Model for the Interaction of Membrane‐Bound Substrates and Enzymes

Abstract

Microsomes and synaptosomes isolated from calf brain contain a sialidase which cleaves ganglioside substrates. The hydrolysis of [3H]ganglioside GD1a by the membrane-bound enzyme was studied under various conditions. The reaction rate decreased with increasing ionic strength in the incubation mixture and was progressively enhanced by increasing concentrations of the primary alcohols n-pentanol to n-octanol. This stimulation correlates quantitatively with an increase in membrane fluidity caused by these alcohols as measured by fluorescence depolarization employing 1,6-diphenyl-1,3,5-hexatriene as probe. The dependence of the reaction rate on the amount of enzyme in the incubation mixture was linear only with water-soluble substrates but not with the lipophilic ganglioside substrate. Lipophilic substrate and enzyme interact mainly within the plane of the membrane presumably by lateral diffusion. Taking this into consideration, Michaelis-Menten theory was modified accordingly. Apparent Km values increased linearly with the amount of membrane-bound enzyme added and decreased with the concentration of n-hexanol in the incubation mixture. In the presence of varying n-hexanol concentrations, the apparent Km value decreased with increasing membrane fluidity, as measured by fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene. V values were not affected by membrane fluidity and increased linearly with the amount of membrane protein.