Abstract 587: Dynamics of the Apolipoproteome: Turnover of HDL- and LDL-Associated Apolipoproteins by LC/MS/MS

The production rate and fractional catabolic rate of apoB and apoAI have been measured using stable isotope infusions of 13C or 2H leucine for many years and has led to our improved understanding of the pathophysiology of dyslipidemia and atherosclerosis. Modern tandem mass spectrometry methods have demonstrated the highly complex complement of proteins that associate with HDL and LDL and alterations in these proteins may impact lipoprotein function and cardiovascular risk. Very little is known about the turnover of the proteins that comprise the apolipoproteome. To study the fesability of measuring the turnover of multiple apolipoproteins associated with HDL and LDL we performed 20 hour IV infusions of U-13C labeled leucine in 20 healthy subjects. VLDL, LDL and HDL were isolated by ultracentrifugation and digested with trypsin. We analyzed peptides containing one leucine with analysis of multiple transitions using an Agilent triple quad. The enrichment of HDL and LDL peptides from multiple apolipoproteins were determined. The fractional synthesis of each protein was calculated by comparison of the average peptide enrichment for each protein to the average enrichment in VLDL apoB after 20 hours, at which time the VLDL apoB is fully turned over and its enrichment reflects the hepatic leucine precursor pool enrichment. Mean and SD values for HDL associated proteins are presented in table 1. The range of protein turnover rates on HDL confirms the dynamic state of the particle components. Fractional synthesis was similar for the majority of proteins on HDL and LDL with the notable exception of apoL1 and apoM. Using these methods, the regulation of the apolipoprotein composition of HDL and LDL can now be investigated.