Galactosyl transferase activity in a variety of sources

The serine protease and RNA-stimulated nucleoside triphosphatase and RNA helicase functional domains of dengue virus type 2 NS3 converge within a region of 20 amino acids

NS3 protein of dengue virus type 2 has a serine protease domain within the N-terminal 180 residues. NS2B is required for NS3 to form an active protease involved in processing of the viral polyprotein precursor. The region carboxy terminal to the protease domain has conserved motifs present in several viral RNA-stimulated nucleoside triphosphatase (NTPase)/RNA helicases. To define the functional domains of protease and NTPase/RNA helicase activities of NS3, full-length and amino-terminal deletion mutants of NS3 were expressed in Escherichia coli and purified. Deletion of 160 N-terminal residues of NS3 (as in NS3del.2) had no detrimental effect on the basal and RNA-stimulated NTPase as well as RNA helicase activities. However, mutagenesis of the conserved P-loop motif of the RNA helicase domain (K199E) resulted in loss of ATPase activity. The RNA-stimulated NTPase activity was significantly affected by deletion of 20 amino acid residues from the N terminus or by substitutions of the cluster of basic residues, 184RKRK-->QNGN, of NS3del.2, although both mutant proteins retained the conserved RNA helicase motifs. Furthermore, the minimal NS3 protease domain, required for cleavage of the 2B-3 site, was precisely defined to be 167 residues, using the in vitro processing of NS2B-NS3 precursors. Our results reveal that the functional domains required for serine protease and RNA-stimulated NTPase activities map within the region between amino acid residues 160 and 180 of NS3 protein and that a novel motif, the cluster of basic residues 184RKRK, plays an important role for the RNA-stimulated NTPase activity.

Evidence of partial localization in Golgi apparatus of UDP-glucose collagen glucosyltransferase from chick embryo liver

1. Collagens are the most important components of the connective tissue. 2. Collagen synthesis involves greater than 12 different enzymes whereas three enzymatic systems are involved in the ordered degradation. 3. Some enzymes are found in the rough endoplasmic reticulum (RER). The subcellular localization of disulfur isomerase, alpha D-glucosidase, proteases, galactosyltransferases and glucosyltransferases specific to collagen is unknown. 4. After having determined the best subcellular fractionation conditions for the chick embryo liver, we demonstrate that the galactosylhydroxylysyl glucosyltransferase specific to collagen is located in the RER and in the Golgi apparatus.

Effect of homeopathic dilutions on subcellular enzymatic activity

The activity of various inhibitors on several subcellular enzymes was studied. First we determined the inhibitory concentration required to reduce maximum enzymatic activity by 50%, then the effect of various hahnemannian dilutions of the same inhibitory agent was tested. Seven inhibitory agents were tested in this way on seven different enzymatic systems. No effects of these hahnemannian dilutions were shown.

Biosynthesis of Lewis fucolipid antigens in human colorectal carcinoma cells. Partial characterization of LcOse4Cer and H-1 fucolipid fucosyl transferase acceptor activities

