Comparative proteomic profiling of plasma very-low-density and low-density lipoproteins

Background: Low-density lipoprotein (LDL) is a natural metabolite of very-low-density lipoprotein (VLDL) in the circulation. Systematic investigation of total protein components and dynamics might provide insights into this normal metabolic process. Methods: VLDL and LDL were purified from normolipidemia pooled plasma by gradient ultracentrifugation with either ionic or non-ionic media. The protein contents were compared by liquid chromatography tandem mass analyses based on isobaric tag for relative and absolute quantitation and two-dimensional gel electrophoresis. Results: Our comparative lipoproteomes revealed 21 associated proteins. Combined with Western blot analysis, and on the basis of the differential expression levels we classified them into 3 groups: (i) VLDL > LDL [apolipoprotein (apo) A-IV, apo(a), apoCs, apoE, apoJ and serum amyloid A-4]; (ii) VLDL < LDL [albumin, α-1-antitrypsin, apoD, apoF, apoM, and paraoxonase-1]; and (iii) VLDL = LDL [apoA-I, apoA-II, apoB-100, apoL-I and prenylcysteine oxidase-1]. The apoA-I level positively correlated with PCYOX1 but negatively with apoM in VLDL and LDL. Furthermore, the two-dimensional maps displayed 5 apoA-I isoforms in which phosphorylation at Ser55, Ser166, Thr185, Thr221 and Ser252 residues were identified. Conclusions: This study revealed the VLDL- and LDL lipoproteomes and the full-spectrum protein changes during physiological VLDL-to-LDL transition. It provides a valuable dataset VLDL and LDL proteomes potentially applied to the development of diagnostics.