Membrane-bound neuraminidase from calf brain: regulation of oligosialoganglioside degradation by membrane fluidity and membrane components.

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Abstract
The degradation of lipophilic ganglioside GD1a and hydrophilic sialyllactitol by membrane-bound neuraminidase (EC 3.2.1.18) from calf brain was studied at substrate concentrations of 0.1 mM. Ganglioside GD1a taken up by cell membranes was hydrolyzed faster by membrane-bound neuraminidase than were water-soluble substrates of the enzyme, sialyllactitol and des-GD1a. Availability and enzymic breakdown of the disialoganglioside were enhanced by general anesthetics such as N2O or halothane; the degradation of the hydrophilic substrate sialyllactitol was not affected or even was decreased by these agents. General anesthetics lowered the microviscosity of membranes as indicated by studies of fluorescence depolarization with the indicator 1,6-diphenylhexatriene. Increased fluidity could result in higher lateral diffusion of ganglioside GD1a, thus increasing its chances of presentation to, and interaction with, membrane-bound neuraminidase. Lipophilic derivatives of the disialoganglioside, gangliosides GM1 and GM2 and gangliotriaosylceramide GA2, were strong inhibitors of the ganglioside degradation; water-soluble derivatives des-GM1, des-GM2, N-acetylneuraminic acid and sialyllactose were not. A model was presented that suggests that the activity of membrane-bound neuraminidase on glangliosides of brain membranes is regulated by the viscosity of these membranes and their monosialoganglioside content.

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