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PubMed1975

The "hidden ligand" of the galactose-binding protein.

Following tryptophan fluorescence of the galactose-binding during dissociation of the ligand it has been found that glucose dissociates with a half life of less than 5 s. Similarly, fast dissociation was also observed by following release of radioactively labelled glucose from Sepharose-coupled galactose-binding protein upon dilution. Accordingly, a previous claim that the galactose-binding protein contains glucose as a non-dissociable "hidden ligand" [G. Richarme and A. Kepes (1974) Eur. J. Biochem. 45, 127-133] has to be reinterpreted

Carrier Proteins↗
PubMed1985

Binding of [1-13C]galactose-labeled N-acetyllactosamine to Erythrina cristagalli agglutinin as studied by 13C-NMR.

The equilibrium binding kinetics of enzymatically prepared N-acetyllactosamine to the lectin from Erythrina cristagalli have been investigated by 13C-NMR spectroscopy. Under the experimental conditions used, NMR signals in the spectrum, corresponding to both the free and bound disaccharide species, were observed for the first time. This has permitted the simultaneous determinations of the equilibrium binding constant and the number of binding sites per lectin molecule. At the relatively high lectin concentrations used (0.3-0.87 mM), the association constants determined at 31 degrees C (approximately 6 X 10(3) M-1) are typically lower then those obtained by other methods employing much lower lectin concentrations. Extrapolation of the experimentally observed values to infinite dilution gave a better fit of the data (Ka approximately 1.4 X 10(4) M-1) with the binding constant determined by other methods (K approximately 1.1 X 10(4) M-1). The sugar residence time on the lectin (approximately 0.2 s) was determined directly from the signal's line-width using total line-shape analysis. Similar NMR experiments may permit an analysis of the interaction of the lectin with glycoproteins and cells labelled with 13C-enriched galactose residues. Moreover, information on lectin-galactose interactions at the binding site may be obtained by using galactose labeled at various carbons.

Amino Sugars↗
PubMed1988

Oxidation of glycolipids in liposomes by galactose oxidase.

Small unilamellar phosphatidylcholine vesicles containing globo-series glycolipids were labeled by the galactose oxidase/NaB[3H]4 procedure. The major glycolipid of human red cells, globoside, was the best substrate for galactose oxidase both in vesicles and in tetrahydrofuran-containing buffer. The oxidation rates of membrane-bound ceramide trihexoside and Forssman glycolipid were one-fourth and one-tenth, respectively, of the oxidation rate of globoside. Membrane-bound ceramide dihexoside was not a substrate for galactose oxidase, although it was readily oxidized in tetrahydrofuran-containing buffer. Soluble sialoglycoproteins and membrane-incorporated glycophorin A stimulated the oxidation of globoside-containing vesicles, whereas membrane-bound GD1a ganglioside had no effect on globoside oxidation.

Animals↗
PubMed1992

Characterization of a recombinant bifunctional enzyme, galactose dehydrogenase/bacterial luciferase, displaying an improved bioluminescence in a three-enzyme system.

The two structural genes encoding galactose dehydrogenase (Pseudomonas fluorescens) and the beta subunit of luciferase (Vibrio harveyi) were fused in-frame in order to prepare and subsequently characterize an artificial bifunctional enzyme complex. This hybrid enzyme exhibited both galactose dehydrogenase activity and bioluminescence when expressed in Escherichia coli together with the alpha subunit of luciferase. The purified conjugate was used to study possible proximity effects in a sequential three-enzyme reaction with the bifunctional enzyme catalyzing the first and the last reaction. The intermediate enzyme, diaphorase, was added separately. The engineered enzyme system, comprising the galactose dehydrogenase/luciferase conjugate, could display a twofold higher bioluminescence in the overall enzyme reaction compared to a corresponding reference system with separate native enzymes. The increased bioluminescence obtained for the engineered enzyme system is proposed to be due to an improved organization of the enzyme in solution.

Dihydrolipoamide Dehydrogenase↗
PubMed1986

Incorporation of tritiated galactose into galactocerebroside by cultured rat oligodendrocytes: effects of cyclic adenosine 3',5'-monophosphate analogues.

