Modulation of bovine milk galactosyltransferase activity by lipids

Probing replacement of pyrophosphate via click chemistry; synthesis of UDP-sugar analogues as potential glycosyl transferase inhibitors

A series of potential UDP-sugar mimics were readily synthesised by copper(I) catalysed modified Huisgen cycloaddition of the corresponding alpha-propargyl glycosides with 5-azido uridine in aqueous solution. None of the compounds accessed displayed significant inhibitory activity at concentrations of up to 4.5mM in an assay against bovine milk beta-1,4-galactosyltransferase.

Galactosylation of N-linked oligosaccharides by human beta-1,4-galactosyltransferases I, II, III, IV, V, and VI expressed in Sf-9 cells

Several studies showed that Sf-9 cells can synthesize the galactosylated N-linked oligosaccharides if beta-1,4-galactosyltransferase (beta-1,4-GalT) is supplied. The full-length human beta-1,4-GalT I, II, III, IV, V, and VI cDNAs were independently transfected into Sf-9 cells, and the galactosylation of endogenous membrane glycoproteins was examined by lectin blot analysis using Ricinus communis agglutinin-I (RCA-I), which preferentially interacts with oligosaccharides terminated with Galbeta1-->4GlcNAc group. Several RCA-I-reactive bands appeared in all of the gene-transfected cells, and disappeared on treatment of blots with beta-1,4-galactosidase or N-glycanase prior to incubation with lectin. Introduction of the antisense beta-1,4-GalT II and V cDNAs separately into human colorectal adenocarcinoma SW480 cells, in which beta-1,4-GalT I, II, and V genes were expressed, resulted in the reduction of RCA-I binding toward N-linked oligosaccharides of the membrane glycoproteins. Differences were found in their K(m) values toward UDP-Gal and GlcNAcbeta-S-pNP and in their acceptor specificities toward oligosaccharides with the GlcNAcbeta1-->4(GlcNAcbeta1-->2)Man branch and with the GlcNAcbeta1-->6(GlcNAcbeta1-->2)Man branch. These results indicate that beta-1,4-GalTs II, III, IV, V, and VI are involved in the N-linked oligosaccharide biosynthesis cooperatively but not in a redundant manner with beta-1,4-GalT I within cells.

Biochemical differences between two types of N-acetylglucosamine:-->6sulfotransferases in human colonic adenocarcinomas and the adjacent normal mucosa: specific expression of a GlcNAc:-->6sulfotransferase in mucinous adenocarcinoma

6-O-Sulfation of beta-GlcNAc is an initial step in the biosynthesis of N-linked and O-linked sulfated glycans, which are widely distributed in colonic tissues. However, the biochemical mechanism of this sulfation in human colonic carcinogenesis was still unclear. In this study, we found two types of GlcNAc:-->6sulfotransferases (SulT) in human colonic adenocarcinomas and the adjacent normal mucosa, and we determined their enzymatic characteristics. One SulT, named SulT-a, was present in the adjacent normal mucosa and in non-mucinous adenocarcinomas, whereas the other SulT, named SulT-b, was present only in mucinous adenocarcinomas and adenocarcinomas with a mucinous component. SulT-a preferentially acted on Galbeta1-->3(GlcNAcbeta1-->6)GalNAc(alpha1)-p-nitrophenyl (pNP) and GlcNAcbeta1-->2Man, whereas SulT-b could act not only on these two glycans, but also on GlcNAcbeta1-->3GalNAc(alpha1)-pNP and GlcNAcbeta1-->3Galbeta1-->4Glc. The levels of SulT-a activity were significantly lower in non-mucinous adenocarcinomas than in the adjacent mucosa. In contrast, SulT-b was expressed in mucinous adenocarcinomas and in adenocarcinomas with a mucinous component. These results indicate that there are at least two types of GlcNAc:-->6SulT, SulT-a and -b, in colonic mucosa and adenocarcinomas, and that the occurrence of these enzymes is closely correlated with colonic cancer and the presence of areas of mucin accumulation.

