Effects of cholesterol on thermal stability of discoidal high density lipoproteins

Shobini Jayaraman,Sangeeta Benjwal,Donald L Gantz,Olga Gursky

doi:10.1194/jlr.m000117

Abstract

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration. To understand physicochemical and functional implications of such variations, we analyzed stability of reconstituted HDL containing human apolipoproteins (apoA-I, apoA-II, or apoC-I), phosphatidylcholines varying in chain length (12-18 carbons) and unsaturation (0 or 1), and 0-35 mol% cholesterol. Lipoprotein heat denaturation was monitored by circular dichroism for protein unfolding/dissociation and by light scattering for particle fusion. We found that cholesterol stabilizes relatively unstable complexes; for example, incorporation of 10-30 mol% cholesterol in apoC-I:dimyristoyl phosphatidylcholine complexes increased their kinetic stability by deltaDeltaG* congruent with 1 kcal/mol. In more stable complexes containing larger proteins and/or longer-chain lipids, incorporation of 10% cholesterol did not significantly alter the disk stability; however, 15% or more cholesterol destabilized the apoA-I-containing complexes and led to vesicle formation. Thus, cholesterol tends to stabilize less stable lipoproteins, apparently by enhancing favorable packing interactions, but in more stable lipoproteins, where such interactions are already highly optimized, the stabilizing effect of cholesterol decreases and, eventually, becomes destabilizing. These results help uncouple the functional roles of particle stability and chain fluidity and suggest that structural disorder in HDL surface, rather than chain fluidity, is an important physicochemical determinant of HDL function.

Highlights

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration
The results show that cholesterol stabilizes some of the less stable complexes, such as apoC-I:dimyristoyl phosphatidylcholine (DMPC), but has no stabilizing effect on more stable disks, such as apoA-II or apolipoprotein A-I (apoA-I)-containing reconstituted HDL (rHDL), and may even destabilize these complexes at 15 mol% and greater concentrations
Our results show that the standard apoAI:DMPC:Ch preparations containing 0–10 mol% cholesterol yield discoidal rHDL, while those containing 15 mol% cholesterol and above yield vesicular complexes

Summary

Introduction

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration. In more stable complexes containing larger proteins and/or longer-chain lipids, incorporation of 10% cholesterol did not significantly alter the disk stability; 15% or more cholesterol destabilized the apoA-I-containing complexes and led to vesicle formation. Cholesterol tends to stabilize less stable lipoproteins, apparently by enhancing favorable packing interactions, but in more stable lipoproteins, where such interactions are already highly optimized, the stabilizing effect of cholesterol decreases and, eventually, becomes destabilizing. These results help uncouple the functional roles of particle stability and chain fluidity and suggest that structural disorder in HDL surface, rather than chain fluidity, is an important physicochemical determinant of HDL function.—Jayaraman, S., S.

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Conclusion