Selective Transfer of Cholesteryl Ester over Triglyceride by Human Plasma Lipid Transfer Protein between Apolipoprotein-Activated Lipid Microemulsions

Abstract

The substrate-specific rate of the human plasma lipid transfer protein (LTP) reaction was studied using pyrene-labeled substrate lipid analogues as probes for various lipids, by monitoring the ratio of the fluorescence intensities of their excimers to those of their monomers as an indicator of pyrene concentration in the microenvironment. Transfer of cholesteryl ester (CE) and triglyceride (TG) was demonstrated between human high-density lipoproteins, between low-density lipoproteins, and between these two lipoprotein, and the specific fractional transfer rate of CE was always higher than that of TG by a factor of 2.4-7.9. On the other hand, the transfer by LTP of CE, TG, and phosphatidylcholine (PC) was also demonstrated between lipid microemulsions having an average diameter of 25-26 nm using the same probes, but only when the emulsions were activated by apolipoproteins A-I, A-II, E, or C-III. The maximally activated rates of the transfer of CE and TG were the same when measured between the emulsions with cores composed exclusively of either lipid. The specific fractional transfer rate of pyrene-CE, however, was inversely proportional to the percentage of CE in the TG core of the emulsions, and the initial transfer of TG was almost completely inhibited by the presence of small percentages of CE in the TG core. Thus, the transfer of CE between the emulsions is highly selective over that of TG by orders of magnitude, much more selective than the reaction between any natural plasma lipoproteins, but this selectivity is not a rate-limiting step of the overall LTP reaction. The maximally activated LTP-catalyzed transfer rate of PC between the emulsions was somewhat higher than that of CE or TG and was not affected by the composition of the core lipids of the emulsion, TG or CE. When an excess amount of LTP was incubated with emulsion containing a small percentage of pyrene-CE in the TG core in the absence of the acceptor particles, excimer fluorescence rapidly decreased to the base line, and this change was suppressed when pyrene-CE was diluted with CE in the core. This result may indicate that LTP selectively disrupts pyrene-CE excimer formation on the basis of its selective interaction with the CE molecule over TG in the emulsion system as a putative background mechanism for the selective transfer of CE.

Physical and kinetic characterization of recombinant human cholesteryl ester transfer protein

“…A recent analysis of recombinant cynomolgus monkey CETP according to a symmetrical lipid exchange model supports the carrier mechanism [19]. The carrier mechanism is further supported by lipid-binding studies [20], the stoichiometry of CE and TG exchange [21], the specificity for certain lipids [22][23][24], preferential inhibition of either CE or TG by monoclonal antibody or other inhibitors [4,19,24,25], and physical studies with lipid interfaces [13]. The present studies are intended to characterize the biochemistry in itro and the physical properties of highly purified recombinant human CETP.…”

Hyperalphalipoproteinemia and Beyond: The Role of HDL in Cardiovascular Diseases

“…The CETP gene maps on the long arm of chromosome 16 (16q12-16q21); it comprises 16 exons [ 40 ] and encodes for a glycosilated plasma protein which catalyze the exchange of triglycerides (TGs) and cholesteryl esters (CEs) among lipoprotein particle cores [ 41 , 42 ]. In more detail, it mediates the transport of CE from HDL to VLDL/chylomicrons and TG from VLDL/chylomicrons to HDL and low-density lipoproteins (LDL) [ 41 , 43 ], thus regulating HDL-C plasma levels.…”

Emerging Risk Biomarkers in Cardiovascular Diseases and Disorders

“…Thus, triacylglycerol transfer by LTP is induced by structural modulation of substrate-carrying lipid particles such as higher integration of apolipoproteins [ 239 ]. The substrate-specific rate of the human plasma lipid transfer protein (LTP) reaction is performed by using pyrene-labeled substrate lipid analogues which function like probes for various classes of lipids [ 240 ]. Hence, relationship between the cholesterol ester (CE) transfer activity of lipid transfer protein (LTP) and its affinity with lipid and lipoprotein particles is too important to find disease prevalence due to metabolic dysfunctions in man [ 241 ].…”