Microsomes and synaptosomes isolated from calf brain contain a sialidase which cleaves ganglioside substrates. The hydrolysis of [3H]ganglioside GDla by the membrane-bound enzyme has been 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-l,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. Evidence is presented that 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.As predicted, apparent K,,, 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 K,-value decreased with increasing membrane 'fluidity', as measured by fluorescence depolarization of 1,6-diphenyl-l,3,5-hexatriene. On the other hand, as expected, I/ values were not affected by membrane 'fluidity' and increased linearly with the amount of membrane protein.Biological membranes contain enzymes acting on phospholipids and sphingolipids which are constituents of these membranes. During the investigation of two such systems we recognized that the reaction rate was highly dependent on the physical state of the membrane: degradation of sphingomyelin by neutral sphingomyelinase and of ganglioside GDI a by sialidase in neuronal membranes was greatly stimulated by general anesthetics which increase membrane 'fluidity' [ 1,2].Centrifugation studies [1] demonstrated that exogenous lipid substrates are adsorbed to or inserted into the membranes. A model was proposed suggesting an interaction between substrate and enzyme within the membrane by lateral diffusion [l]. Fluorescence depolarization studies with the probe 1,6-diphenyl-I ,3,5-hexatriene indicated a qualitative correlation between increase of membrane 'fluidity and stimulation of lipid degradation in the presence of general anesthetics. In the case of halothane this correlation was demonstrated at clinical concentrations [2,3]. In this paper a quantitative correlation between membrane 'fluidity' and the stimulation of degradation of ganglioside GDla employing the homologous series of primary alcohols will be presented.Attempts to determine kinetic parameters for the system mentioned above met with systematic difficulties, such as nonlinear protein-dependence and strong variations of the Dedicated to Professor Horst Jatzkewitz on the occasion of his 70th birthday.

No abstract

This paper presents the effect that the traditional cooking process of black beans (Phaseolus vulgaris, Tamazulapa variety) has on the quantity and composition of soluble (SDF) and insoluble (IDF) dietary fiber of beans, as well as on its protein digestibility and protein quality. There was an increase of IDF from 18.1% in cooked beans to 22.4% in fried beans, and a decrease in SDF from 8.4% to 6.6%, respectively. Starch content decreased from 34.5% to 31.3%. No change was found in lignin. The xylose content was higher in IDF than in SDF and decreased to some extent from cooked to fried beans. Arabinose content was similar in IDF and SDF with no change caused by processing. The fraction containing glucose, mannose and galactose in IDF was higher than in SDF, the content increasing in IDF and decreasing in SDF, with processing. Protein content in IDF was higher than in SDF, with no major change when processing. About 29.5% of the total protein of beans was bound in DF. Protein digestibility and protein quality decreased from cooked to fried beans and was positively related to IDF.