The Lowry Method for Protein Quantitation
Jakob H. Waterborg
1. Introduction
The most accurate method of determining protein concentration is probably acid
hydrolysis followed by amino acid analysis. Most other methods are sensitive to the
amino acid composition of the protein, and absolute concentrations cannot be obtained
(1). The procedure of Lowry et al. (2) is no exception, but its sensitivity is moderately
constant from protein to protein, and it has been so widely used that Lowry protein
estimations are a completely acceptable alternative to a rigorous absolute determination
in almost all circumstances in which protein mixtures or crude extracts are
involved.
The method is based on both the Biuret reaction, in which the peptide bonds of
proteins react with copper under alkaline conditions to produce Cu+, which reacts with
the Folin reagent, and the Folin–Ciocalteau reaction, which is poorly understood but in
essence phosphomolybdotungstate is reduced to heteropolymolybdenum blue by the
copper-catalyzed oxidation of aromatic amino acids. The reactions result in a strong
blue color, which depends partly on the tyrosine and tryptophan content. The method is
sensitive down to about 0.01 mg of protein/mL, and is best used on solutions with
concentrations in the range 0.01–1.0 mg/mL of protein.
2. Materials
1. Complex-forming reagent: Prepare immediately before use by mixing the following stock
solutions in the proportion 100:1:1 (by vol), respectively:
Solution A: 2% (w/v) Na2CO3 in distilled water.
Solution B: 1% (w/v) CuSO4·5H2O in distilled water.
Solution C: 2% (w/v) sodium potassium tartrate in distilled water.
2. 2 N NaOH.
3. Folin reagent (commercially available): Use at 1 N concentration.
4. Standards: Use a stock solution of standard protein (e.g., bovine serum albumin fraction V)
containing 2 mg/mL protein in distilled water, stored frozen at –20°C. Prepare standards
by diluting the stock solution with distilled water as follows:
Stock solution (μL) 0 2.5 5 12.5 25 50 125 250 500
Water (μL) 500 498 495 488 475 450 375 250 0
Protein conc. (μg/mL) 0 10 20 50 100 200 500 1000 2000
3. Method
1. To 0.1 mL of sample or standard (see Notes 1–4), add 0.1 mL of 2 N NaOH. Hydrolyze at
100°C for 10 min in a heating block or boiling water bath.
2. Cool the hydrolysate to room temperature and add 1 mL of freshly mixed complex-forming
reagent. Let the solution stand at room temperature for 10 min (see Notes 5 and 6).
3. Add 0.1 mL of Folin reagent, using a vortex mixer, and let the mixture stand at room
temperature for 30–60 min (do not exceed 60 min) (see Note 7).
4. Read the absorbance at 750 nm if the protein concentration was below 500 μg/mL or at
550 nm if the protein concentration was between 100 and 2000 μg/mL.
5. Plot a standard curve of absorbance as a function of initial protein concentration and use it
to determine the unknown protein concentrations (see Notes 8–13).
4. Notes
1. If the sample is available as a precipitate, then dissolve the precipitate in 2 N NaOH and
hydrolyze as described in Subheading 3, step 1. Carry 0.2-mL aliquots of the hydrolyzate
forward to Subheading 3, step 2.
2. Whole cells or other complex samples may need pretreatment, as described for the Burton
assay for DNA (3). For example, the perchloroacetic acid (PCA)/ethanol precipitate from
extraction I may be used directly for the Lowry assay, or the pellets remaining after the
PCA hydrolysis step (Subheading 3, step 3 of the Burton assay) may be used for Lowry.
In this latter case, both DNA and protein concentration may be obtained from the same
sample.
3. Peterson (4) has described a precipitation step that allows the separation of the protein
sample from interfering substances and also consequently concentrates the protein sample,
allowing the determination of proteins in dilute solution. Peterson’s precipitation step is
as follows:
a. Add 0.1 mL of 0.15% deoxycholate to 1.0 mL of protein sample.
b. Vortex-mix, and stand at room temperature for 10 min.
c. Add 0.1 mL of 72% trichloroacetic acid (TCA), vortex-mix, and centrifuge at 1000–
3000g for 30 min.
d. Decant the supernatant and treat the pellet as described in Note 1.
