Litman B J, Mitchell D C
Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcoholism and Alcohol Abuse, National Institutes of Health, Rockville, Maryland 20852, USA.
Lipids. 1996 Mar;31 Suppl:S193-7. doi: 10.1007/BF02637075.
Visual transduction is one of the best characterized G protein--coupled signalling systems. In addition, about 50% of the disk membrane phospholipid acyl chains are 22:6n-3, making this system ideal for determining the role of polyunsaturation in modulating membrane-signalling systems. The extent of formation of metarhodopsin II (MII), the G protein--activating photointermediate of rhodopsin, was studied in phospholipid vesicles composed of a variety of phosphatidylcholines, differing in their acyl chain composition at the sn-2 position. The amount of MII formed increased progressively with the level of acyl chain unsaturation at the sn-2 position. The effect of added cholesterol was to reduce the amount of MII formed. The acyl chain packing free volume of the rhodopsin containing lipid vesicles was characterized by a fractional volume parameter fv derived from measurements of the time-resolved fluorescence anisotropy decay of the hydrophobic membrane probe 1,6-diphenyl-1,3,5-hexatriene. The relationship among sn-2 acyl chain unsaturation, cholesterol content, and MII formation is explained on the basis of variation in fv with bilayer lipid composition and a novel model for the packing of phospholipids containing polyenoic acyl chains, such as 22:6n-3.
视觉转导是特征最为明确的G蛋白偶联信号系统之一。此外，约50%的盘膜磷脂酰基链为22:6n-3，这使得该系统成为确定多不饱和脂肪酸在调节膜信号系统中作用的理想选择。在由多种磷脂酰胆碱组成的磷脂囊泡中研究了视紫红质的G蛋白激活光中间体——变视紫红质II（MII）的形成程度，这些磷脂酰胆碱在sn-2位的酰基链组成有所不同。在sn-2位形成的MII量随着酰基链不饱和度的增加而逐渐增加。添加胆固醇的作用是减少MII的形成量。含有视紫红质的脂质囊泡的酰基链堆积自由体积通过一个分数体积参数fv来表征，该参数源自对疏水膜探针1,6-二苯基-1,3,5-己三烯的时间分辨荧光各向异性衰减的测量。基于fv随双层脂质组成的变化以及一种含多烯酰基链（如22:6n-3）的磷脂堆积的新模型，解释了sn-2酰基链不饱和度、胆固醇含量与MII形成之间的关系。