Cells dissociated from the forebrains of 21-day-old rats were enriched in oligodendroglia by Percoll gradient centrifugation, seeded on polylysine-coated surfaces, and cultured in a serum-containing medium. Incorporation by the cultures of tritium from D-[3H]galactose into the galactosyl residue of galactocerebroside (galC) increased in an almost linear fashion for 48 h with 1-8 muCi of D-[3H]galactose (30 mCi/mumol) per milliliter medium. Treatment for 2 days (day 1-3 after seeding) with 10(-4) M or 10(-3) M dibutyryl cyclic adenosine 3',5'-monophosphate (db cyclic AMP) or 10(-4) M 8-bromo cyclic AMP stimulated galC radiolabelling. Incorporation of D-[3H]galactose into galC during a terminal 48-h radiolabelling period was not stimulated when the cells were continuously treated with these cyclic AMP analogues for 8 rather than 2 days.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed2002

Cloning, expression and characterization of L-arabinose isomerase from Thermotoga neapolitana: bioconversion of D-galactose to D-tagatose using the enzyme.

Gene araA encoding an L-arabinose isomerase (AraA) from the hyperthermophile, Thermotoga neapolitana 5068 was cloned, sequenced, and expressed in Escherichia coli. The gene encoded a polypeptide of 496 residues with a calculated molecular mass of 56677 Da. The deduced amino acid sequence has 94.8% identical amino acids compared with the residues in a putative L-arabinose isomerase of Thermotoga maritima. The recombinant enzyme expressed in E. coli was purified to homogeneity by heat treatment, ion exchange chromatography and gel filtration. The thermophilic enzyme had a maximum activity of L-arabinose isomerization and D-galactose isomerization at 85 degrees C, and required divalent cations such as Co(2+) and Mn(2+) for its activity and thermostability. The apparent K(m) values of the enzyme for L-arabinose and D-galactose were 116 mM (v(max), 119 micromol min(-1) mg(-1)) and 250 mM (v(max), 14.3 micromol min(-1) mg(-1)), respectively, that were determined in the presence of both 1 mM Co(2+) and 1 mM Mn(2+). A 68% conversion of D-galactose to D-tagatose was obtained using the recombinant enzyme at the isomerization temperature of 80 degrees C.

Aldose-Ketose Isomerases↗
PubMed1985

Estimation of the hepatic blood "flow" by galactose plasma clearance in patients with liver disease.

Following intravenous administration of 500 mg/kg b.wt. galactose, Galactose Elimination Capacity (GEC, mg/min/kg) was determined in 24 subjects with chronic non-cirrhotic liver disease (CLD), 33 with liver cirrhosis and 11 controls. GEC was significantly (P less than 0.01) reduced in both CLD and cirrhosis. A statistically significant difference (P less than 0.01) was present between these two groups. Following the plasma disappearance curve at concentrations below 1.25 mmol/l, at which the extraction coefficient is assumed to be equal to one, the "Efficient Hepatic Blood Flow" (EHBF, ml/min) was determined in 11 consecutive cirrhosis patients, seven patients with CLD and 11 controls. EHBF was normal or slightly reduced in CLD as compared to controls (1046 +/- 216 vs. 1471 +/- 156 ml/min, mean +/- SEM, n.s.) whereas it was markedly reduced in cirrhosis (846 +/- 96 ml/min, mean +/- SEM, p less than 0.001). Interestingly, a significant linear correlation (r = 0.757, p less than 0.001) was present between EHBF and the plasma clearance of sulfobromophthalein. No correlation was present, on the other hand, between the value of GEC and that of EHBF. These data indicate that after a single intravenous injection of galactose, the hepatic blood flow passing through the enzymatically active parts of the liver (i.e. excluding shunts) can be measured.

Adolescent↗
PubMed1987

Evaluation of an oral bile acid loading test for assessment of liver function in chronic hepatitis. A comparison with fasting serum bile acids and i.v. galactose elimination test.

