Lipid scrambling

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Bevers, E.M., Comfurius, R, Dekkers, D.W., Harmsma, M. andZwaal, R.F. (1998) Regulatory mechanisms of transmembrane phospholipid distributions and pathophysiological implications of transbilayer lipid scrambling. Lupus 7 (Suppl), S126-S131. 

Upon fractionation of vesicles reconstituted from LDAO-depleted RC s at a molar lipid/RC ratio of 560 on Biogel A-50 it turns out that the sizes of the vesicles vary from 1200 to 500 nm (as measured with light scattering). In these fractions the net-orientation of the RC s decreases with vesicle size from 80/20 to 60/40. This suggests that the RC s spontaneously orient themselves with their cytochrome binding site on the outside of large vesicles, whereas an increased curvature of the bilayer leads to an increased scrambling of the RC s, possibly due to increased steric repulsion. 

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