Purification and characterization of a glucuronyltransferase involved in the biosynthesis of the HNK-1 epitope on glycoproteins from rat brain

The glucuronyltransferase involved in the biosynthesis of the HNK-1 epitope on glycoproteins was purified to an apparent homogeneity from the Nonidet P-40 extract of 2-week postnatal rat forebrain by sequential chromatographies on CM-Sepharose CL-6B, UDP-GlcA-Sepharose 4B, asialo-orosomucoid-Sepharose 4B, Matrex gel Blue A, Mono Q, HiTrap chelating, and HiTrap heparin columns. The purified enzyme migrated as a 45-kDa protein upon SDS-polyacrylamide gel electrophoresis under reducing conditions, but eluted as a 90-kDa protein upon Superose gel filtration in the presence of Nonidet P-40, suggesting that the enzyme forms homodimers under non-denatured conditions. The enzyme transferred glucuronic acid to various glycoprotein acceptors bearing terminal N-acetyllactosamine structure such as asialo-orosomucoid, asialo-fetuin, and asialo-neural cell adhesion molecule, whereas little activity was detected to paragloboside, a precursor glycolipid of the HNK-1 epitope on glycolipids. These results suggested that the enzyme is specifically associated with the biosynthesis of the HNK-1 epitope on glycoproteins. Sphingomyelin was specifically required for expression of the enzyme activity. Stearoyl-sphingomyelin (18:0) was the most effective, followed by palmitoyl-sphingomyelin (16:0) and lignoceroyl-sphingomyelin (24:0). Interestingly, activity was demonstrated only for sphingomyelin with a saturated fatty acid, i.e. not for that with an unsaturated fatty acid, regardless of the length of the acyl group.

Membrane-bound states of alpha-lactalbumin: implications for the protein stability and conformation

alpha-Lactalbumin (alpha-LA) associates with dimyristoylphosphatidylcholine (DMPC) or egg lecithin (EPC) liposomes. Thermal denaturation of isolated DMPC or EPC alpha-LA complexes was dependent on the metal bound state of the protein. The intrinsic fluorescence of thermally denatured DMPC-alpha-LA was sensitive to two thermal transitions: the Tc of the lipid vesicles, and the denaturation of the protein. Quenching experiments suggested that tryptophan accessibility increased upon protein-DMPC association, in contrast with earlier suggestions that the limited emission red shift upon association with the liposome was due to partial insertion of tryptophan into the apolar phase of the bilayer (Hanssens I et al., 1985, Biochim Biophys Acta 817:154-166). On the other hand, above the protein transition (70 degrees C), the spectral blue shifts and reduced accessibility to quencher suggested that tryptophan interacts significantly with the apolar phase of either DMPC and EPC. At pH 2, where the protein inserts into the bilayer rapidly, the isolated DMPC-alpha-LA complex showed a distinct fluorescence thermal transition between 40 and 60 degrees C, consistent with a partially inserted form that possesses some degree of tertiary structure and unfolds cooperatively. This result is significant in light of earlier findings of increased helicity for the acid form, i.e., molten globule state of the protein (Hanssens I et al., 1985, Biochim Biophys Acta 817:154-166). These results suggest a model where a limited expansion of conformation occurs upon association with the membrane at neutral pH and physiological temperatures, with a concomitant increase in the exposure of tryptophan to external quenchers; i.e., the current data do not support a model where an apolar, tryptophan-containing surface is covered by the lipid phase of the bilayer.

Further evidence for an active site polypeptide of galactosyl transferase

In a previous report it was shown that galactosyl transferase activity after blotting from acrylamide gel was present in a molecular weight range of less than 14 kDa, in Triton X-100 (1). Molecular sieve chromatography on Superose 12, in the presence of Triton X-100, gave the same result. The low molecular weight activity peak was eluted together with peptides as a part of the covalent structure of the enzyme or as absolutely requires effectors. Peptide mapping showed a new poly-lysine-like peptide and a new hydrophobic peptide in this low molecular weight activity peak as effectors of the enzyme inside its hydrophobic environment.

