Protein Determination: Lowry Method

BIOLOGY 371

INDEPENDENT RESEARCH

WEEK 5

Protein Determination - Lowry Procedure

Introduction

The principle behind the Lowry method of determining protein concentrations lies in the reactivity of the peptide nitrogen[s] with the copper [II] ions under alkaline conditions and the subsequent reduction of the Folin-Ciocalteay phosphomolybdicphosphotungstic acid to heteropolymolybdenum blue by the copper-catalyzed oxidation of aromatic acids [Dunn, 13]. The Lowry method is sensitive to pH changes and therefore the pH of assay solution should be maintained at 10 - 10.5.

The Lowry method is sensitive to low concentrations of protein. Dunn [1992] suggests concentrations ranging from 0.10 - 2 mg of protein per mL while Price [1996] suggests concentrations of 0.005 - 0.10 mg of protein per mL. The major disadvantage of the Lowry method is the narrow pH range within which it is accurate. However, we will be using very small volumes of sample which will have little or no effect on pH of the reaction mixture.
A variety of compounds will interfere with the Lowry procedure. These include some amino acid derivatives, certain buffers, drugs, lipids, sugars, salts, nucleic acids and sulphydryl reagents [Dunn, 1992]. Price [1996] notes that ammonium ions, zwitterionic buffers, nonionic buffers and thiol compounds may also interfere with the Lowry reaction. These substances should be removed or diluted before running Lowry assays.

Lowry, O. H., N. J. Rosebrough, A.L. Farr and R. J. Randall, 1951. Protein measurement with the Folin-Phenol reagents. J. Biol. Chem. 193: 265-275.
Dunn, M. J., 1992. Protein determination of total protein concentration. Harris, E. L. V., Angal, S., [Eds], Protein Purification Methods, Oxford: IRL Press.
Price, N. C., 1996. Proteins, Labfax, Oxford: Academic Press.

Reagents

A. 2% Na2CO3 in 0.1 N NaOH
B. 1% NaK Tartrate in H₂O
C. 0.5% CuSO4.5 H₂O in H₂O
D. 48 mL of A, 1 mL of B, 1 mL C
E. Phenol Reagent - 1 part Folin-Phenol [2 N] : 1 part water

[Reagents A, B and C may be stored indefinitely]

BSA Standard - 1 mg/ mL

Bovine Serum Albumin: 5 mg in 5 mL of water [1 µg / µl].
Freeze 1 mL aliquots.

Procedure

[Run triplicate determination for all samples.]

1. Set up eleven sets of three 16 x 150 mm test tubes in rack.
2. Add BSA [0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 µl] to these tubes.
3. Add 2 mL of solution D to each test tube.
4. Incubate for 10 minutes at room temperature.
5. Add 0.2 mL of dilute Folin-phenol solution to each tube.
6. Vortex each tube immediately.
9. Incubate at room temperature for 30 minutes.
10. Determine absorbance of each sample at 600 nm.
11. Plot absorbance vs mg protein to obtain standard curve.
12. Set up triplicate assays for all "unknowns".

Figure 6. Protein concentrations of a crude homogenate and a 45-65% pellet.

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