I have a recombinant protein interleukin as a powder that I need to run in an SDS PAGE of a required concentration. Before mixing it with loading buffer how should I liquefy to get the required concentration?
If the protein is lyophylized, the only thing you need is to add ddH2O (water). Try do dissolve in about 1mg/mL and divide into aliquots.
Dilute to your desired concentration and then add sample buffer.
However, do be aware that this is a very general guideline. It is best to double-check this by whomever supplied you the proteins. If you got it from some company, ask for a datasheet with a clear protocol on how to reconstitute your product.
Good luck.
If the protein is crystallized, precipitated, or etc. and not lyophilized, the buffer you choose to dissolve the protein in would be highly dependent on the properties of the protein. For example, a very hydrophobic or membrane protein probably won't dissolve very well in water or PBS. You could just dissolve the protein in your SDS-PAGE loading buffer (as this will dissolve most proteins), and use the weight of the powdered protein to determine the concentration. If you weigh out 1 mg of your protein and dissolve it in 1 ml of your loading buffer, then you have a 1mg/ml solution and can load it directly on the gel.
For a coomassie stained protein gel 1mg/ml is way too concentrated. You need around 1-10ug/ml and load 10ul to get a good band without too much overloading. If you are using a more sensitive stain (eg silver staining) then you need even less. Try not to dissolve all your protein in loading buffer because it cannot be effectively recovered. IF necessary make a suspension in water and use a small amount of that and then redry the remnant for future experiments.
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