BOVINE-MILK SPHINGOMYELIN AT THE AIR WATER SURFACE AND ITS INTERACTION WITH XANTHINE/OXIDASE/

The monolayer properties of two different sphingomyelin samples from b ovine milk (one pure with respect to sphingomyelin (>99%) and one sphi ngolipid fraction containing 68% sphingomyelin) at the air/water inter face and the interaction between the lipid monolayer and xanthine oxid ase (XO) from the milk fat globule membrane were investigated by the f ilm balance technique. For comparison, similar measurements were perfo rmed on distearoylphosphatidylcholine (DSPC) monolayers. Bovine milk s phingomyelin formed monolayers comparable with those of other natural sphingomyelins. A liquid-condensed phase transition at 20-22 mN/m refl ected the high amount of long saturated fatty acids, as in the case fo r monolayers of egg sphingomyelin. Monolayers of bovine milk sphingomy elin were metastable, as opposed to those of DSPC. The observed decrea se in surface area with time at constant pressure indicated that disso lution/expulsion into the subphase took place. A pure sphingomyelin mo nolayer was significantly more stable at a surface pressure of 10 nN/m than at 20 nN/m. The discontinuity in the surface area change versus time and the low solubility of the lipid show that the instability can not be explained by simple desorption of lipid monomers. The present o f XO in the subphase increased the maximal surface pressure at an area per molecular of sphingomyelin of 30 Angstrom(2) (maximal compression ) by 15 mN/m for the sphingolipid sample and 20 mN/m for pure sphingom yelin, indicating a stabilization of sphingomyelin monolayers in the p resence of XO at high surface pressure. This effect was not observed f or DSPC monolayers, which suggests a specific interaction between sphi ngomyelin and XO.