Cross-linking of the components of lactose synthetase with dimethylpimelimidate.

The cross-linking of the two components of lactose synthetase, alpha-lactalbumin and a galactosyltransferase, with dimethylpimelimidate was examined. The extent of the cross-linking at pH 8.1 was found to be dependent upon the presence of substrates or inhibitors for the galactosyltransferase. N-acetylglucosamine and mixtures of either N-acetylglucosamine, Mn-2+ and UDP, or UDP-galactose and Mn-2+ promoted the formation of cross-linked species. Glucose or a mixture of UDP and Mn-2+ were much less effective in promoting cross-linking. Two types of intermolecularly cross-linked species of alpha-lactalbumin and the galactosyltransferase were obtained. Each was a 1:1 cross-linked complex of alpha-lactalbumin and either of the two forms of the transferase with molecular weights of about 42,000 and 48,000, respectively. Cross-linked complexes were not observed with more than 1 molecule each of alpha-lactalbumin and the transferase. The cross-linked complexes were obtained in homogeneous form by gel filtration on Sephadex and absorption of uncross-linked enzyme by affinity chromatography on alpha-lactalbumin-Sepharose in the presence of N-acetylglucosamine. They migrated on gel electrophoresis in sodium dodecyl sulfate with mobilities in accord with their predicted molecular weights as 1:1 complexes of alpha-lactalbumin and the transferase. The amino acid composition of the cross-linked complex was in reasonable agreement with the expected composition of a 1:1 mixture of alpha-lactalbumin and galactosyltransferase. The enzymic properties of the cross-linked and uncross-linked enzymes were compared. The cross-linked complex had a much higher intrinsic lactose synthetase activity than did uncross-linked enzyme although only about 1% of the potential activity of uncross-linked enzyme in the presence of optimal concentrations of alpha-lactalbumin. The lactose synthetase activity of the cross-linked complex, however, was unaffected by exogenous alpha-lactalbumin. In addition, the complex readily catalyzed the transfer of galactose from UDP-galactose to xylose in the absence of exogenous alpha-lactalbumin. The N-acetyllactosamine synthetase activity of the complex was low compared to its activity with other monosaccharides. Ovalbumin, which is a good acceptor for the uncross-linked transferase, was not an acceptor for the cross-linked complex. Kinetic studies of the complex suggest that its modified catalytic activity is not the result of the modification by dimethylpimelimidate but reflects the expected effects of is provided, and that     

Photoaffinity labeling of lactose synthase with a UDP-galactose analogue

A photoaffinity analogue of UDP-galactose, 4-azido-2-nitrophenyluridylyl pyrophosphate (ANUP), has been synthesized for the investigation of the binding topography of alpha-lactalbumin on galactosyltransferase. Results obtained from steady state kinetics show that ANUP is an effective competitive inhibitor against UDP-galactose in the reactions of lactose and N-acetyllactosamine syntheses. The specific binding of ANUP to the UDP-galactose-binding site is further demonstrated by its ability to facilitate the formation of the lactose synthase complex on solid supports, either alone or in the presence of glucose or N-acetyl-glucosamine. ANUP inactivates galactosyltransferase on irradiation. One mole of ANUP was incorporated per mol of enzyme inactivated. This process is Mn2+-dependent and can be prevented by UDP-galactose. Glucose and N-acetylglucosamine render only partial protection. Photoaffinity labeling of lactose synthase either free in solution or immobilized on Sepharose does not result in any reduction of the alpha-lactalbumin modifier activity. In addition, no incorporation of radioactivity into alpha-lactalbumin was observed when radioactive ANUP was used, whereas galactosyltransferase was labeled. These data indicate that alpha-lactalbumin does not bind to galactosyltransferase in the region of the ANUP site, suggesting that the location of protein-protein interaction between the two subunits of lactose synthase may be removed from the UDP-galactose-binding domain.     

