Streptavidin (Streptomyces avidinii)
≥ 14.0 U/mg protein
~52,000 Daltons (based on MS), composed of 4 identical polypeptide chains
Predominantly a single band by SDS PAGE.
Supplied lyophilized, contains ~0.9 mg protein/mg lyophilizate.
Size: Available in 10 mg, 100 mg and 1,000 mg pack sizes.
Product Code: SA10
Q. Can I use water to dissolve the material, or do I have to use PBS?
A. Streptavidin is readily soluble in water or salt-containing buffers, up to 50 mg/ml or more.
There is a tendency for lyophilized streptavidin to aggregate when it is re-dissolved in water or other low ionic strength buffers at neutral or acidic pH. ProZyme Streptavidin has been lyophilized from a dilute sodium chloride solution at mildly alkaline pH in order to minimize aggregate formation. In rare cases Streptavidin may contain a small amount of insoluble material when dissolved in de-ionized water or low ionic strength buffers, either at the time it is dissolved or after freezing and re-thawing. This effect is generally not seen in the presence of salt-containing buffers such as PBS. If undissolved material is observed, it can be removed by centrifugation and does not constitute a significant fraction of the total protein.
Our SA10 Technical Data Sheet contains further information.
Q. How do I reconstitute the streptavdin to avoid aggregate formation?
Our standard technique for a 1 gram or 100 mg bottle is to add enough buffer to dissolve to 20 – 50 mg/ml, and immediately begin swirling the bottle (for instance holding it on a counter-top and moving the bottle fairly rapidly in a tight circular motion). We keep this up until we see the protein has dissolved, which might take a couple of minutes. This minimizes aggregate formation, but there is often a little cloudiness that should be removed by centrifugation prior to using the material. When dissolved this way, the recovery after centrifugation should be as stated on the bottle when measured by A280.
This is most important when re-dissolving in water or low ionic strength buffers (as mentioned in the SA10 Tech Data Sheet); dissolving in PBS and higher ionic strength buffers seems to be less sensitive to constant agitation. But even when using PBS, one should not just add the buffer and leave the bottle sitting undisturbed on the bench while waiting for the streptavidin to dissolve.
For 10 mg samples and other samples dispensed to microcentrifuge tubes, the usual procedure is to add enough buffer to dissolve to 5 – 50 mg/ml, and mix by up and down pipetting until the contents appear to have dissolved. With these small samples, re-dissolving is rapid.
Q. What molecular weight should I observe for SA10 on an SDS-PAGE gel? The SA10 Certificate of Analysis and Tech Data sheet state that the product should run as predominantly a single band by SDS-PAGE.
A. Streptavidin is a homotetramer of ~52 kDa. With heat denaturation in SDS-PAGE sample buffer, streptavidin tetramers dissociate into monomers that run at ~13 kDa on an SDS-PAGE gel. If the streptavidin is not boiled in sample loading buffer before SDS-PAGE (e.g., 20 minutes at 100°C in buffer containing 0.2% SDS), you may not achieve complete denaturation of the tetramer into monomers and may see the tetrameric form. As there are no cysteine residues in streptavidin (thus no disulfide bonds) the presence or absence of a reducing agent in the sample (e.g., DTT, β-mercaptoethanol) does not make a difference.
Q. Are there any amine-containing compounds in the product?
A. There are no amine containing-compounds in the SA10 formulation. SA10 can be dissolved directly into an amine-free buffer of choice and reacted with amine-reactive reagents without the need for dialysis or desalting (unless the NaCl is undesirable, which is not usually the case).
Q. Does the product have any additives such as salt?
A. SA10 is supplied lyophilized and contains ~0.9 mg protein/mg lyophilizate; the balance is sodium chloride. This means that every mg of lyophilized product contains 0.1 mg sodium chloride (10%). However, we dispense by protein content – if you purchase a gram of streptavidin, you receive a gram of streptavidin + additional salt, rather than a gram of lyophilizate. We include the information on the NaCl balance in case our customers weigh out the product.
Product Safety Documentation for SA10: