Proetin Quantification of Biologics

Protein Quantification: Methods, Suitability, and Limitations

The determination of protein content is one of the core analytical tasks in quality control as well as in the research and development of protein-based products. Whether dealing with monoclonal antibodies, recombinant proteins, enzymes, or other protein therapeutics, reliable and reproducible quantification is essential for assessing product quality, dosage, stability, process consistency, and batch comparability. Furthermore, protein concentration often serves as the starting point for subsequent characterization and release testing.

To address these requirements, HWI employs a broad range of established and advanced protein quantification techniques. Depending on the sample type and analytical objective, we determine either the total protein content of a sample or the concentration of a specific target protein. The selection of the most suitable method is made on a case-by-case basis, taking into account the sample matrix as well as the required sensitivity, selectivity, and regulatory expectations.

Colorimetric Methods

Among the established colorimetric methods for protein quantification are the Biuret assay, the Lowry assay, the Bradford assay, and the BCA assay. All of these techniques rely on a colorimetric reaction whose intensity is proportional to the amount of protein present.  The Biuret assay is one of the oldest methods for protein quantification. It is based on the formation of complexes between copper ions and peptide bonds under alkaline conditions. The method is robust and straightforward to perform but offers relatively low sensitivity and is therefore often complemented today by more sensitive techniques.

An advancement of this approach is the Lowry assay. It combines the Biuret reaction with an additional redox reaction using the Folin-Ciocalteu reagent. As a result, the assay achieves significantly higher sensitivity but is also more susceptible to matrix effects and interfering substances.
One of the most widely used methods today is the Bradford assay, which is also employed at HWI in microplate-based analytical workflows. In this assay, Coomassie Brilliant Blue binds to protein structures and produces a measurable color change proportional to the protein concentration. The Bradford assay is valued for its high sensitivity, ease of use, and suitability for high-throughput applications. In a microplate format, large numbers of samples can be analyzed simultaneously, making it a standard tool in routine analytics and quality control. For samples with more complex matrices, the BCA assay provides a robust alternative. It is based on the reduction of copper ions by proteins, followed by a colorimetric reaction with bicinchoninic acid. The assay delivers linear and precise results and is more tolerant of many detergents than the Bradford assay. As with the Bradford method, the microplate format enables efficient processing of high sample numbers while minimizing sample consumption.

Fluorescence-Based Methods

In addition to colorimetric approaches, fluorescence-based assays are frequently used for protein quantification. These methods typically offer higher sensitivity and are particularly suitable for low protein concentrations. Well-known examples include NanoOrange®, Qubit™ Protein Assays, and other dye-based fluorescence assays.

At HWI, the NanoOrange® assay is among the methods utilized. The reagents employed exhibit virtually no intrinsic fluorescence in aqueous solution and generate a strong fluorescent signal only after binding to protein structures. This enables highly sensitive quantification of very small protein amounts. In addition, the NanoOrange assay is less dependent on protein composition and less susceptible to interference from contaminants such as nucleic acids. Combined with microplate readers, it allows accurate analyses using minimal sample volumes and is particularly well suited for demanding quantitative applications.

UV-Based Methods

One of the most direct methods for protein quantification is UV absorption measurement at 280 nm. Proteins absorb light in this wavelength range primarily due to the aromatic amino acids tryptophan and tyrosine, and to a lesser extent phenylalanine and disulfide bonds. This method is particularly suitable for purified and well-characterized proteins and is characterized by its rapid execution and the absence of additional reagents. However, because the signal depends on the number of aromatic amino acids present, it does not represent a classical determination of total protein content. Instead, it provides a concentration measurement of defined proteins with known extinction coefficients.

A modern advancement of this approach is the soloVPE® technology, which is also based on UV absorption but utilizes a variable path length. This allows highly concentrated protein samples to be quantified directly and often without the need for additional dilution, thereby reducing potential dilution errors. The main limitations of UV-based methods arise from potential interfering substances within the sample matrix and their dependence on the amino acid composition of the protein. Consequently, these methods are only of limited suitability for complex matrices or heterogeneous protein mixtures.