Purified glycolipids were tested for their ability to serve as acceptors of [14C]fucose from GDP-[14C]fucose as catalyzed by cell-free extracts and purified membrane fractions of human colorectal carcinoma cells, SW1116, cultured in serum-free medium. Purified lactotetraosyl ceramide (Gal beta 1----3GlcNAc beta 1----3Gal beta 1----4Glc-Cer or LcOse4Cer) and H-1 glycolipid (Fuc alpha 1----2Gal beta 1----3GlcNAc beta 1----3Gal beta 1----4Glc-Cer or IV2 Fuc alpha LcOse4Cer) stimulated incorporation of radioactivity into lipid-soluble glycolipid at a rate greater than ten times that of Lea glycolipid [Gal beta 1----3(Fuc alpha 1----4)GlcNAc beta 1----3Gal beta 1----4Glc-Cer or III4 Fuc alpha LcOse4Cer]. The enzymatic activities in crude and purified membrane fractions were optimized for substrate concentrations (glycolipid and GDP-fucose), detergent requirement (taurocholate), pH, time and protein. The radioactive product of H-1 fucosylation migrated as discrete and distinct bands on high-performance thin-layer chromatograms (HPTLC). Evidence for their identity with Leb fucolipid described previously [Fuc alpha 1----2Gal beta 1----3(Fuc alpha 1----4)GlcNAc beta 1----3Gal beta 1----4Glc-Cer or III4IV2 (Fuc alpha) LcOse4Cer] is presented. The radioactive product of LcOse4Cer fucosylation was mainly Lea fucolipid as determined by co-migration with authentic Lea fucolipid in three HPTLC systems as native and acetylated derivatives. Our results also indicated a low level of H-1 and Leb glycolipid synthesis from LcOse4Cer. On the basis of the optima, linearity for time, and enzyme-limiting conditions, we obtained a 12-19-fold purification of the LcOse4Cer and H-1 fucosyl transferase acceptor activities in three peaks of a sucrose gradient. The peak with the highest specific activity (peak 3) was highest in density and in Na+, K+, ATPase specific activity, although NADH-cytochrome-c reductase and UDP-GalNac transferase were also present in peak 3. The apparent Km values of LcOse4Cer acceptor activity and H-1 acceptor activity in peak 3 were significantly different (p less than 0.01) by statistical tests, 2.4 microM and 0.5 microM, respectively. These apparent Km values were much lower (10(3) X) and the pH optima were lower (4.8-5.3), than the corresponding properties reported for the alpha 1----3/alpha 1----4 fucosyl transferase purified from human milk. Our results suggest a role for the non-glycosidic moieties of the acceptors and/or the tissue-specific or primitive expression of these fucosyl transferase activities.

Topology of glucosylceramide synthesis in Golgi membranes from porcine submaxillary glands

The topology of ceramide glucosyltransferase and de novo synthesized glucosylceramide was studied in sealed and 'right-side-out' vesicles of porcine submaxillary glands derived from Golgi apparatus. Pronase treatment which did not cause any breakdown of the luminal glycoprotein galactosyltransferase activity, inhibited the ceramide glucosyltransferase to more than 50% at a ratio proteinase to Golgi protein 1:100. Trypsin at the same concentration, while producing no inactivation of luminal galactosyltransferase, caused a complete loss of ceramide glucosyltransferase activity. The membrane-impermeable compound, DIDS, which did not cause any inhibition of the galactosyltransferase, inhibited the ceramide glucosyltransferase (70% reduction at 80 microM DIDS). Thus, the enzyme ceramide glucosyltransferase is accessible from the cytoplasmic side of the Golgi vesicles. The orientation of the newly synthesized glucosylceramide is studied by the ability of the enzyme glucosylceramidase to hydrolyse this compound both on intact and on disrupted vesicles. The same percentage (respectively, 36 and 30%) of hydrolysis was obtained during an incubation of 3 h, showing that glucosylceramide is not at all protected from external hydrolysis. Pronase-treated vesicles revealed an increase in glucosylceramidase hydrolysis (up to 45%), which indicates that glucosylceramide that glucosylceramide may be cryptic. All these results indicate that the ceramide glucosyltransferase, as well as related glucosylceramide, are cytoplasmically oriented in Golgi vesicles from porcine submaxillary glands.

