Technical Notes: Recombinant Protein Frequently Asked Questions

R&D Systems takes pride in a knowledgeable technical service staff that strives to find solutions for a wide variety of customer questions. We wanted to take this opportunity to address several of those questions that appear to come up more regularly than others.

What does “CF” on the end of some catalog numbers mean?

CF stands for Carrier-Free. We add 50 µg BSA (carrier protein) per 1 µg of protein for stability. The carrier-free version does not contain BSA. Usually proteins with BSA can be stored at a more dilute concentration than their carrier-free counterparts and may have a longer shelf-life. Generally, one will want to purchase the version with BSA when adding the protein to cell/tissue culture or when used as an ELISA standard. Some situations where carrier-free protein might be called for include in vivo applications, when labeling the protein, or any other application where the BSA might interfere with the experiment.

Why do some proteins require reconstitution in an acidic buffer?

The acid reconstitution of certain proteins is critical. Some proteins have a high isoelectric point causing the molecule to be hydrophobic at neutral pH. If the suggested buffer is not used, the cytokine may not go into solution fully, or it may be lost on the surface of labware such as pipet tips, vials, and test tubes. We also recommend storing the aliquots in the reconstitution buffer for the same reason. One can dilute the aliquots of protein to working concentration in the appropriate buffer for one’s application. This will buffer the small amount of acid added and make it safe to use with live cells.

How are units calculated for your protein?

Because there are several ways to determine unit values, units can be a confusing way to describe activity of a protein. To follow a protocol given in units, or to compare to another vendor’s product sold in units, one must verify how the author/source defined the unit. The commonly accepted definition is that 1 unit is equal to the ED50 in a given bioassay. Varying conditions can cause a different ED50. To avoid the obvious problems resulting from this, R&D Systems intentionally sells nearly all of its proteins by mass rather than units. If you find that the author/vendor has compared their “unit” to a WHO standard unit or International Unit (IU), you may find the conversion information contained in the “Unit Conversion Table” listed on our website. This table contains side-by-side comparison data of R&D Systems and WHO standard proteins.