Interactions of galactosyltransferase with serum and secretory immunoglobulins and their component chains

Assay of the activity of beta-1,4-galactosyltransferase (beta-1,4-GT) revealed that in addition to serum, milk, colostrum, amniotic and cerebrospinal fluids and malignant effusions, this enzyme is present also in tears and saliva. Molecular-sieve chromatography of human colostral whey and serum and subsequent assay of beta-1,4-GT activity have shown that beta-1,4-GT was present as a free enzyme (55 kDa) and associated with components of larger molar mass. The elution pattern did not change when the chromatography was carried out in a buffer devoid of, or enriched with, Mn2+, a cofactor of beta-1,4-GT activity. However, the activity associated with the large molar mass components was absent when the chromatography was carried out in the presence of a chelating agent (EDTA). Analyses of the eluted material by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE), and by immunodiffusion indicated that the major colostral component in beta-1,4-GT activity-containing fractions was secretory IgA (S-IgA); in addition, the beta-1,4-GT activity was detected in fractions that contained lactoferrin and alpha-lactalbumin. Interactions of beta-1,4-GT with S-IgA and lactoferrin in colostrum were also demonstrated by the detection of radioactivity in precipitin lines obtained by immunoelectrophoresis and autoradiography of the colostral whey after it had been incubated with UDP-[3H]-galactose. Furthermore, radioactively labeled S-IgA and alpha-chain were detected when colostral whey incubated with UDP-[3H]-galactose was analyzed by SDS-PAGE under non-reducing and reducing conditions, respectively. In serum, the beta-1,4-GT-binding components identified in fractions after molecular-sieve chromatography were IgG, IgA, IgM and transferrin. The binding of beta-1,4-GT to immunoglobulins (Ig) was also demonstrated by assaying the beta-1,4-GT activity associated with Sepharose-4B-immobilized Ig of various isotypes and molecular forms, which were incubated with colostral beta-1,4-GT in the presence of Mn2+. Beta-1,4-GT measured by enzyme activity was bound to these Ig in order: polymeric IgA2 > monomeric IgA1 = polymeric IgA1 = secretory IgA = pentameric IgM > IgG. Immobilized component chains, namely alpha, mu and J chains, bound beta-1,4-GT more effectively than native Ig. Incubation of the IgA1 myeloma protein with crude human colostral galactosyltransferase in the presence of UDP[3H]-galactose and Mn2+ resulted in galactosylation of both N- and O-linked carbohydrate side chains.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of IgA-binding receptors on murine T cells: relative importance of Fc alpha R, beta-galactosyltransferase and anti-secretory component reactive proteins (ASCP)

Murine T cells and T-cell lines express receptors for the Fc of IgA (Fc alpha R); however, their molecular properties remain to be elucidated. In the present study, we examined three candidate molecules for IgA-binding receptors including Fc alpha R, beta-galactosyltransferase (beta-GT) and anti-secretory component (SC) reactive proteins (ASCP) expressed on T cells which might participate in the binding of different molecular forms of IgA. T-cell lines derived from CD4+ T cells of mouse Peyer's patches (PP) (designated PPT 4-6 and PPT 4-16) and from cloned PP T helper (Th) cell lines (ThHA1 #9 and #10) bound both monomeric and dimeric IgA (mIgA and dIgA), while the fusion partners (BW 5147 and R1.1) did not. In contrast, both Fc alpha R+ and Fc alpha R- cell lines bound to high molecular weight polymeric or aggregated IgA (pIgA). All cell lines reacted with a monoclonal anti-beta-GT (MoAb) and beta-GT enzyme activity was associated with the cell lysates and membrane fractions of all cells tested. The anti-beta-GT MoAb stained a 47-kDa band on immunoblots which was identical to that seen with native enzyme. mRNA analysis with beta-GT cDNA showed that all cell lines constitutively produced enzyme-specific mRNA. Both Fc alpha R+ T cells and Fc alpha R- control cell lines showed cell surface specific beta-GT activity. This is the first study which shows that mouse T cells produce beta-GT. However, Fc alpha R and beta-GT appear to be separate receptors, because Fc alpha R+ T cells bound mIgA and dIgA, and this treatment did not affect staining with biotinylated anti-beta-GT MoAb. Further, preincubation of the Fc alpha R+ cells with anti-beta-GT MoAb did not block mIgA binding. However, the anti-beta-GT MoAb partially blocked binding of pIgA to both Fc alpha R+ and Fc alpha R- T cells, suggesting that beta-GT may be a receptor for pIgA. Others have shown that T cells may bind IgA through a receptor serologically related to SC. We found that antibodies both to human SC and to rat SC specifically bound to both Fc alpha R+ and Fc alpha R- T cells. Further, a 72-kDa band was detected when cell membrane fractions were analysed with these antisera (ASCP) by solid phase immunoisolation technique and immunoblot analysis. The ASCP is not an IgA-binding receptor, since anti-SC did not block either mIgA or pIgA binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of lindane on the fluidity of the rat ventral prostate membranes