In 40 patients with histologically verified chronic hepatitis, (chronic persistent hepatitis, n = 13; chronic active hepatitis without, n = 14; or with cirrhosis, n = 13), of viral and autoimmune origin, serum bile acids (SBA) were measured before and during 3 h after oral ingestion of 1 g chenodeoxycholic acid (CDA). Fasting SBA were elevated in 22 (55%) patients, whereas the CDA loading test was abnormal in 15 (38%) patients and the galactose elimination was prolonged in 16 (40%) patients. In patients with chronic active hepatitis, 20/27 had elevated SBA either in the fasting state (18/27) or after the CDA loading test (13/27). Normal SBA values were found in 9/13 (70%) patients with chronic persistent hepatitis. Thus, fasting SBA is not sensitive enough to detect mild chronic inflammatory liver disease as chronic persistent hepatitis, but seems to be as sensitive as the galactose elimination or CDA loading tests in detecting potential severe liver disease. Fasting SBA may thus be used as a complement of conventional liver tests in the follow-up of chronic hepatitis as assessment of liver function. An oral CDA loading and an i.v. galactose elimination test add no further information to that given by fasting serum bile acids.

Adult↗
PubMed1989

Cytotoxicity of galactose, tyrosine and methionine in cultured suckling rat hepatocytes: relation to liver immaturity.

Monolayers of suckling rat hepatocytes cultured for 24 hours were treated with galactose, I-tyrosine and I-methionine. The purpose was to study the reasons for the clinical improvement of patients with neonatal hepatitis after dietary restriction of these nutrients. Galactose, tyrosine, and methionine was cytotoxic on suckling rat hepatocytes, yet had no effect on adult rat hepatocytes. Furthermore, the pretreatment of suckling rat hepatocytes with dexamethasone ameliorated the cytotoxicity and induced a differentiation of the cells. These results suggested that the cytotoxicity resulted from the immaturity of suckling rat hepatocytes and therefore dietary restriction of galactose, tyrosine and methionine might be a useful treatment for patients with neonatal hepatitis.

Animals↗
PubMed1996

Interactions of cephradine and cefaclor with the intestinal absorption of D-galactose.

Oral cephalosporins are frequently prescribed beta-lactam antibiotics. Although it has been well established that cephalosporins compete with dipeptides for absorption in the intestine, using the same transport mechanism, little is known about the action of the drugs on the absorption of other nutrients. In this work the effect of cephradine and cefaclor on the absorption of D-galactose has been studied. Intestinal sugar uptake was measured in-vitro in pieces of intestine (50 mg) and brush-border membrane vesicles, and in-vivo in intestinal loops. Galactose uptake was inhibited by cephalosporins in a dose-related, time-dependent manner. In-vivo the inhibition appeared when the antibiotics were on the luminal side of the enterocyte and when they reached the gut from the basolateral side. Only the active transport of the sugar was modified; passive transfer did not change in the presence of cephalosporins. In brush-border membrane vesicles, cephradine and cefaclor did not alter sugar uptake in either sodium or potassium gradients. Both antibiotics non-competitively inhibited basolateral Na+,K(+)-ATPase activity. These findings show that cephradine and cefaclor inhibit the active-transport component of galactose absorption because they reduce the activity of the basolateral Na+,K(+)-ATPase.

Animals↗
PubMed1960

Effect of galactose-1-phosphate on glucose oxidation by normal and galactosemic leukocytes.

During incubation with galactose, galactosemic leukocytes accumulated more galactose-l-phosphate than did normal leukocytes. Concomitant determination of glucose oxidation, with C(14) glucose, revealed no inhibition of the hexosemonophosphate pathway. These results are at variance with recent studies in rat lens tissue, which suggests that intracellular galactose-1-phosphate depressed glucose-6-phosphate dehydrogenase activity and the oxidative pathway.

Animals↗
PubMed1971

Galactose toxicity in the chick: hyperosmolality.

Galactose-fed chicks have been found to develop severe hyperosmolar dehydration. Although biochemical abnormalities have been observed in the brain of the galactose-toxic chick, the observed physiologic alteration of serum osmolality could be the major factor responsible for the galactose toxicity syndrome in the chick.

Animals↗
PubMed1996

Activation of Gal4p by galactose-dependent interaction of galactokinase and Gal80p.

