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PubMed1993

Proliferative changes of lens epithelial cells in rat and mouse galactose cataracts--examination using whole-mount preparations.

The proliferative activity of the lens epithelium in the early stages of cataract crisis was investigated in rats and mice using the 3H-thymidine autoradiographic method with the whole-mount preparations of total lens epithelial cells. Three-week-old Sprague-Dawley (SD) rats were given a diet which included galactose in three different concentrations (15%, 25%, 50%) to produce sugar cataracts of three different degrees. Seven-week-old ICR mice were given a 50% galactose-diet. In the lenses of control rats and mice, 3H-thymidine labeled cells were observed mainly in the germinative zone at the lens equator; a few labeled cells were detected in the anterior subcapsular central zone. In the lenses of the SD rats on the 4th day of the diet, labeled cells increased remarkably in the central zone. However, labeled cells decreased as the cataract progressed. The peak in the number of labeled cells was observed on the 4th day of the diet regardless of the galactose concentrations, and was not proportional to the degree of the cataracts. In the galactose-fed ICR mice, the blood galactose level was high, but there was no increase in the number of labeled cells or the development of galactose cataract. The marked increase of labeled cells in the central zone in the SD rat lenses had probably occurred because of the accumulation of galactitol.

Animals↗
PubMed1993

Effects of a high-galactose diet on the interphotoreceptor matrix of two strains of rat.

PURPOSE: The authors previously reported that a diet containing 30% galactose retards the development of the late-onset photoreceptor dystrophy in the spontaneously hypertensive (SHR) rat. It was suggested that the dystrophy might result from faulty galactosylation of a critical glycoprotein or glycosaminoglycan (GAG) in the interphotoreceptor matrix (IPM). In the current study, this hypothesis was tested by studying IPM protein and GAG composition in SHR and normotensive Wistar-Kyoto (WKy) control strain rats fed a standard or a high-galactose (30%) diet. METHODS: The authors performed biochemical analyses of the IPM of SHR and of WKy control rats fed either the basal diet or a 30% galactose diet for 14 mo and labeled at the termination of the experiment with 3H-glucosamine and 35S-sulfate. Analyses included high-performance liquid chromatography and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing (IEF) of extracted proteoglycans with identification of the GAG by selective enzymatic degradation and immunoblotting of SDS-PAGE gels for interphotoreceptor retinol-binding protein (IRBP), with slot blots for additional quantitation. RESULTS: Although several differences were detected in GAG radiolabeling between the two strains, only one, a decreased overall synthesis of 3H-glucosamine-labeled GAG in the WKy rats, was altered by the high-galactose diet. There was no apparent difference in the protein patterns of the IPM as evaluated by two-dimensional SDS-PAGE and IEF. However, slot blots of that portion of the IPM extracted from the neural retinas showed less reactivity per microgram of protein for IRBP in the galactose-fed animals of both strains than for the rats fed a basal diet. This was statistically significant, however, only in the WKy rats. CONCLUSIONS: The composition of the IPM of these two strains of rat appears similar to the composition of the IPM of other species that have been studied. Whether the quantitative alteration in IRBP that appears to be produced by galactose feeding has functional significance, with particular relevance to retarding the development of the photoreceptor cell dystrophy of the SHR rat, is unclear at this time.

Animals↗
PubMed1997

Free lysine, glycine, alanine, glutamic acid and aspartic acid reduce the glycation of human lens proteins by galactose.

The amino acids lysine, glycine, alanine, glutamate and aspartate formed adducts with galactose at physiological pH and temperature as shown by incorporation of U[14C] galactose. The percentage of galactose reacting with lysine, glycine, alanine, glutamate and aspartate was 4.5 to 7.8, 7.9 to 10.8, 3.2 to 4.6, 2.8 to 4.8 and 3 to 5.2, respectively. Studies with lysine showed that the extent of glycation of the free amino acid increased with time. Incubation of lens homogenate with galactose, effected glycation of proteins. Addition of lysine in concentrations of 5 and 10 mM to equimolar concentrations of galactose decreased the glycation of lens proteins by 64% to 71%; glycine, alanine, glutamate and aspartate decreased glycation by 23 to 68%, 32 to 61%, 35 to 56% and 26 to 61% respectively. Under similar conditions, glycine reacts to a greater extent than lysine, alanine, glutamic and aspartic acids. However, lysine was more effective than glycine, alanine, aspartic and glutamic acids in decreasing glycation of lens proteins by galactose. The decrease of glycation with added lysine increased with time. In general increase of amino acid concentration rather than that of sugar augmented the decrease of glycation of lens proteins.

