Serum apolipoprotein A-1 quantification by LC–MS with a SILAC internal standard reveals reduced levels in smokers

BACKGROUND Absolute quantification of protein biomarkers such as serum apolipoprotein A1 by both immunoassays and LC-MS can provide misleading results. RESULTS Recombinant ApoA-1 internal standard was prepared using stable isotope labeling by amino acids in cell culture with [(13)C6(15)N2]-lysine and [(13)C9(15)N1]-tyrosine in human cells. A stable isotope dilution LC-MS method for serum ApoA-1 was validated and levels analyzed for 50 nonsmokers and 50 smokers. CONCLUSION The concentration of ApoA-1 in nonsmokers was 169.4 mg/dl with an 18.4% reduction to 138.2 mg/dl in smokers. The validated assay will have clinical utility for assessing effects of smoking cessation and therapeutic or dietary interventions in high-risk populations.