Florio E, Jarrell H, Fenske D B, Barber K R, Grant C W
Department of Biochemistry, University of Western Ontario, London, Canada.
Biochim Biophys Acta. 1990 Jun 27;1025(2):157-63. doi: 10.1016/0005-2736(90)90093-4.
Glycosphingolipid fatty acids commonly have up to eight methylene carbons more than do their surrounding phospholipid-attached counterparts. The resultant 'extra' segment may very well modulate glycosphingolipid function as receptor and structural element. As part of an investigation of this phenomenon, galactosylceramide was prepared with a deuterated 18-carbon fatty acid chain. Deuterium-labelled galactosylceramide was assembled at 10 mol% into unsonicated phosphatidylcholine bilayers having all 14-carbon or all 18-carbon saturated fatty acid chains (DMPC and DSPC, respectively). The systems were studied by 2H-NMR spectroscopy above and below the phase transition temperatures, Tm, of the host matrices. At comparable reduced temperatures in fluid membranes the degree of motional order exhibited by the glycolipid fatty acid was significantly higher in the phospholipid host matrix that was four carbons shorter. The fatty acid chain segment least affected by the change from long to short chain host matrix was the terminal (deutero)methyl group (an increase of 8% in quadrupolar splitting for the terminal methyl vs. 16% for deuterons at C17 and 23-28% for the remainder of the chain). Order parameter profiles for galactosylceramide were qualitatively very similar in the two host membranes, arguing against any major conformational difference between the arrangement of the 18-carbon glycolipid fatty acid in the 18-carbon vs. 14-carbon host matrices. Similarly a nitroxide spin probe covalently attached to carbon-12 of the galactosylceramide fatty acid gave clear indication of greater order in the fluid 14-carbon fatty acid phospholipid bilayer. These results are consistent with 'tethering' of the extra length of fatty acid via interdigitation into the opposing monolayer. There was no spectroscopic evidence of any intrinsic difference in glycolipid behaviour in the two fluid host matrices. 2H-NMR spectra of galactosylceramide at comparable reduced temperatures below Tm of the phospholipid bilayer were very different for 14-carbon vs. 18-carbon host matrices. The glycolipid fatty acid showed evidence of relatively reduced mobility in the shorter chain matrix.
糖鞘脂脂肪酸通常比与其相连的周围磷脂脂肪酸多八个亚甲基碳。由此产生的“额外”片段很可能会调节糖鞘脂作为受体和结构元件的功能。作为对这一现象研究的一部分，制备了带有氘代18碳脂肪酸链的半乳糖神经酰胺。将氘标记的半乳糖神经酰胺以10摩尔%的比例组装到具有全14碳或全18碳饱和脂肪酸链（分别为二肉豆蔻酰磷脂酰胆碱和二硬脂酰磷脂酰胆碱）的未超声处理的磷脂酰胆碱双层中。在主体基质的相变温度Tm上下，通过2H-核磁共振光谱对这些体系进行了研究。在流体膜中，在可比的降低温度下，糖脂脂肪酸表现出的运动有序程度在碳链短四个碳的磷脂主体基质中明显更高。受从长链到短链主体基质变化影响最小的脂肪酸链段是末端（氘代）甲基（末端甲基的四极分裂增加8%，而C17处的氘核增加16%，链的其余部分增加23 - 28%）。在两种主体膜中，半乳糖神经酰胺的序参数分布在定性上非常相似，这表明18碳糖脂脂肪酸在18碳与14碳主体基质中的排列不存在任何主要的构象差异。同样，共价连接到半乳糖神经酰胺脂肪酸碳-12上的氮氧自由基自旋探针清楚地表明，在流体14碳脂肪酸磷脂双层中有序程度更高。这些结果与脂肪酸的额外长度通过叉指作用进入相对的单层而“拴系”一致。没有光谱证据表明两种流体主体基质中糖脂行为存在任何内在差异。在低于磷脂双层Tm的可比降低温度下，14碳与18碳主体基质的半乳糖神经酰胺的2H-核磁共振光谱非常不同。糖脂脂肪酸在较短链基质中显示出流动性相对降低的迹象。
