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Biophys J, January 1999, p. 281-290, Vol. 76, No. 1

The Membrane-Permeabilizing Effect of Avenacin A-1 Involves the Reorganization of Bilayer Cholesterol

C. N. Armah,* A. R. Mackie,* C. Roy,* K. Price,# A. E. Osbourn, P. Bowyer, and S. Ladha§

 *Food Biophysics Department and  #Biochemistry Department,  §Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, and  Sainsbury Laboratory, John Innes, Centre Norwich Research Park, Colney, Norwich NR4 7UH, England

Avenacin A-1 is a member of a group of naturally occurring compounds called saponins. It is found in oat plants, where it protects against fungal pathogens. A combined electrical and optical chamber was used to determine the interaction of avenacin A-1 with Montal-Mueller planar lipid bilayers. This system allowed simultaneous measurement of the effect of avenacin A-1 on the fluorescence and lateral diffusion of a fluorescent lipid probe and permeability of the planar lipid bilayer. As expected, cholesterol was required for avenacin A-1-induced bilayer permeabilization. The planar lipid bilayers were also challenged with monodeglucosyl, bis-deglucosyl, and aglycone derivatives of avenacin A-1. The results show that the permeabilizing activity of the native avenacin A-1 was completely abolished after one, two, or all three sugar residues are hydrolyzed (monodeglucosyl, bis-deglucosyl, and aglycone derivatives, respectively). Fluorescence recovery after photobleaching (FRAP) measurements on cholesterol-containing planar lipid bilayers revealed that avenacin A-1 caused a small but significant reduction in the lateral diffusion of the phospholipid probe N-(7-nitrobenzoyl-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (NBD-PE). Similarly, with the sterol probe (22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta -ol (NBD-Chol), avenacin A-1, but not its derivatives, caused a more pronounced reduction in the lateral diffusion than that observed with the phospholipid probe. The data indicate that an intact sugar moiety of avenacin A-1 is required to reorganize membrane cholesterol into pores.

Biophys J, January 1999, p. 281-290, Vol. 76, No. 1
© 1999 by the Biophysical Society   0006-3495/99/01/281/10  $2.00


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