Plasma PCSK9 measurement by liquid chromatography-Tandem mass spectrometry and comparison with conventional ELISA

The combination of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and trypsin proteolysis is an effective tool for accurate quantitation of multiple proteins in a single run. However, expensive samples pre-treatment as immunoenrichment are often required to analyze low abundant proteins. Plasma proprotein convertase subtilisin/kexin type 9 (PCSK9), a circulating regulator of low-density lipoprotein metabolism, was studied as an example of a low abundant plasma protein. We investigated post-proteolysis solid-phase extraction (SPE) as an alternative strategy to improve its detection. After optimization of pretreatment, including denaturation, reduction, alkylation, tryptic digestion and selective SPE concentration, 91 ± 7% of PCSK9 was recovered from human plasma samples and coefficients of variation were less than 13.2% with a lower limit of quantification of 37.5 ng/ml. This LC-MS/MS method was compared with standard enzyme-linked immunosorbent assay in 30 human plasma samples with a broad range of PCSK9 concentrations. Both methods were significantly correlated (r = 0.936, p < 0.001) with less than 7% of the values out of the 95% confidence interval and similar concentrations were measured using either LC-MS/MS or ELISA methods (514.2 ± 217.2 vs. 504.2 ± 231.0 ng/ml, respectively- p = NS). This method involving SPE is an effective measurement tool for low abundant plasma protein analysis that could be easily included in multiplexed assays.