The influence of lindane upon the dynamic properties of plasma membranes from rat ventral prostate has been investigated using a fluorescence polarization technique. Preincubation with lindane decreased the fluorescence polarization in a dose dependent manner. This effect, which is associated with an increased membrane fluidity, occurred in a very short period of time. Lindane also provoked a number of changes in lipid biosynthesis from acetate in the membrane. Less [1-14C]acetate was incorporated into cholesterol and more into phospholipids when this liposoluble toxicant was added to the preincubation medium. However, not all phospholipid classes were equally increased, because while the rate of acetate incorporation was greater into choline glycerophospholipids than into ethanolamine glycerophospholipids, both were higher than the rates of acetate incorporation into serine glycerophospholipids and sphingomyelin.

Effect of lindane upon the beta-adrenergic stimulation of cyclic AMP accumulation in rat renal cortical tubules caused by alterations in membrane fluidity

The influence of lindane (gamma-hexachlorocyclohexane) on fluidity and lipid composition in rat renal cortical tubules has been investigated. Lindane increased membrane fluidity as measured by a fluorescence polarization technique using the probe diphenylhexatriene. This effect was dose-dependent and was accompanied by a 70% inhibition of the beta-adrenergic stimulatory activity upon cyclic AMP accumulation after 30 min of preincubation with lindane at 25 degrees C. Experiments with increasing concentrations of isoproterenol indicated that the efficacy, but not the potency, of the beta-adrenergic effect upon cyclic AMP accumulation was affected by lindane. Lindane toxicity could also be associated with variations in the incorporation of acetate into various lipid classes. Lindane increased acetate incorporation into phospholipids and decreased that into cholesterol.

Effects of glycosylation enzymes from membrane fractions, induced by chronic ethanol administration

To study the effect of chronic ethanol administration on the enzyme activities involved in the glycosylation processes (glycosidases and glycosyltransferases), we used 6-week ethanol-treated female Wistar rats and pair-control rats. Biological material was membrane fractions of microsomes and plasma membrane obtained by subcellular fractionation technique. Ethanol treatment increased with statistical significance the Vmax of N-acetyl-beta-D-glucosaminidase (P less than 0.10), beta-D-glucuronidase and alpha-D-mannosidase (P less than 0.001) activities from microsomal fractions and likewise it increased the Km value of beta-D-glucuronidase (P less than 0.001). In vitro doses of ethanol (0.1-1.7 M) were added to delucidate hypothetical membrane tolerance responses. However in vitro addition of ethanol provoked no differential effects in treated and untreated rats. Glycosyltransferase assays were carried out using [14C]sugar derivatives. We detected several glycosyltransferase activities in both microsome and plasma membrane fractions. Chronic ethanol exposure appeared to affect N-acetyl-neuraminyltransferase activity from both membrane fractions, producing a greatly increased incorporation in the presence of asialofetuin. Glucosyl and mannosyltransferase activities from plasma membrane fractions were also altered by ethanol treatment, producing an increased enzyme activity when reaction was performed in the presence of phosphatidylcholine + dolichol-phosphate liposomes.

Effects of chronic ethanol administration on plasma-membrane-bound glycosyltransferase activities

Physiological and membranous modifications induced by a 4-week ethanol administration in mouse liver plasma membrane were studied. Galactosyl- and glucosyltransferase activities were stimulated in the presence of dolichylphosphate alone or with phosphatidyl-choline. The galactosyltransferase activity was inhibited by chronic ethanol administration. These enzymes were modulated by different phospholipids. The phosphatidic acid was the most efficient activator. Ethanol provoked an inhibition of the galactosyltransferase activity whatever the phospholipid used, as well as an inhibition of the glucosyltransferase activity, chiefly in presence of phosphatidyl-inositol. The preincubation of control or treated mouse liver plasma membranes with liposomes loaded by dolichylphosphate and cholesterol greatly enhanced the enzymatic activities without removing the inhibition by ethanol treatment.

