Bradford Protein Assay vs. Lowry Protein Assay

Attribute	Bradford Protein Assay	Lowry Protein Assay
Principle	Uses Coomassie Brilliant Blue G-250 dye to bind to proteins and form a complex that can be measured spectrophotometrically.	Uses the reaction between proteins and copper ions in an alkaline environment to form a colored complex that can be measured spectrophotometrically.
Sensitivity	Less sensitive compared to the Lowry assay.	More sensitive compared to the Bradford assay.
Interference	May be affected by certain detergents, reducing agents, and high salt concentrations.	May be affected by certain detergents, reducing agents, and high salt concentrations.
Linear Range	Has a limited linear range for protein quantification.	Has a wider linear range for protein quantification.
Time Required	Generally faster compared to the Lowry assay.	Generally slower compared to the Bradford assay.
Cost	Relatively inexpensive.	Relatively inexpensive.

Introduction

Protein quantification is a crucial step in many biological and biochemical experiments. Two commonly used methods for protein quantification are the Bradford Protein Assay and the Lowry Protein Assay. Both assays are widely used due to their simplicity, sensitivity, and compatibility with a variety of sample types. In this article, we will compare the attributes of these two protein assays, highlighting their differences and similarities.

Principle

The Bradford Protein Assay is based on the binding of Coomassie Brilliant Blue dye to proteins. The dye undergoes a color change from brown to blue when it binds to proteins, resulting in an increase in absorbance at 595 nm. The intensity of the blue color is directly proportional to the protein concentration in the sample.

On the other hand, the Lowry Protein Assay is a colorimetric assay that relies on the reduction of copper ions by proteins in an alkaline medium. The reduced copper ions react with Folin-Ciocalteu reagent, resulting in the formation of a blue color complex. The intensity of the blue color is directly proportional to the protein concentration in the sample.

Sensitivity

Both the Bradford and Lowry assays are known for their high sensitivity in protein quantification. However, the Bradford Protein Assay is generally considered to be more sensitive than the Lowry Protein Assay. The Bradford assay can detect protein concentrations as low as 1-5 μg/mL, while the Lowry assay has a lower detection limit of around 10 μg/mL. This higher sensitivity of the Bradford assay makes it more suitable for samples with low protein concentrations.

Linearity and Range

Linearity refers to the ability of an assay to provide accurate and proportional results over a range of protein concentrations. Both the Bradford and Lowry assays exhibit good linearity, but the range of linearity differs between the two.

The Bradford Protein Assay has a wider linear range compared to the Lowry Protein Assay. The linear range of the Bradford assay typically spans from 0.02 to 2 mg/mL, allowing for accurate quantification of a wide range of protein concentrations. On the other hand, the Lowry assay has a narrower linear range, usually ranging from 0.02 to 0.2 mg/mL. Therefore, if you have samples with protein concentrations outside the linear range of the Lowry assay, the Bradford assay would be a better choice.

Interference

Interference from various substances can affect the accuracy of protein quantification. Both the Bradford and Lowry assays are susceptible to interference, but the nature of interference differs between the two.

The Bradford Protein Assay is known to be more susceptible to interference from detergents, reducing agents, and certain buffer components. These substances can bind to the Coomassie dye or compete with proteins for binding sites, leading to inaccurate protein quantification. On the other hand, the Lowry Protein Assay is less affected by detergents and reducing agents, making it more suitable for samples containing these substances.

Sample Compatibility

Both the Bradford and Lowry assays are compatible with a wide range of sample types, including cell lysates, purified proteins, and biological fluids. However, the Bradford Protein Assay is more tolerant of common contaminants found in biological samples.

The Bradford assay is less affected by substances such as salts, lipids, and nucleic acids, which can interfere with protein quantification. This makes it a preferred choice for samples with complex matrices, such as crude cell lysates or serum samples. On the other hand, the Lowry assay may require additional sample preparation steps, such as lipid removal or nucleic acid precipitation, to minimize interference and obtain accurate protein quantification.

Speed and Convenience

Both the Bradford and Lowry assays are relatively quick and convenient methods for protein quantification. However, the Bradford Protein Assay offers some advantages in terms of speed and ease of use.

The Bradford assay can be performed in a single step, where the protein sample is mixed with the Coomassie dye and the absorbance is measured directly. In contrast, the Lowry assay involves multiple steps, including protein precipitation, copper ion reduction, and color development. These additional steps make the Lowry assay more time-consuming and labor-intensive compared to the Bradford assay.

Cost

Cost is an important consideration when choosing a protein quantification method, especially for laboratories with limited budgets. In terms of cost, the Bradford Protein Assay is generally more economical compared to the Lowry Protein Assay.

The Bradford assay requires fewer reagents and has a simpler protocol, resulting in lower overall costs. Additionally, the Coomassie dye used in the Bradford assay is less expensive compared to the Folin-Ciocalteu reagent used in the Lowry assay. Therefore, if cost-effectiveness is a priority, the Bradford assay may be a more suitable choice.

Conclusion

Both the Bradford Protein Assay and the Lowry Protein Assay are widely used methods for protein quantification. While they share similarities in terms of sensitivity, compatibility, and convenience, they also have distinct attributes that make them suitable for different applications.

The Bradford assay offers higher sensitivity, wider linear range, and better tolerance to common contaminants, making it a preferred choice for samples with low protein concentrations or complex matrices. On the other hand, the Lowry assay may be more suitable for samples containing detergents or reducing agents, and it can provide accurate results within its narrower linear range.

Ultimately, the choice between the two assays depends on the specific requirements of the experiment, such as the protein concentration range, sample type, and presence of interfering substances. Understanding the attributes and limitations of each assay will help researchers make an informed decision and ensure accurate protein quantification in their experiments.