Temporal effect of prolactin on the activities of lactose synthetase, alpha-lactalbumin, and galactosyl transferase in mouse mammary gland explants

The onset of the prolactin (PRL) stimulation of lactose synthesis is between 4 and 8 hr after adding PRL to cultured mouse mammary tissues. The synthesis of lactose is catalyzed by the enzyme lactose synthetase, which is composed of two parts, alpha-lactalbumin and galactosyl transferase. In time-sequence studies, it was found that the activity of galactosyl transferase is enhanced by PRL in concert with the onset of the PRL stimulation of lactose synthesis. In contrast, the earliest detectable effect of PRL on alpha-lactalbumin activity occurred 24 hr after adding PRL to the cultures. It is, therefore, apparent that the rate-limiting component for the PRL stimulation of lactose synthesis in cultured mouse mammary tissues is galactosyl transferase activity.     

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.     

The presence of the milk protein, alpha-lactalbumin and its mRNA in the rat epididymis

alpha-Lactalbumin, a modifier protein that changes the substrate specificity of galactosyltransferase, to promote the synthesis of lactose, is found in the mammary glands of lactating mammals and in milk. Molecules similar to mammary gland alpha-lactalbumin but distinct in their modifier activity have been found in rat epididymal fluid. We report here, using a rat mammary gland alpha-lactalbumin cDNA clone as a hybridization probe, RNA sequences homologous to alpha-lactalbumin mRNA were detected in total RNA from the rat epididymis. This finding suggests that alpha-lactalbumin or similar molecules, in addition to regulating lactose synthesis in the mammary gland, may have other important functions in mammalian reproduction.     

Immobilized bovine lactose synthase. A method of topographical analysis of the active site.

Bovine galactosyltransferase (UDPgalactose: D-glucose 4beta-galactosyltransferase, EC 2.4.1.22) was covalently coupled to Sepharose 4B by reaction at pH 5.0 with the activated mixed disulfide Sepharose-glutathione-2(5-nitropyridyl)-disulfide. The Sepharose-protein conjugate was presumably coupled via the unique highly reactive cysteine of those thiols on the bovine enzyme. The gel-bound N-acetyllactosamine and lactose synthase activity of about 0.4% was consistent with the affects of diffusion and the 90% activity reduction noted upon thiol modification of the dissolved enzyme. The residual lactose biosynthetic activity of the bound enzyme appeared possible only if the reactive thiol were physically distinct from the active site since the bulky Sepharose-glutathione group must not obscure the alpha-lactalbumin binding region.     

Physiological roles of zinc and calcium binding to alpha-lactalbumin in lactose biosynthesis

Bovine apo-alpha-lactalbumin was shown to be severalfold more efficient than its calcium conformer as a cofactor in lactose biosynthesis. This rate enhancement was manifested in a 3.5-fold increase in Vmax, with no differences in Km(app) between the two alpha-lactalbumin forms. In the presence of zinc, which shifts Ca(II)-alpha-lactalbumin toward the "apo-like" conformation [Musci, G., & Berliner, L.J. (1985) Biochemistry 24, 3852-3856], the catalytic rate constant for lactose synthesis was identical for both the Ca(II) and apo conformers. Activity measurements at different temperatures, on the other hand, confirmed that calcium is important in stabilizing the protein (alpha-lactalbumin) against thermal denaturation. The stabilizing effect of calcium was independent of the presence of Zn(II), i.e., of the protein conformation. The physiological implications of these results are discussed.     

The calcium-dependent electrophoretic shift of alpha-lactalbumin, the modifier protein of galactosyl transferase

alpha-Lactalbumin, the modifier protein of galactosyl transferase in the synthesis of lactose by the mammary gland, has been shown to undergo a Ca2+-dependent electrophoretic shift. Such shifts, characteristic of most calcium modulated proteins, are related to gross conformational changes upon binding calcium when detected in the presence of detergent (SDS-PAGE). However, we detected the calcium shift for alpha-lactalbumin using non-denaturing PAGE (ND-PAGE) where electrical charge changes are observed upon binding calcium. In order for a shift to be observed between the apo and calcium bound protein, calcium ion binding to proteins must have minimal dissociation constants (Kdiss) of 10(-7) M; alpha-lactalbumin is reported to bind calcium at Kdiss = 10(-10) to 10(-12) M. The electrophoretic shift identifies alpha-lactalbumin in complex milk whey patterns of many species of mammals.     

