Galactose oxidase action on GM1 ganglioside in micellar and vesicular dispersions

GM1 ganglioside was dispersed in different membrane-mimicking systems and the effect of dispersion on GM1 oxidation by galactose oxidase was studied. The following membrane-mimicking systems were used: homogeneous micelles of GM1; mixed micelles (at different proportions of constituents) of GM1 with either GD1a ganglioside (which is resistant to the enzyme), or the non-ionic detergent Triton X-100, or bovine serum albumin; small unilamellar vesicles of egg phosphatidylcholine (PC), containing various proportions of GM1. As a reference substrate not involved in membranous systems and freely interacting with the enzyme, the oligosaccharide portion of GM1 (DesGM1) was employed. The apparent Vmax of the enzyme was dramatically dependent on the type of GM1 dispersion. The lowest value was recorded on homogeneous micelles of GM1 and on mixed GM1-GD1a micelles. From this value, the Vmax increased 2-fold with GM1-bovine serum albumin lipoprotein micelles, up to 1400-fold with mixed GM1-Triton X-100 (optimal molar ratio, 1:13.8) micelles, and up to 14000-fold on PC vesicles containing 8 mol% GM1 (this proportion was optimal for enzyme activity on vesicles). The activity developed on these latter vesicles turned out to be still greater (2-fold) than that displayed on DesGM1. The apparent Km had very similar values in all different membrane systems; in contrast, it was markedly greater on DesGM1. Both Triton X-100 micelles and PC vesicles did not appreciably alter the kinetics of galactose oxidase action on pure galactose, indicating that the above effects are dependent on the intrinsic characteristics of the membrane-like systems containing gangliosides.