Yeast galactokinase (Gal1p) is an enzyme and a regulator of transcription. In addition to phosphorylating galactose, Gal1p activates Gal4p, the activator of GAL genes, but the mechanism of this regulation has been unclear. Here, biochemical and genetic evidence is presented to show that Gal1p activates Gal4p by direct interaction with the Gal4p inhibitor Gal80p. Interaction requires galactose, adenosine triphosphate, and the regulatory function of Gal1p. These data indicate that Gal1p-Gal80p complex formation results in the inactivation of Gal80p, thereby transmitting the galactose signal to Gal4p.

Amino Acid Sequence↗
PubMed1996

Substitution of L-fucose by L-galactose in cell walls of Arabidopsis mur1.

An Arabidopsis thaliana mutant (mur1) has less than 2 percent of the normal amounts of L-fucose in the primary cell walls of aerial portions of the plant. The survival of mur1 plants challenged the hypothesis that fucose is a required component of biologically active oligosaccharides derived from cell wall xyloglucan. However, the replacement of L-fucose (that is, 6-deoxy-L-galactose) by L-galactose does not detectably alter the biological activity of the oligosaccharides derived from xyloglucan. Thus, essential structural and conformational features of xyloglucan and xyloglucan-derived oligosaccharides are retained when L-galactose replaces L-fucose.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1988

Sugar and signal-transducer binding sites of the Escherichia coli galactose chemoreceptor protein.

D-galactose-binding (or chemoreceptor) protein of Escherichia coli serves as an initial component for both chemotaxis towards galactose and glucose and high-affinity active transport of the two sugars. Well-refined x-ray structures of the liganded forms of the wild-type and a mutant protein isolated from a strain defective in chemotaxis but fully competent in transport have provided a molecular view of the sugar-binding site and of a site for interacting with the Trg transmembrane signal transducer. The geometry of the sugar-binding site, located in the cleft between the two lobes of the bilobate protein, is novel in that it is designed for tight binding and sequestering of either the alpha or beta anomer of the D-stereoisomer of the 4-epimers galactose and glucose. Binding specificity and affinity are conferred primarily by polar planar side-chain residues that form intricate networks of cooperative and bidentate hydrogen bonds with the sugar substrates, and secondarily by aromatic residues that sandwich the pyranose ring. Each of the pairs of anomeric hydroxyls and epimeric hydroxyls is recognized by a distinct Asp residue. The site for interaction with the transducer is about 18 A from the sugar-binding site. Mutation of Gly74 to Asp at this site, concomitant with considerable changes in the local ordered water structures, contributes to the lack of productive interaction with the transmembrane signal transducer.

Bacterial Proteins↗
PubMed1986

Retinal dystrophy: development retarded by galactose feeding in spontaneously hypertensive rats.

Retinal photoreceptor cell dystrophies have been widely observed in humans and in animals, but pathogenetic mechanisms are known in only a few such disorders, and successful therapeutic intervention has been reported in fewer still. Spontaneously hypertensive albino rats develop a retinal photoreceptor cell dystrophy with onset late in the first year or early in the second year of life. Between 60 and 70 percent of the animals are affected. A substantial reduction in the prevalence and severity of the dystrophy occurred in such animals whose diet contained 30 percent (by weight) D-galactose. Neither an inhibitor of the enzyme aldose reductase, present in the diet, nor diabetes mellitus, induced by streptozotocin, had any statistically significant influence on the dystrophy. Ambient light and systolic blood pressure levels also did not seem to influence the course of the disorder. The mechanism by which galactose exerts its effect is unknown, but a mutant enzyme with an elevated Michaelis constant (Km) for galactose is plausible.

Age Factors↗
PubMed1981

Reduction in oocyte number following prenatal exposure to a diet high in galactose.

When pregnant rats were fed a 50 percent galactose diet there was a striking reduction in oocyte number in the offspring. The most prominent effects were noted after exposure to galactose during the premeiotic stages of oogenesis. Prenatal exposure to galactose or its metabolites may contribute to the premature ovarian failure characteristic of human galactosemia.

Animals↗