The interaction of bovine milk galactosyltransferase with lipid and alpha-lactalbumin

The activation of galactosyltransferase (UDPgalactose: N-acetyl-D-glucosaminyl-glycopeptide 4-beta-D-galactosyltransferase, EC 2.4.1.38) by alpha-lactalbumin has been studied at low concentrations of alpha-lactalbumin where the relationship is sigmoidal. The sigmoidal shape of the activation curve was eliminated by neutral lipids such as phosphatidylcholine and phosphatidylethanolamine, detergents such as Triton X-100 or by an aggregated form of alpha-lactalbumin generated by crosslinking alpha-lactalbumin with dithiobissuccinimidylpropionate. It is proposed that these different reagents present a hydrophobic surface to the enzyme which is necessary for lactose synthase activity. In competition experiments, large amounts of alpha-lactalbumin were able to displace lipid from the enzyme as suggested by the loss of the lipid-activating effect in the presence of an excess of alpha-lactalbumin. Optimal lactose synthase activity was obtained when the ratio of lipid/alpha-lactalbumin/enzyme was 60:6:1. The mechanism by which the lipid effect was obtained probably involved a phase transition in the enzyme which was detected as a sharp break in the Arrhenius curve. The presence of phosphatidylcholine abolished the break demonstrating that full activity of the enzyme required both alpha-lactalbumin and lipid.

Galactosyltransferase activities in mitochondrial outer membrane: biosynthesis of dolichylmonophosphate-galactose

Mitochondrial outer membranes were prepared from mouse liver homogenates by swelling purified mitochondria in phosphate buffer and were purified on a discontinuous sucrose gradient. Assays for marker enzymes and controls in electron microscopy confirmed the purity and homogeneity of this subfraction. Mitochondrial outer membranes had significant galactosyltransferase activity when incubated with UDP-[14C]galactose: 14C-labelling was found in products extractable with organic solvents and in a residual precipitate. Addition of exogenous dolichylmonophosphate loaded into phosphatidylcholine liposomes strongly enhanced the incorporation of [14C]galactose into chloroform/methanol (2:1, v/v) -extractable products. Thin-layer chromatography of these 2:1 extracts showed that the increase of [14C]galactose incorporation was attributable to the synthesis of a new galactosylated lipid, 'lipid L'. This 'lipid L' has been purified on silicic acid columns by elution with chloroform/methanol (1:1, v/v). The purified 'lipid L' was labile in acid and released [14C]galactose. It had the same chromatographic behaviour as dolichylmonophosphate-mannose in neutral, acid and alkaline solvent systems. Upon incubation in presence of [3H]dolichylmonophosphate and UDP-[14C]galactose, purified 'lipid L' contained both 3H- and 14C-labelling. 'Lipid L', synthesized by mitochondrial outer membranes, was therefore characterized as dolichylmonophosphate-galactose.

Purification, properties and cation activation of galactosyltransferase from lactating-rat mammary Golgi membranes

Galactosyltransferase was purified from Golgi membranes of lactating-rat mammary gland and studied with respect to its physical and enzymic (lactose synthetase) properties. The enzyme occurred in both monomeric (43-46 kDa) and apparently dimeric (90 kDa) forms. It was very unstable except in the presence of phospholipid, detergent, or cations binding to site 2. The amino acid composition and the N-terminal sequence closely resembled that of the human and bovine milk enzymes, particularly in respect to a Pro-Pro-Pro-Pro sequence. Kinetic studies demonstrated a high-affinity Mn2+-binding site (1) essential for activity, and a low-affinity Mn2+-binding site (2) that could also bind spermidine or clupeine. Mn2+ binding at site 2 raised Vmax fivefold. Spermidine binding at site 2 enhanced Mn2+ binding at site 1, and influenced binding of glucose. At physiological glucose concentration, clupeine or spermidine activated nearly as well as 15 mM MnCl2 and are regarded as models of a natural cation activator that remains to be isolated. Evidence is given for an essential histidine residue in the galactosyltransferase. It is proposed that site 1 Mn2+ participates directly in the reaction mechanism, whereas site 2 is a regulator site allosterically activated by a basic protein.