The sulfhydryl group microenvironment of lactose synthase from bovine milk

Galactosyltransferase from bovine milk was inactivated by a series of sulfhydryl group specific reagents of different structures and sizes. The inactivation rate constants suggest that the thiol is located in a nonpolar microenvironment. The ESR spectrum of a spin labeled galactosyltransferase showed that the sulfhydryl group is in a region of non-restricted rotation, consistent with its broad reactivity towards various thiol reagents. Galactosyltransferase immobilized onto agarose through its sulfhydryl group retained its ability to catalyze the synthesis of N-acetyllactosamine and lactose. Thus the residual activity of the sulfhydryl group modified enzyme is not due to an isozyme lacking such a group. In addition, the active thiol can not be located at the active site nor the protein-protein interaction site between galactosyltransferase and alpha-lactalbumin.     

Lactose synthase: effect of alpha-lactalbumin on substrate activity of N-acylglucosamines

N-Acetyl-, N-propionyl-, N-butyryl- and N-valerylglucosamines were synthesized as topographical probes to localize further the interaction site of alpha-lactalbumin on galactosyltransferase. All these compounds were found to be substrates for galactosyltransferase with Km values in the millimolar range. In the presence of alpha-lactalbumin, the Michaelis-Menten constants were diminished. However, the effect on the initial rates of these reactions varied. Thus, at low N-acylglucosamine concentrations, alpha-lactalbumin activated the enzyme activity, but at high concentrations, alpha-lactalbumin became inhibitory. This mixed-type inhibition kinetics indicated that a quaternary complex between galactosyltransferase, alpha-lactalbumin, Mn2+-UDPgalactose and N-acylglucosamine existed during the catalytic process. The ability of these N-acylglucosamine substrates to bind to lactose synthase complex was further substantiated by the physical association of galactosyltransferase onto the solid-bound alpha-lactalbumin in the presence of any one of these compounds. The data revealed that the presence of the N-acyl group up to five carbons in length did not interfere with the interaction between alpha-lactalbumin and galactosyltransferase, suggesting that alpha-lactalbumin was not bound in the vicinity of the C-2 region of the monosaccharide site. The inhibitory effect of alpha-lactalbumin on N-acyllactosamine formation is probably a consequence of conformational changes of galactosyltransferase.     

Glycosyltransferases in the Golgi membranes of onion stem

Cell fractions consisting largely of Golgi membranes were prepared from the meristematic region of the onion. Several enzyme activities were found to be localized in these fractions: inosine diphosphatase, galactosyltransferases and glucosyltransferases. The fractions catalysed the transfer of [(14)C]galactose from UDP-galactose to endogenous and cell-sap acceptors, to N-acetylglucosamine and to ovalbumin. In the presence of bovine alpha-lactalbumin, transfer to glucose (lactose synthesis) was catalysed. [(14)C]Glucose was transferred from UDP-glucose to endogenous and cell-sap acceptors, to cellobiose and to fructose (sucrose synthesis). All these activities were latent, being potentiated by detergents (Triton X-100 or sodium deoxycholate). The characteristics of some of these enzyme activities are described and their biological significance is discussed.     

The reaction of ethanolamine O-sulphate with 4-aminobutyrate aminotransferase.

Homogeneous rat alpha-lactalbumin was prepared from whey by chromatography on DEAE-Sephadex A-50 and Ultrogel AcA 44. Two biologically active forms of alpha-lactalbumin were apparent after ion-exchange chromatography, but on gel filtration the combined forms were eluted as a single peak with a molecular weight of approx. 33000. The molecular weight when determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis was 15100. Antiserum to alpha-lactalbumin was prepared from rabbits, and single radial immunodiffusion was used to measure the concentration of alpha-lactalbumin in milk expressed from rats during lactation and for 2 days after the cessation of lactation. A significant positive correlation (r = + 0.89) between the concentrations of alpha-lactalbumin and lactose was obtained for the first 20 days of lactation. This is consistent with the suggestion that alpha-lactalbumin may control the concentration of lactose in milk. However, a significant negative correlation (r = -0.91) between the concentration of alpha-lactalbumin and lactose was obtained for 2 days after the cessation of lactation on day 20.     

