Stable Isotope-Labeled ApoA-1 as a Global Standard for Quantitative Proteomic Studies

Cardiovascular disease (CVD) is the number one cause of morbidity and mortality worldwide.¹ However, reliable diagnostic tests of CVD risk are lacking, and in nearly 1/3 of patients the first indication of CVD is an acute, often fatal, cardiovascular event (i.e., myocardial infarction). Epidemiological studies have demonstrated an inverse association of CVD risk with plasma concentration of high-density lipoprotein (HDL), a complex comprising protein and lipids.² HDL is thought to mitigate atherosclerosis through a number of mechanisms (e.g., cholesterol efflux, anti-thrombosis, and antiinflammation). ^3,4 However, whether HDL proteins (>80 identified)^4,6 are associated with the cardiovascular protection remains unknown. Because HDL is in the causal pathway of atherosclerosis and CVD, and is a less complex mixture than plasma to analyze (e.g., ca. 100 vs. thousands of proteins and 4 vs. >10 order of magnitude concentration range),^7,8 HDL is an attractive target for quantifying the potentially cardioprotective proteins. The major analytical challenge stems from the phospholipids present in HDL (phospholipids represent about 30% of HDL by weight and are present at ca. 100× molar excess over an average protein),⁷ which are recognized electrospray ionization (ESI) suppressants. If the HDL proteins can be quantified in a precise and accurate manner, they can potentially serve as a diagnostic tool of CVD and be used to help address the deficiencies of current clinical practices.