Characterization of a glucosyltransferase activity in liver plasma membrane: modulation by cations and lipidic effectors

1. Glucosyltransferase activity incorporating [14C]glucose from UDP-[14C]glucose onto endogenous lipidic acceptors was localized primarily in the plasma membrane of liver. 2. Incubation of plasma membrane by phosphatidyl-choline liposomes loaded with dolichyl-phosphate stimulated the enzymatic activity. 3. This enzyme required Mg2+ for maximal catalitic activity. Ca2+ could substitute Mg2+. 4. Mn2+ acted as a partial non-competitive inhibitor of the Mg2+-activated glucosyltransferase. 5. This enzyme can be modulated by neutral and acidic phospholipids; the most efficient were phosphatidyl-serine and phosphatidyl-inositol. 6. The enzymatic activity was not significantly changed by cholesterol alone but it is greatly enhanced by liposomes loaded with dolichyl-phosphate and cholesterol.

Different reactivity to lysophosphatidylcholine, DIDS and trypsin of two brain sialyltransferases specific for O-glycans: a consequence of their topography in the endoplasmic membranes

Some properties of two distinct rat brain sialyltransferases, acting on fetuin and asialofetuin, respectively, were investigated. These two membrane-bound enzymes were both strongly inhibited by charged phospholipids. Neutral phospholipids were without effect except lysophosphatidylcholine (lysoPC) which modulated these two enzymes in a different way. At 5 mM lysoPC, the fetuin sialyltransferase was solubilized and highly activated while the asialofetuin sialyltransferase was inhibited. Preincubation of brain microsomes with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), known as a specific anion inhibitor and a non-penetrating probe, led to a moderate inhibition of the asialofetuin sialyltransferase just as in the case of the ovomucoid galactosyltransferase (used here as a marker for the luminal side of the Golgi membrane); under similar conditions, the fetuin sialyltransferase was strongly inhibited. In the presence of Triton X-100, which induced a disruption of membranes, all three enzymes were strongly inhibited by DIDS. Trypsin action on intact membranes showed that asialofetuin sialyltransferase, galactosyltransferase and fetuin sialyltransferase were all slightly inhibited. After membrane disruption by Triton X-100, the first two enzymes were completely inactivated by trypsin while the fetuin sialyltransferase was quite insensitive to trypsin treatment. From these data, we suggest that the fetuin sialyltransferase, accessible to DIDS, is an external enzyme, oriented closely towards the cytoplasmic side of the brain microsomal vesicles (endoplasmic and Golgi membranes), whereas the asialofetuin sialyltransferase is an internal enzyme, oriented in a similar manner to the galactosyltransferase. Moreover, the anion site (nucleotide sugar binding site) of the fetuin sialyltransferase must be different from its active site, as this enzyme, when solubilized, is strongly inhibited by DIDS while no degradation is observed in the presence of trypsin.

The modulating effects of lipids on purified rat liver Golgi galactosyltransferase

