(Sandermann and Stromiger. 1972. J. Biol. Chem. 247:5123-5131.)

The advantage of this assay over the Lowry assay from which it is derived, is the presence of SDS in reagent A. This allows one to perform protein assays in nearly any detergent (eg., when detergents are used in the purification of outer membrane proteins). In addition, SDS separates membrane proteins from contaminating membrane constituents and denatures the proteins allowing more reproducible results.
 

SOLUTIONS:

2% Na2CO3, 0.02% NaK tatrate, 0.1M NaOH, 1% SDS (added last).

0.5% CuSO4.

On the day of the assay mix 25ml of solution A with 1 ml of solution B.

Dilute Folins reagent to 1N with deionized water.

ASSAY:

1. Wash all glassware with distilled water to remove detergents.
2. In 13mm borosilicate glass test tubes, add 5 and 10 µl of a ten-fold dilution of outer membrane preparation.
3. Prepare a standard curve by adding 0, 5, 10, 15, 20 and 25 µl of 0.1% BSA in water to test tubes as in step 2. Make sure you prepare 2 samples of 0 BSA so you have enough sample for the reference cuvette (1 mg/ml).
4. Add 1 ml of solution C, allow to stand for 15 minutes at room temperature.
5. Add 0.1 ml of solution D and vortex the test tube immeadiately, allow to stand for 30 minutes at room temperature.
6. Read at 650 nm against a reagent blank.