McMullen T P, McElhaney R N
Department of Biochemistry, University of Alberta, Edmonton, Canada.
Biochim Biophys Acta. 1995 Mar 8;1234(1):90-8. doi: 10.1016/0005-2736(94)00266-r.
We have investigated the effects of cholesterol on the thermotropic phase behavior of annealed and unannealed aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) using high-sensitivity differential scanning calorimetry (DSC), concentrating particularly on the cholesterol concentration range from 0 to 20 mol%. We find that the incorporation of cholesterol into low-temperature annealed DPPC bilayers decreases the enthalpy of the subtransition without affecting the transition temperature, such that the subtransition is abolished by 20 mol% cholesterol. Similarly, the incorporation of cholesterol progressively decreases the temperature and enthalpy of the pretransition and abolishes it entirely at cholesterol concentrations above 5 mol%. The incorporation of increasing quantities of cholesterol also alters the main or chain-melting phase transition. At cholesterol concentrations of 2 to 20 mol% cholesterol, the DSC endotherm arising from the main transition consists of superimposed sharp and broad components, the former due to the melting of cholesterol-poor and the latter to the melting of the cholesterol-rich DPPC domains. The temperature and cooperativity of the sharp component decreases slightly with increasing cholesterol concentration whereas the enthalpy decreases markedly, becoming zero at 20-25 mol% cholesterol. In contrast, the temperature and enthalpy of the broad component increases, and the cooperativity decreases markedly over this same range of cholesterol concentrations. An apparent increase in cooperativity of the overall DPPC endotherm at 7 mol% cholesterol is shown to arise because of a convergence in the transition temperatures of the sharp and broad components of the DSC endotherms. Some of our experimental findings, particularly the absence of any evidence for the existence of a triple point near 7.5 mol% cholesterol, do not accord with a recently proposed DPPC/cholesterol phase diagram derived from DSC and 2H-NMR data (see Vist, M.R. and Davis, J.H. (1990) Biochemistry 29, 451-464). In addition, we examined the effect of cholesterol on phosphatidylcholines (PCs) of different chain lengths and confirm that a eutectic point does not exist for any of these PC/cholesterol mixtures. We then propose a new, more complete DPPC/cholesterol phase diagram based on our high-sensitivity DSC data as well as some recent spectroscopic data on PC/cholesterol mixtures and explore some of its biological implications.
我们使用高灵敏度差示扫描量热法（DSC）研究了胆固醇对二棕榈酰磷脂酰胆碱（DPPC）退火和未退火水分散体热致相行为的影响，特别关注胆固醇浓度范围为0至20摩尔％的情况。我们发现，将胆固醇掺入低温退火的DPPC双层中会降低亚转变的焓，而不影响转变温度，以至于20摩尔％的胆固醇会消除亚转变。同样，胆固醇的掺入逐渐降低预转变的温度和焓，并在胆固醇浓度高于5摩尔％时完全消除它。掺入越来越多的胆固醇也会改变主要的或链熔化相变。在胆固醇浓度为2至20摩尔％时，主要转变产生的DSC吸热峰由叠加的尖锐和宽泛成分组成，前者归因于贫胆固醇区域的熔化，后者归因于富胆固醇DPPC区域的熔化。尖锐成分的温度和协同性随着胆固醇浓度的增加而略有降低，而焓则显著降低，在胆固醇浓度为20 - 25摩尔％时变为零。相反，宽泛成分的温度和焓增加，并且在相同的胆固醇浓度范围内协同性显著降低。结果表明，在7摩尔％胆固醇时DPPC整体吸热峰的协同性明显增加是由于DSC吸热峰尖锐和宽泛成分的转变温度趋同。我们的一些实验结果，特别是没有任何证据表明在7.5摩尔％胆固醇附近存在三相点，与最近根据DSC和2H - NMR数据提出的DPPC/胆固醇相图不一致（见Vist，M.R.和Davis，J.H.（1990）Biochemistry 29，451 - 464）。此外，我们研究了胆固醇对不同链长磷脂酰胆碱（PC）的影响，并确认这些PC/胆固醇混合物中任何一种都不存在低共熔点。然后，我们基于高灵敏度DSC数据以及最近关于PC/胆固醇混合物的一些光谱数据提出了一个新的、更完整的DPPC/胆固醇相图，并探讨了其一些生物学意义。
