also provided carrier-free and additive-free.
The endotoxin level of our recombinant proteins is guaranteed to be less than 0.1 ng/µg of protein, or 1 EU/µg. However, for many proteins, the actual measured endotoxin values are consistently below this stated endotoxin level.
Our recombinant proteins are lyophilized, and based on the way they are formulated (carrier- and additive-free) they end up as a very thin, sometimes invisible, film that is deposited within the vial. We recommend performing a quick spin (20-30 seconds) in a microcentrifuge prior to opening to ensure all material migrates to the bottom of the vial.
The formula for converting the activity as an ED50 in ng/ml to specific activity in units per mg is:
Note: ED50 is defined as the cytokine concentration at which the activity is 50% of the maximum response. ED50 should only be used for cytokines whose dose-response curves are sigmoidal in shape.
Please refer to the Technical Data Sheet and/or Certificate of Analysis (C of A) for specific information on each individual recombinant protein. Our recombinants are analyzed for the following factors prior to release to ensure their quality:
- Authenticity: Verified by N-terminal and Mass Spectrometry analyses (when applicable).
- Purity: Content Verified by UV Spectroscopy and/or SDS-PAGE gel (refer to C of A).
- Biological Activity: Tested by a relevant in vitro or in vivo bioassay (refer to C of A).
- Protein Content: Content Verified by UV Spectroscopy and/or SDS-PAGE gel.
- Endotoxin Content: Tested by kinetic LAL method.
- Membrane Filtration: Products are sterile-filtered (membrane filtration) prior to vialing.
Most human cytokines are active on mouse cells and while many mouse cytokines are active on human cells, they may show lower specific activity than a human cytokine would. There are some exceptions - the interferons, GM-CSF, IL-3 and IL-4 are very species-specific and demonstrate little activity on cells from a different species. On the other hand, most neurotrophins and the FGF proteins show excellent cross-species activity.