Galactosyltransferase was purified from rat liver Golgi membranes. The Triton X-100, used to solubilize the enzyme was removed immediately prior to the lipid interaction studies. In lipid vesicles, prepared from a variety of phosphatidylcholines (PCs), including egg PC, DOPC, DMPC, DPPC and DSPC, the ability of the lipids to stimulate the enzyme decreased in the order egg PC greater than DOPC greater than DMPC greater than DPPC greater than DSPC, i.e. the lower the transition temperature (Tc) the greater the stimulation of the enzyme. A second, neutral lipid, phosphatidylethanolamine was used to permit a comparison of the effect of a different head group of the same net charge at neutral pH. The PEs included, egg PE, soy PE, Pl-PE, PE(PC) and DPPE in order of increasing Tc. The effect of the PEs was opposite to that of the PCs, i.e. the higher the Tc, the greater the stimulation of the enzyme. In fact egg PE and soy PE which have the lowest Tc values were inhibitory. Thus the modulation of the Golgi membrane galactosyltransferase by these lipids was different from that reported earlier for the bovine milk galactosyltransferase. The effects of two acidic lipids, egg phosphatidic acid (PA) and egg phosphatidylglycerol (PG) were studied also. Both totally inhibited the enzyme even at low concentrations of lipid, however, the PA was more effective than PG. In mixtures of neutral lipid (PC) and acidic lipid (PA or PG), the effect of the acidic lipid dominated. Even in the presence of excess PC, total inhibition of the enzyme was observed. It was concluded that the enzyme bound the acidic lipid preferentially to itself. The choice of the lipids allowed us to make several direct comparisons concerning the effect of the nature of the lipid head group on the activity of the enzyme. For example PE(PC), egg PA and egg PG would have fatty acid chains identical to egg PC since these three lipids are all prepared by modification of egg PC. As well, DPPE differs from DPPC only by nature of the head group. These comparisons indicated that not only the net charge but also chemical nature of the head group were important in the lipid modulation of Golgi galactosyltransferase.

Galactosyltransferase purified from rat milk is distinct from the human and bovine enzyme

N-Acetylglucosaminide beta 1, 4-galactosyltransferase (EC 2.4.1.22) was purified from rat milk by affinity chromatography on N-acetylglucosamine-Sepharose and alpha-lactalbumin-Sepharose columns. The purified enzyme migrated as three polypeptides of relative molecular weight 59,000, 54,000, and 27,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antiserum raised against the 54K rat protein immunoprecipitated all three polypeptides, suggesting that they share common antigenic sites. The human milk galactosyltransferase, purified under similar conditions, was electrophoretically homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with relative molecular weight 54K, and was not immunoprecipitated by the antiserum to the 54K rat milk protein. In addition, Michaelis constants for the enzyme from rat and human milk differed. The apparent Michaelis constant for N-acetylglucosamine and uridine 5'-diphosphate-galactose were 4.8 and .3 mM, respectively, for the rat enzyme, and 1.8 and .028 mM, respectively, for the human enzyme.

The modulation of bovine milk D-galactosyltransferase by various phosphatidylethanolamines

To investigate the possible role of nonbilayer phases in the modulation of glycosyltransferase activity, bovine milk D-galactosyltransferase has been studied in phosphatidylethanolamine (PE) membranes, including soybean PE, egg PE, PE prepared by transphosphatidylation of egg PC, bovine brain PE, plasmalogen PE, and DPPE. The gel-to-liquid crystalline transition (TC) and the lamellar-to-hexagonal transitions (TH) are known for most of the PE compounds. The lower the TC (or TH) value, the greater the stimulation of galactosyltransferase activity in both the lactose- and N-acetyllactosamine-synthetase reactions. No correlation was found between either TC or TH value and the break in the Arrhenius plots for the N-acetyllactosamine synthetase. In membranes consisting of mixtures of PE with PC, the dominant effect was that of PC. The stimulation of activity in the mixed-lipid systems was never greater than that produced by PC alone, therefore the enzyme showed a definite preference for PC in the mixtures.

The effect of retinoids on the activity of the membrane form of galactosyltransferase, studied in an enzyme/liposome model system

In the present study the effect of retinoids on the membrane form of galactosyltransferase was tested. A model system consisting of pure bovine milk galactosyltransferase and phosphatidylserine vesicles was used for this investigation. Retinol, retinal and retinylphosphate were able to overcome the modulating effect of phosphatidylserine, that is, activated the enzyme. Retinoic acid and retinylpalmitate were ineffective in this system.

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.

Stimulation of a neutral triacylglycerol hydrolase from rat heart by phosphatidylethanolamine and lysophosphatidylethanolamine

Triacylglycerol hydrolase activity measured at pH 7.5 in a pH 5.2 precipitate fraction from rat heart was increased two- to three-fold by the presence of phosphatidylethanolamine (PE) or lysophosphatidylethanolamine (LPE). This stimulatory effect also could be obtained in assays with particulate and soluble subcellular fractions and was observed with two different methods of preparing triolein substrate emulsions. Ethanolamine and glycerophosphorylethanolamine had no effect on hydrolase activity, whereas phosphatidylcholine (PC) and acidic phospholipids such as cardiolipin were inhibitory. Palmitic acid, palmityl CoA and palmityl carnitine inhibited PE-stimulated hydrolase activity, but ethyl esters of palmitate had no effect. The preparation of acetone-ether powders resulted in a marked reduction of triacylglycerol hydrolase activity, but PE and LPE now stimulated hydrolase activity by ten-fold or greater, suggesting that these phospholipids may have an obligatory role in modulating triacylglycerol hydrolase activity. Triton X-100 also stimulated hydrolase activity in acetone-ether powders.

