Quantification of Thyroglobulin, a Low-Abundance Serum Protein, by Immunoaffinity Peptide Enrichment and Tandem Mass Spectrometry

• Published in: Clinical chemistry (Baltimore, Md.) Vol. 54; no. 11; pp. 1796 - 1804
• Main Authors: Hoofnagle, Andrew N, Becker, Jessica O, Wener, Mark H, Heinecke, Jay W
• Format: Journal Article
• Language: English
• Published: Washington, DC Am Assoc Clin Chem 01.11.2008; American Association for Clinical Chemistry; Oxford University Press
• Subjects: Amino Acid Sequence; Analytical, structural and metabolic biochemistry; Antibodies; Biological and medical sciences; Biomarkers; Chromatography; Chromatography, Affinity - methods; Chromatography, High Pressure Liquid - methods; Fundamental and applied biological sciences. Psychology; Humans; Immunoassay; Immunoassays; Immunoglobulins; Investigative techniques, diagnostic techniques (general aspects); Liquid chromatography; Mass spectrometry; Medical sciences; Patients; Peptide Fragments - blood; Peptide Fragments - chemistry; Peptide Fragments - isolation & purification; Peptide Mapping; Peptides; Proteins; Reference materials; Reference Standards; Reproducibility of Results; Scientific imaging; Sensitivity and Specificity; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Tandem Mass Spectrometry - methods; Thyroglobulin - blood; Thyroglobulin - chemistry; Thyroglobulin - isolation & purification; Thyroid cancer; Tumors; Enrichment; Peptides; Thyroglobulin; Mass spectrometry MS/MS; Clinical biology; Quantization; Thyroid hormone; Biochemistry; Molecular biology; Serum protein; Quantitative analysis
• ISSN: 0009-9147; 1530-8561
• DOI: 10.1373/clinchem.2008.109652

Quantification of serum tumor markers plays an important role in determining whether patients treated for cancer require further therapy. Whereas large-scale proteomic efforts aim to identify novel tumor markers to facilitate early detection, optimization of methods for quantifying known tumor markers offers another approach to improving management of malignancies. For example, immunoassays used in clinical practice to measure established tumor markers suffer from potential interference from endogenous immunoglobulins and imperfect concordance across platforms-problems that also plague many other immunoassays. To address these important limitations, this study used peptide immunoaffinity enrichment in concert with liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify thyroglobulin, a well-characterized tumor marker. We identified 3 peptides in tryptic digests of thyroglobulin that were detected at low concentrations by tandem mass spectrometry, raised polyclonal antibodies to those peptides, and used the antibodies to extract the 3 corresponding peptides from tryptic digests of human serum. We quantified each endogenous peptide using LC-MS/MS and multiple reaction monitoring with external calibrators. The detection limit for endogenous thyroglobulin in serum was 2.6 microg/L (4 pmol/L). Direct comparison with immunoassay revealed good correlation (r(2) = 0.81). Immunoaffinity peptide enrichment-tandem mass spectrometry can detect tryptic peptides of thyroglobulin at picomolar concentrations while also digesting the endogenous immunoglobulins that can potentially interfere with traditional immunoassays. Our observations suggest a general analytical strategy for using immunoaffinity isolation together with tandem mass spectrometry to quantify tumor antigens and other low-abundance proteins in human serum.