Membranes of mammary gland. IX. Concentration of glycosphingolipid galactosyl and sialytransferases in Golgi apparatus from bovine mammary gland

Ganglioside biosynthesis in Golgi apparatus: new perspectives on its mechanism

The synthesis of gangliosides GM1 and GM2 in intact rat liver Golgi vesicles is stimulated by phosphatidylglycerol as much as or even more than by detergents (Triton X-100 and octyglucoside, respectively). The antibiotic tunicamycin, known as an inhibitor of the N-glycosylation of proteins, strongly inhibits the synthesis of the above gangliosides, in the presence as well as in the absence of the phospholipid. Both phosphatidylglycerol dependence and tunicamycin inhibition disappear when the Golgi vesicles are solubilized by addition of detergents or disrupted by ultrasonication or pretreated with pronase. Transport studies with UDP-[3H]Gal show that tunicamycin blocks the penetration of the sugar nucleotide into the Golgi vesicles in a concentration-dependent manner up to 80%. The results show that tunicamycin inhibits ganglioside biosynthesis by blocking the transport of the nucleotide sugar and not by inhibiting the transferase directly. Studies on glycoprotein-galactosyltransferase with ovalbumin as exogenous acceptor showed that phosphatidylglycerol does not destroy the integrity of the Golgi vesicles. So this phospholipid is an excellent tool for studying ganglioside biosynthesis at optimal transferase activities without solubilizing the Golgi membranes.

Disposition of gangliosides and sialosylglycoproteins in neuronal membranes

Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP-[3H]N-acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H]sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 M-KCl, 125 mM-EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of th neuraminidase-labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase-labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labelled in vivo by an intracerebral injection of N-[3H]acetylmannosamine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with behaviour of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

Glycosylation of kappa-casein. I. Localization and characterization of sialyltransferase in bovine mammary gland

A sialyltransferase (CMP-N-acetylneuraminate:D-galactosyl-glycoprotein N-acetylneuraminyltransferase, EC 2.4.99.1) which attaches N-acetylneuraminic acid to the terminal end of the carbohydrate chain of kappa-casein was found to be concentrated in Golgi apparatus-enriched fractions of bovine mammary gland. Maximum sialyltransferase activity was obtained at pH 5.5 and 37 degrees C in the presence of 1 mM dithiothreitol and Triton X-100. A Km of 0.19 mg asialo-kappa-casein/ml (0.01 mM) was obtained for the sialyltransferase. Native kappa-casein also served as acceptor for N-acetylneuraminic acid transferase of Golgi apparatus-enriched fractions although at a slower rate than did asialo-kappa-casein. The sialyltransferase has a divalent cation requirement for maximum activity which was best satisfied by the presence of 10 mM Mn2+.

Characterization of a soluble glycolipid galactosyltransferase which occurs in bovine milk

Bovine milk was found to contain, in soluble form, an enzyme which transfers galactose from UDPgalactose to glucosylceramide. This enzyme was partially purified by the same procedure used to isolate the galactosyltransferase of lactose synthetase. The partially purified enzyme required detergents for activity, had a pH optimum of 7.2--7.3 and required Mn2+. The apparent Km calculated for glucosylceramide was 1.33 . 10(-4) M. With glucosylceramide as acceptor the product of the reaction was identified as lactosylceramide by autoradiography on thin-layer chromatograms. Lactosylceramide was also an effective acceptor for the transferase reaction but neutral glycosphingolipids or gangliosides with terminal galactose of N-acetylgalactosamine residues were ineffective or poorly effective as acceptors. Addition of alpha-lactalbumin inhibited the transferase reaction.