Chromatographic and Mass Spectrometric Methods

A special position among chromatographic approaches is occupied by amino acid analysis (AAA). In this method, proteins are first completely hydrolyzed. The released amino acids are subsequently separated chromatographically, quantified, and used to calculate the original protein concentration. Due to its high accuracy and traceability, amino acid analysis is considered a reference method for determining the concentration of proteins and monoclonal antibodies. In the biopharmaceutical field, it is frequently used to verify or calibrate other quantification methods and is described as a suitable analytical approach in the ICH Q6B guideline. Compared with assay-based or UV-based techniques, amino acid analysis is considerably more time- and labor-intensive and is therefore primarily used for reference measurements and protein characterization studies.

In addition, chromatographic methods such as reversed-phase high-performance liquid chromatography (RP-HPLC) are used for the quantification of specific recombinant proteins, while size-exclusion chromatography with UV detection (SEC-UV) and size-exclusion chromatography coupled to multi-angle light scattering (SEC-MALS) are applied to investigate protein aggregates and high molecular weight species. These methods enable the quantification of specific proteins and provide information on critical quality attributes such as purity, monomer content, and aggregation. In contrast to classical protein assays, their primary focus is not the determination of total protein content but the characterization of defined protein species.

For comprehensive structural characterization, chromatographic information alone is often insufficient. This is where mass spectrometric methods become essential. Intact mass analysis enables determination of the molecular mass of intact proteins and provides an initial assessment of product heterogeneity. LC-MS/MS techniques are particularly valuable for protein and peptide identification as well as for the characterization of post-translational modifications. Furthermore, they allow detailed purity and heterogeneity profiling and the investigation of product-related variants. For complex biologics, mass spectrometry has become an indispensable tool for structural characterization and quality assessment.
LC-MS/MS-based approaches encompass various acquisition strategies with different objectives. Techniques such as Multiple Reaction Monitoring (MRM) allow highly specific quantification of individual proteins or defined protein markers. By contrast, approaches such as Data-Dependent Acquisition (DDA) and Data-Independent Acquisition (DIA) support not only protein identification but also the investigation of structural features and modifications.

Immunological Methods

For the highly sensitive determination of specific proteins, immunological techniques such as the Enzyme-Linked Immunosorbent Assay (ELISA) are frequently employed. Detection is achieved through highly specific antibodies, enabling quantification of target proteins even in complex matrices and at very low concentrations. As with other target-specific methods, ELISA does not determine total protein content. Instead, it selectively detects and quantifies a specific protein of interest.

Analytical Classification

The methods described above provide different types of information about a sample. Colorimetric and fluorescence-based assays are commonly used to determine total protein content or total protein amount. UV-based methods, such as A280 measurements, enable direct concentration determination of purified and characterized proteins, provided that the extinction coefficient is known and interfering matrix components are largely absent.
Chromatographic methods provide varying information depending on the detection principle employed. Amino acid analysis serves as a reference method for precise and traceable determination of absolute protein content. RP-HPLC and SEC-UV are frequently applied for quantification, purity assessment, and the detection of degradation products and aggregates. SEC-MALS additionally enables determination of absolute molecular mass and aggregate characterization. Mass spectrometric methods provide further information on protein identity, sequence, and structure, while immunological methods are primarily used for the sensitive and selective detection and quantification of specific target proteins.
No single analytical method can answer all relevant questions simultaneously. Therefore, the selection of an appropriate technique always depends on the sample type, analytical objective, required sensitivity, and regulatory requirements.

Conclusion

Protein quantification encompasses a broad range of analytical technologies, from classical colorimetric assays and fluorescence- or UV-based methods to chromatographic, mass spectrometric, and immunological approaches. The most suitable method is determined by the specific analytical question as well as the required levels of accuracy, selectivity, and informational value.
While protein assays and UV-based techniques provide rapid and efficient quantification, the comprehensive characterization of biologics, biosimilars, and antibody-drug conjugates requires the application of additional orthogonal methods. Only the combination of complementary analytical techniques enables a scientifically sound assessment of critical quality attributes of modern biopharmaceutical products, including identity, stability, purity, and structural integrity.