Purification and characterization of human serum galactosyltransferase (lactose synthetase A protein)

A galactosyltransferase, which transfers galactose from UDP-galactose to N-acetylglucosamine, was purified 286,000-fold to homogeneity with 40% yield from human plasma by repeated affinity chromatography on alpha-lactalbumin-Sepharose. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band with molecular weight of 49,000. The enzyme is a glycoprotein with 11% by weight carbohydrate, which seems to have only asparagine-N-acetylglucosamine linkage-type carbohydrate chains. The enzyme showed characteristic changes in activity at different alpha-lactalbumin concentrations, indicating that the enzyme is the A protein of lactose synthetase. Km values for the substrates were found to be 0.056 mM for UDP-galactose, 3.2 mM for GlcNAc, and 0.44 mM for Mn2+, and in the presence of alpha-lactalbumin, 3.4 mM for Glc, and 0.20 mM for Mn2+. The activity of the enzyme was neutralized by anti-enzyme antibody, but the antibody did not neutralize the bovine milk galactosyltransferase (A protein) activity.     

Calcium regulates folding and disulfide-bond formation in alpha-lactalbumin

Refolding and disulfide bond formation in reduced denatured bovine alpha-lactalbumin is shown to be Ca2+-dependent. Whereas in the absence of Ca2+ only about 2% of the native active protein is regenerated, in the presence of Ca2+, almost quantitative renaturation is obtained. A close coupling between Ca2+-binding and native disulfide bond formation is also indicated by spontaneous disulfide scrambling in the apoprotein in the presence of low concentrations of thiols. This phenomenon is not found in other disulfide-containing proteins including the homologous chicken lysozyme. It is proposed that the alpha-lactalbumin Ca2+-binding site has the in vivo function of imposing Ca2+ regulation on the folding of nascent alpha-lactalbumin and thereby on lactose synthesis.     

Purification and properties of beta-galactosidase from fungus, Curvularia inaequalis]

Highly purfied beta-galactosidase from fungus Curvularia inaequalis cultural fluid with a specific activity of 50 units per mg of protein was obtained by 2-fold purification of the enzyme, using chromatography on DEAE-cellulose and on hydroxylapatite. The enzyme was found to hydrolyze o-nitrophenyl-beta-D-galactopyranoside (pH optimum of 3.7--4.5) and lactose (pH optimum 3.9--5.3). The isoelectric point was observed at pH 4.4 the temperature optimum was 60 degrees C. The molecular weight (115 000--126 000) and the amino acid composition of the enzyme were determined. Km values for o-nitrophenyl-beta-D-galactopyranoside and lactose were 0.55-10(-3) M and 4.5-10(-3) M respectively. Disc-electrophoresis in polyacrylamide gel revealed a single band with a specific activity. The homogeneity of the enzyme was found in ultracentrifuge.     

Human serum contains N-acetyllactosamine: beta 1-3 N-acetylglucosaminyltransferase activity

Human serum was shown to contain N-acetyllactosamine: N-acetylglucosaminyltransferase activity. The reaction product was hydrolyzed by beta-N-acetylglucosaminidase and released [14C]N-acetylglucosamine, indicating that the N-acetylglucosaminyl residue was beta-linked to N-acetyllactosamine. Methylation and hydrolysis of the reaction product yielded 2,4,6-trimethyl[3H]galactose, indicating that the N-acetylglucosaminyl residue was introduced at position C-3 of the terminal galactose of N-acetyllactosamine. In our experiments, 2,3,4-trimethyl[3H]galactose was not detected. Substrate competition studies between N-acetyllactosamine and lactose showed that this enzyme also catalyzed the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to lactose. Since the Km value for N-acetyllactosamine, which was 7.0 mM, was approximately a fourth of that for lactose (29.8 mM), N-acetyllactosamine was more effective than lactose as an acceptor.     

