Sialyltransferases in Rat Brain: Intracellular Localization and Some Membrane Properties

Abstract

Abstract— Total rat cerebral homogenate, with nuclei removed, yielded sialyltransferase activity peaks that were distinct from the protein distribution profile in a continuous sucrose density gradient. Marker enzyme studies and electron microscopic examinations on the gradient fractions suggested that most of the sialyltransferase activities were not associated with the synaptosomes. The sialyltransferases appeared to be localized in the smooth microsomal membranes and the Golgi complex derivatives. The sialyltransferase activities were stimulated by non‐ionic detergent mixture, Triton CF‐54/Tween 80 (2/1, w/w), the effect being much more pronounced with exogenous substrates. The stimulatory effect was dependent on detergent concentration. With 1 mg detergent mixture per mg enzyme protein, the percent increases in enzyme activities with the different substrates were: endogenous glycolipids, 100; endogenous glycoproteins, 50; exogenous GM1a, 700; exogenous DS‐fetuin, 230. The action of the nonionic detergents appears to be on a hydrophobic segment of the enzyme molecule, bearing the active site, which is buried in the membrane lipid bilayer. This was substantiated by the partial trypsin resistance of the sialyltransferase activities and the abolition of that resistance when trypsiniza‐tion was performed in the presence of nonionic detergents. Furthermore, the sialyltransferase activities were markedly inhibited by organic solvents; and these inhibitory effects were inversely proportional to the solvent dielectric constants.

Occurrence of Sialyltransferase Activity in the Synaptosomal Membranes Prepared from Calf Brain Cortex

“…Our results, while in agreement with those of Den et al (1975), contrast with those of other authors (Van den Eijnden and van Dijk, 1974; Landa et al, 1977;Ng and Dain, 19776) with reference to the presence of sialyltransferase in the nerve endings. This contradiction may partially be explained by animal species or age differences.…”

Evidence for the Presence of CDP‐Ethanolamine: 1,2‐diacyl‐sn‐glycerol Ethanolaminephosphotransferase in Rat Central Nervous System Myelin

“…This was born out by the negligible activities observed for three microsomal marker enzymes in the purified myelin. These markers are thought to represent various components of the heterogeneous microsomal fraction, including plasma membranes, endoplasmic reticulum, and Golgi complexes of both neurons and glia Gielen and Hinzen, 1974;Ng and Dain, 1977;Benjamins and Iwata, 1979).…”

Disposition of Gangliosides and Sialosylglycoproteins in Neuronal Membranes

“…The accessibility of the unlabeled glycoconjugates to neuraminidase was explored in the synaptosomes in the conditions used to investigate the from the main source of synthesizing enzymes 1977;Ng and Dain, 1977;Landa et al, 1977;. In contrast .with the [3H]NeuNAc glycoconjugates of the NP membrane fraction, incubation of the synaptosomes with neuraminidase for 180 min caused a release of 55% and 65% of the total NeuNAc in gangliosides and glycoproteins, respectively.…”

“…This knowledge should increase the understanding of the mechanism by which a ganglioside reaches its position in the neuronal plasma membrane. Since an extrasynaptosomal origin of synaptosomal sialosylglycoproteins was also suggested (Rosner, 1975;Ng and Dain, 1977) their topography was examined as well. We labeled the endogenous gangliosides and sialosylglycoproteins of the neuronal perikaryon membrane fraction by incubation of homogenized neuronal perikarya with CMP-[3H]N-acetyl neuraminic acid (CMP-[3H]NeuNAc) and studied the stability of the association between the labeled glycoconjugates and the membranes.…”