UDPglucose-ceramide glucosyltransferase from porcine submaxillary glands is associated with the Golgi apparatus

Subcellular distribution of pig submaxillary gland UDPglucose-ceramide glucosyltransferase (EC 2.4.1.80), the enzyme which catalyses the first step during the sequential addition of carbohydrate moieties for ganglioside biosynthesis, was studied. The results presented strongly suggest that in pig submaxillary gland, the transfer of glucose on endogenous or exogenous ceramides takes place in the Golgi apparatus: the specific activity of UDPglucose-ceramide glucosyltransferase increased in parallel with the activity of a known marker of the Golgi apparatus, UDPgalactose-ovomucoid galactosyltransferase. The specific activity of the glucosyltransferase was 18-times higher in the purified Golgi membranes than in the postnuclear supernatant and the yield was over 30%. An apparent Km of 22 microM for UDPglucose and 54 microM for ceramides was determined. Maximal glucosylation of endogenous ceramides was achieved at pH 6.5 in the presence of NADH (1 mM) as inhibitor of pyrophosphatases and with Mn2+ (5 mM). It was found that the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) is an efficient activator for the glucosylation of exogenous ceramides.

Purification by affinity chromatography and some properties of microsomal galactosyltransferase from pig thyroid

Membrane-bound 4-beta-galactosyltransferase (lactose synthase; UDP galactose: D-glucose 4-beta-galactosyltransferase, EC 2.4.1.22) was purified 1500-fold to near homogeneity from pig thyroid microsomes with about 30% yield. The purified enzyme behaved as a lipophilic protein, rapidly losing activity and aggregating if not supplemented with either Triton X-100 or serum albumin (both of these were equally effective for long-term stabilization). The enzyme preparation showed an absolute requirement for Mn2+, which could not be replaced by other cations. Catalytic properties were very similar to those reported for soluble forms of the enzyme in biological fluids. The purified galactosyltransferase showed a major protein band of approx. 74,000 daltons on sodium dodecyl sulfate gel electrophoresis. On gel filtration, enzyme activity was eluted at approx. 70,000 daltons. It is concluded that the membrane-bound thyroid galactosyltransferase is a monomeric protein significantly larger than the soluble forms of this enzyme described earlier; but it resembles recently reported galactosyltransferases from sheep mammary Golgi membranes and liver microsomes.

[Galactose transfer in the membranes of human milk fat globules (author's transl)]

Galactosyltransferase which catalyzes the transfer from UDP-galactose to either endogeneous glycoproteins, free N-acetylglucosamine or N-acetylglucosaminyl residues in the carbohydrate portion of glycoproteins, or to glucose when alpha-lactalbumin is added, occurs in human milk fat globule membranes. Various treatments (washing of membranes, freezing and thawing) did not affect this activity. In the presence of Triton X-100, the enzyme shows appreciable latency, This detergent was then used to solubilize the enzyme and to study its main characteristics. A competition and a heat stability experiment show that only one enzyme acts on two substrates (free N-acetylglucosamine or desialyzed and degalactosylated fetuin). UDP-galactose hydrolase activities were very low compared to those of the bovine milk fat globule membranes. Other characteristic enzymes of Golgi vesicles were found in human milk fat globules membranes. It is of interest to find out whether this is the result of contamination with cytoplasmic particles or whether it reflects the participation of Golgi vesicles in human milk fat globule secretion.

Studies on the thiol group of lactose synthetase A protein from human milk and on the binding of uridine diphosphate galactose to the enzyme

The lactose synthetase activity of A protein from human milk was much decreased but not abolished by reaction with thiol-group reagents. Protection experiments indicated that a free thiol group on the enzyme is situated near the UDP-galactose binding site and inactivation of the enzyme with p-hydroxymercuribenzoate was probably due to prevention of UDP-galactose binding. Affinity chromatography showed that the mercuribenzoate substituent also decreased the affinity of A protein for N-acetylglucosamine but complex-formation between A protein-N-acetylglucosamine and alpha-lactalbumin was relatively unaffected. UDP-galactose appears to be bound to the enzyme mainly through its pyrophosphate group with Mn(2+) ion and through the cis hydroxyls of ribose, whereas its hexose moiety has little if any affinity for the enzyme. Lactose synthetase activity remaining after the reaction with thiol-group reagents indicates that a free thiol group is not an essential part of the A protein active site.