Bio-Rad Protein Assay - Manual (Page 10)

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Section 3
Common Questions
1. The buffer that I normally use is not listed in the reagent compatibility
list. How will I know if it interferes with the Bio-Rad Protein Assay?
It is best to run two standard curves, one with protein in the same
buffer as your sample and one with protein in water, and then plot a
graph of protein concentration versus absorbance. If the buffer does
not interfere, the two graphs of the standard curve will have identical
slope. Partial interference can be compensated for by adding the
buffer or interfering component to the standard curve for the actual
protein assay.
2. My sample contains a detergent concentration which is incompatible
with the Bio-Rad Protein Assay. How can I assay for protein?
If the protein concentration is high enough, a sample with detergent
can be diluted so that the concentration of detergent is reduced to
0.1% or less. Excess detergent can also be removed from a protein
solution with Bio-Beads
® SM-2 adsorbent.
Alternatively, the Bio-Rad DC (detergent compatible) Protein Assay
can be used. The DC Protein Assay is a modified Lowry assay which
works in the presence of 1% ionic or nonionic detergent. This two-
step method is ready to evaluate in just 15 minutes, and each kit will
assay up to 500 samples with the standard method, or 10,000 samples
with the microplate method.
3. Which protein assay method should I choose, the Bio-Rad (Bradford)
Protein Assay or the DC (detergent compatible) Protein Assay?
There are distinct advantages for each assay depending upon the
application. The Bio-Rad Protein Assay, based on the Bradford
method, can be used in the presence of sugars, DTT, and 2-mercap-
toethanol which may interfere with the Bio-Rad DC Protein Assay.
Alternatively, the Bio-Rad DC Protein Assay can be used in the pres-
ence of detergents and sodium hydroxide, two components known to
interfere with the Bradford assay. If the sample is in a buffer that is
compatible with both assays, then either may be used.
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