Stimulation of bovine milk galactosyltransferase activity by bovine colostrum N-acetylglucosaminyltransferase I

Purified bovine milk galactosyltransferase was stimulated by purified bovine colostrum N-acetylglucosaminyltransferase I by more than 10-fold. Only slight stimulation of the N-acetylglucosaminyltransferase I by galactosyltransferase was observed. Heat inactivation destroyed the ability of the N-acetylglucosaminyltransferase I to stimulate the galactosyltransferase. The stimulation of galactosyltransferase was accompanied by a decrease in Km of this enzyme from 9.7 to 3.3. mM and an increase in Vmax from 1.87 to 3.71 nmol galactose transferred/min per mg galactosyltransferase when GlcNAc was the substrate. When the Km for UDPgalactose was determined, it increased from 0.19 to 0.42 mM in the presence of N-acetylglucosaminyltransferase I and the Vmax increased from 0.66 to 2.76 nmol galactose transferred/min per mg galactosyltransferase. In phosphatidylcholine vesicles, no effect on Km values with GlcNAc as substrate was noted, while an increase in the Km of UDPgalactose was observed. The Vmax values were generally higher in the lipid vesicles. Complex formation between galactosyltransferase and N-acetylglucosaminyltransferase I was demonstrated both by glycerol density gradient centrifugation and Bio-Gel P-100 column chromatography. An approximate molecular weight for the complex was obtained on a calibrated Sephadex G-200 column and found to be about 75 000, consistent with a 1:1 complex. The stimulation of galactosyltransferase involved the N-acetyllactosamine synthetase activity of this enzyme and not the lactose synthetase activity, since the latter activity was only slightly affected. Since N-acetylglucosaminyltransferase I is not involved in the lactose synthetase reaction, the stimulation is consistent with the known biosynthetic role of N-acetylglucosaminyltransferase I in the biosynthesis of asparagine-linked oligosaccharides.

The effect of acidic lipids on the activity of bovine milk galactosyltransferase in vesicles of different phosphatidylethanolamines

Bovine milk galactosyltransferase was incorporated into vesicles prepared from different phosphatidylethanolamines which varied widely in both their gel-liquid crystalline and their lamellar-hexagonal phase transition temperatures. Although all phosphatidylethanolamines stimulated the activity of the enzyme the extent of stimulation varied. Acidic lipids phosphatidylserine and phosphatidic acid inhibited the activity of the enzyme incorporated into all of the phosphatidylethanolamines except when the enzyme was in soya PE in which the acidic lipids had no effect.

Modulation of solubilized brain fucosyltransferase activity by phospholipids

Phospholipids interact on Triton X-100 solubilized GDP-fucose: asialofetuin fucosyltransferase (EC 2.4.1.68) isolated from sheep brain. This enzymatic activity is modulated by charged phospholipids. In particular, phosphatidic acid and analogues markedly inhibit the transfer of fucose from GDP-[14C]fucose. Kinetic studies show that phosphatidic acid interacts as a mixed inhibitor: the velocity and affinity of fucosyltransferase for the GDP-fucose and asialofetuin substrates are strongly decreased. However, this inhibitory effect is not related to stereospecificity, and the different parameters involved in the enzymatic reaction of glycosylation are not modified. The nature of fatty acids and chemical bond (ester or ether) occurring in the carbohydrate chain does not modify the behaviour of phosphatidic acid with respect to fucosyltransferase activity. Further, the physical state of phosphatidic acid (gel phase or liquid crystalline phase) has no influence. However, as the inhibition is closely pH-dependent, these data suggest that phosphatidic acid might directly interact with the active site of the enzyme and induce a conformational change.