Galactosyltransferase from buffalo milk: Further characterization

Buffalo milk galactosyltransferase is a single poly-peptide of molecular weight 55,000 to 56,000. The enzyme is specific for glucose as an acceptor substrate in the presence of 8-lactalbumin, L-Arabinose. L-xylose, D-ribose and D-fructose did not serve as acceptor substrates even at concentration as high as 0.13 M, while N-acetylglucosamine and ovalbumin served as good acceptors of galactosyl moiety in the absence of ∞ -lactalbumin. UDP-galacturonic acid did not serve as a donor substrate; on the contrary, it inhibited the reaction. Lactose synthetase reaction was inhibited by D-ribose, L-arabinose and L-xylose, whereas D-fructose did not show any inhibition. Buffalo milk ∞ -lactalbumin enhanced the synthesis of lactose but inhibited the synthesis of N-acetyllactosamine. Cations like Ca²⁺, Mg²⁺, Cu²⁺, Ba²⁺ and Co²⁺ could not replace Mn²⁺ in the N-acetyllactosamine synthetase reaction. Except Co²⁺, these cations had no effect on this reaction. Co²⁺ was found to be a competitive inhibitor of Mn²⁺. The observed inhibition of the reaction by-EDTA also confirmed the absolute requirement of Mn²⁺ for the reaction. Lactose synthetase reaction had an optimum pH of 8.5, whereas N-acetyllactosamine synthetase reaction was maximal at pH 8.0.     

Evolution of Pinnipedia lactation strategies: a potential role for α-lactalbumin?

Despite the considerable variation in milk composition found among mammals, a constituent common across all groups is lactose, the main sugar and osmole in most eutherians milk. Exceptions to this are the families Otariidae (fur seals and sea lions) and Odobenidae (walruses), where lactose has not been detected. We investigated the molecular basis for this by cloning alpha-lactalbumin, the modifier protein of the lactose synthase complex. A mutation was observed which, in addition to preventing lactose production, may enable otariids to maintain lactation despite the extremely long inter-suckling intervals during the mother's time at sea foraging (more than 23 days in some species).     

Alpha-lactalbumin as a lysosomal enzyme-releasing factor

In the early stage of mammary gland involution, biochemically detectable lysosomal damage occurs. The mechanism(s) underlying this damage is not well understood. We found that alpha-lactalbumin from mouse milk induced the release of enzymes from the lysosomes of mouse mammary epithelial cells in vitro, and this induction also occurred with bovine alpha-lactalbumin. This enzyme release was accelerated by the addition of whey proteins with a molecular weight of 50 000 to 60 000. We also found that the lysosomal membrane of mammary epithelial cells had a strong affinity for alpha-lactalbumin.     

Changes in alpha-lactalbumin, total lactose, UDP-galactose hydrolase and other factors in tammar wallaby (Macropus eugenii) milk during lactation

alpha-Lactalbumin was isolated from milk of M. eugenii and its concentration in milk samples taken at various times during lactation (0-40 weeks post partum) was determined by single radial immunodiffusion using rabbit antiserum to the purified protein. The alpha-lactalbumin concentration remained almost constant throughout lactation even though the concentration of total lactose (free lactose plus lactose contained in oligosaccharides) fell to zero after 34 weeks post partum. This fall in lactose was accompanied by a rise in the free galactose and glucose concentrations and marked increases in UDP-galactose hydrolase, nucleotide pyrophosphatase, alkaline phosphatase and acid beta-galactosidase activities. It is suggested that the in vitro hydrolysis of UDP-galactose was due to nucleotide pyrophosphatase and that this enzyme may also play a role in vivo late in lactation by making UDP-galactose unavailable for the synthesis of lactose. Alternatively, lactose and lactose-containing oligosaccharides might be degraded by the acid beta-galactosidase during or after secretion.