EVIDENCE FOR TRANSBILAYER, TAIL-TO-TAIL CHOLESTEROL DIMERS IN DIPALMITOYLGLYCEROPHOSPHOCHOLINE LIPOSOMES
EVIDENCE FOR TRANSBILAYER, TAIL-TO-TAIL CHOLESTEROL DIMERS IN DIPALMITOYLGLYCEROPHOSPHOCHOLINE LIPOSOMES
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DOI：
10.1021/bi00011a043
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发表时间：
1995-03-21
期刊：
影响因子：
2.9
通讯作者：
KEZDY, FJ
中科院分区：
文献类型：
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作者：
HARRIS, JS;EPPS, DE;KEZDY, FJ
在小于或等于 5 mol% 的两亲性溶质油酸甲酯、胆固醇、孕烯醇酮和脱氢雄酮存在下，通过差示扫描量热法 (DSC) 研究了 2,3-二棕榈酰-sn-甘油-1-磷酸胆碱 (DPPC) 多层脂质体的行为。 DSC 热分析图表明，溶质仅与液体无序 (1(d)) 相混溶，而不与固体有序 (S-o) 相混溶。 T-m 与溶质浓度曲线的斜率证实了这一结论：看来 DPPC 的 s(o)-1(d) 相变（对应于磷脂链的熔化）可以被视为一个简单的熔化过程，因此可以用作低温系统。在这种情况下，其熔点下降常数 K-f 可以根据实验测量的每克 DPPC 的熔化热 l(f) 和纯 DPPC 的相变温度 T-o 计算出来，通过方程 K-f = RT(o)(2)/(1000l(f)) = 12.3 +/- 0.9 K g M(-1) cm(3)。以油酸甲酯为溶质，T-m 与油酸甲酯浓度图呈线性，根据斜率我们计算出 K-f 12.9 +/- 0.8 K g M(-1) cm(3)。因此，油酸甲酯似乎与二棕榈酰卵磷脂脂质体形成理想的冷冻系统：它与id相完全混溶，但显然不溶于s(o)相。孕烯醇酮和脱氢雄酮也与二棕榈酰卵磷脂脂质体形成理想的冷冻系统：T-m 与溶质浓度图是线性的，并产生这些溶质的正确 MW。相反，胆固醇的 T-m 与浓度图是弯曲的；低于 2 mol% 溶质的斜率对应于单体分子量，但高于 3 mol% 则接近二聚体的斜率。完整曲线与 1(d) 相中溶质的简单平衡二聚一致。因此，胆固醇似乎不溶于 s(o) 相，并且很容易在 1(d) 相中形成二聚体。空间填充考虑表明二聚体是跨双层二聚体。这种胆固醇二聚体与 Sankaram 和 Thompson 评论的各种观察结果一致 [Sankaram, M. B., and Thompson, T. E. (1990) Biochemistry 29, 10676-10684]。由于胆固醇不溶于 s(o) 相，因此它必须在 T-m 以下形成单独的相或至少单独的域。然而，微域不应该太小，因为微域相对较大的边界区域会影响固相的热力学稳定性。
The behavior of multilamellar liposomes of 2,3-dipalmitoyl-sn-glycero-1-phosphocholine (DPPC) was studied by differential scanning calorimetry (DSC) in the presence of less than or equal to 5 mol % of the amphiphilic solutes methyl oleate, cholesterol, pregnenolone, and dehydroandrosterone. The DSC thermograms indicate that the solutes are miscible only with the liquid-disordered (1(d)) phase, and not with the solid-ordered (S-o) phase. The slopes of the T-m vs solute concentration curves confirm this conclusion: It appears that the s(o)-1(d) phase transition of DPPC, which corresponds to the melting of the phospholipid chains, can be treated as a simple melting process and, thus, could be used as a cryoscopic system. In that case, its melting point depression constant, K-f, can be calculated a priori from the experimentally measured heat of fusion per gram of DPPC, l(f), and the temperature of the phase transition of pure DPPC, T-o, by the equation K-f = RT(o)(2)/(1000l(f)) = 12.3 +/- 0.9 K g M(-1) cm(3). With methyl oleate as the solute, the T-m vs methyl oleate concentration plot is linear, and from the slope we calculate K-f 12.9 +/- 0.8 K g M(-1) cm(3). Thus, methyl oleate appears to form an ideal cryoscopic system with dipalmitoyllecithin liposomes: It is fully miscible with the id phase but is apparently insoluble in the s(o) phase. Pregnenolone and dehydroandrosterone also form ideal cryoscopic systems with dipalmitoyllecithin liposomes: The T-m vs solute concentration plots are linear and yield the correct MWs for these solutes. In contrast, the T-m vs concentration plot of cholesterol is curved; the slope below 2 mol % solute corresponds to the monomeric MW, but above 3 mol % it approaches that of a dimer. The full curve is consistent with a simple equilibrium dimerization of the solute in the 1(d) phase. Thus, cholesterol appears to be insoluble in the s(o) phase, and it readily forms a dimer in the 1(d) phase. Space-filling considerations indicate that the dimer is a transbilayer one. Such a dimer of cholesterol would be consistent with a variety of observations reviewed by Sankaram and Thompson [Sankaram, M. B., and Thompson, T. E. (1990) Biochemistry 29, 10676-10684]. Since cholesterol is insoluble in the s(o) phase, it must form below T-m a separate phase or at least separate domains. The domains, however, should not be too small since the relatively large boundary region of the microdomains would affect the thermodynamic stability of the solid phase.