4. Detergents such as sodium dodecyl sulfate (SDS) are often present in protein preparations,
added to solubilize membranes or remove interfering substances (5–7). Protein precipitation
by TCA may require phosphotungstic acid (PTA) (6) for complete protein
recovery:
a. Add 0.2 mL of 30% (w/v) TCA and 6% (w/v) PTA to 1.0 mL of protein sample.
b. Vortex-mix, and stand at room temperature for 20 min.
c. Centrifuge at 2000g and 4°C for 30 min.
d. Decant the supernatant completely and treat the pellet as described in Note 1.
5. The reaction is very pH dependent, and it is therefore important to maintain the pH between
10 and 10.5. Therefore, take care when analyzing samples that are in strong buffer outside
this range.
6. The incubation period is not critical and can vary from 10 min to several hours without
affecting the final absorbance.
7. The vortex-mixing step is critical for obtaining reproducible results. The Folin reagent is
reactive only for a short time under these alkaline conditions, being unstable in alkali, and
great care should therefore be taken to ensure thorough mixing.
8. The assay is not linear at higher concentrations. Ensure that you are analyzing your sample
on the linear portion of the calibration curve.
9. A set of standards is needed with each group of assays, preferably in duplicate. Duplicate
or triplicate unknowns are recommended.
10. One disadvantage of the Lowry method is the fact that a range of substances interferes
with this assay, including buffers, drugs, nucleic acids, and sugars. (The effect of some of
these agents is shown in Table 1 in Chapter 3.) In many cases, the effects of these agents
can be minimized by diluting them out, assuming that the protein concentration is sufficiently
high to still be detected after dilution. When interfering compounds are involved,
it is, of course, important to run an appropriate blank. Interference caused by detergents,
sucrose, and EDTA can be eliminated by the addition of SDS (5) and a precipitation step
(see Note 4).
11. Modifications to this basic assay have been reported that increase the sensitivity of the
reaction. If the Folin reagent is added in two portions, vortex-mixing between each addition,
a 20% increase in sensitivity is achieved (8). The addition of dithiothreitol 3 min
after the addition of the Folin reagent increases the sensitivity by 50% (9).
12. The amount of color produced in this assay by any given protein (or mixture of proteins)
is dependent on the amino acid composition of the protein(s) (see Introduction). Therefore,
two different proteins, each for example at concentrations of 1 mg/mL, can give
different color yields in this assay. It must be appreciated, therefore, that using bovine
serum albumin (BSA) (or any other protein for that matter) as a standard gives only an
approximate measure of the protein concentration. The only time when this method gives
an absolute value for protein concentration is when the protein being analyzed is also used
to construct the standard curve. The most accurate way to determine the concentration of
any protein solution is amino acid analysis.
13. A means of speeding up this assay using raised temperatures (10) or a microwave oven
(see Chapter 5) has been described.
References
1. Sapan, C. V., Lundblad, R. L., and Price, N. C. (1999) Colorimetric protein assay techniques.
Biotechnol. Appl. Biochem. 29, 99–108.
2. Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) Protein measurement
with the Folin phenol reagent. J. Biol. Chem. 193, 265–275.
3. Waterborg, J. H. and Matthews, H. R. (1984) The Burton assay for DNA, in Methods in
Molecular Biology, Vol. 2: Nucleic Acids (Walker, J. M., ed.), Humana Press, Totowa, NJ,
pp. 1–3.
4. Peterson, G. L. (1983) Determination of total protein. Methods Enzymol. 91, 95–121.
5. Markwell, M.A.K., Haas, S. M., Tolbert, N. E., and Bieber, L. L. (1981) Protein determination
in membrane and lipoprotein samples. Methods Enzymol. 72, 296–303.
6. Yeang, H. Y., Yusof, F., and Abdullah, L. (1998) Protein purification for the Lowry assay:
acid precipitation of proteins in the presence of sodium dodecyl sulfate and other biological
detergents. Analyt. Biochem. 265, 381–384.
7. Chang, Y. C. (1992) Efficient precipitation and accurate quantitation of detergent-solubilized
membrane proteins. Analyt. Biochem. 205, 22–26.
8. Hess, H. H., Lees, M. B., and Derr, J. E. (1978) A linear Lowry-Folin assay for both watersoluble
and sodium dodecyl sulfate-solubilized proteins. Analyt. Biochem. 85, 295–300.
9. Larson, E., Howlett, B., and Jagendorf, A. (1986) Artificial reductant enhancement of the
Lowry method for protein determination. Analyt. Biochem. 155, 243–248.
10. Shakir, F. K., Audilet, D., Drake, A. J., and Shakir, K. M. (1994) A rapid protein determination
by modification of the Lowry procedure. Analyt. Biochem. 216, 232–233.