Adult↗
PubMed1978

Galactose transport in human erythrocytes. The transport mechanism is resolved into two simple asymmetric antiparallel carriers.

The kinetic properties of the mediated transport of galactose in human erythrocytes are investigated at 20 degrees C. Different methodological procedures are used to acquire a complete kinetic description of the system. Under zero-trans conditions the uptake of galactose is mediated by two distinctly different carriers (defined as alpha and beta) having significantly different Mic;aelis parameters: alpha K = 12.7 mM and beta K = 81.5 mM, but similar maximal velocities, approx. 40 nM.min-1. The zero-trans efflux procedure reveals apparently one single carrier with K = 74.4 mM and V = 241 mM.min-1. Under equilibrium-exchange conditions the galactose transport is mediated apparently by a single site with K = 146 mM and V = 521 mM.min-1. The data for the alpha-carrier are analyzed in terms of the simple carrier model as formulated by Lieb and Stein (Biochim. Biophys. Acta (1974) 373, 178). Application of several rejection criteria for the simple carrier failed to indicate lack of fitness of the alpha-carrier to a simple asymmetric carrier. From the analysis of the kinetic data it is inferred that the transport of galactose across the human erythrocyte membrane is mediated by two simple asymmetric carriers operating in antiparallel fashion. Using this model and the data of zero-trans and equilibrium-exchange, it is shown that the predicted half-saturation constants for both uptake and efflux in infinite-cis conditions fully agree with the experimentally derived values. Further analysis of the kinetic data indicate that the translocation of the unloaded alpha-carrier is the rate-limiting step in galactose uptake. Under equilibrium-exchange conditions the unloaded carrier is asymmetrically distributed across the membrane so that its concentration is 8 times higher on the inner side of the membrane. Using the value of 3.3.10(5) hexose carriers per cell, the turnover number of galactose exchange is 6.5.10(4) molecules/carriers per min.

Biological Transport, Active↗
PubMed1992

American Leishmania spp. and Trypanosoma cruzi: galactosyl alpha(1-3) galactose epitope localization by colloidal gold immunocytochemistry and lectin cytochemistry.

Patients with Chagas' disease or different clinical forms of leishmaniasis (cutaneous or visceral) have elevated galactosyl alpha (1-3)galactose antibodies. Using colloidal gold immunocytochemistry--monoclonal antibody gal-13 (specific for lipid-linked galactosyl alpha (1-3)galactose residues) and anti-nidogen antibodies and lectin cytochemistry (Bandeiraea simplicifolia IB4), both techniques specific for demonstrating galactosyl alpha (1-3)galactose residues--we have found terminal disaccharide residues on the Trypanosoma cruzi external surface of Vero cell-derived trypomastigotes but not in intact epimastigotes (although disrupted epimastigotes strongly stained), in the lips of the flagellar pocket, and on the parasitic side exactly opposite to the flagellar pocket in amastigote and promastigote forms of American Leishmania. These results resemble those obtained using anti-laminin antibodies in both trypanosomatids. In addition, results obtained with anti-nidogen antibodies seem to recognize in Trypanosoma cruzi and American Leishmania culture forms another different unknown terminal disaccharide. These results confirm the presence of terminal galactosyl alpha (1-3)galactose residues in both trypanosomatids, and that rabbit anti-laminin antibodies are indeed also recognizing galactosyl alpha (1-3)galactose residues as demonstrated for human circulating antibody. The presence of abundant galactosyl alpha (1-3)galactose residues on Trypanosomatid family members suggests a specific unknown role in parasite physiology for this terminal disaccharide.

Animals↗
PubMed1991

Galactose-containing epitopes on the surface of IgG model immune complexes are accessible for specific binding with the high molecular weight ligand, Ricinus agglutinin, in solution-light-scattering studies.

Immunoglobulin G (IgG) molecules contain covalently linked carbohydrate chains with galactose residues in their branched "antennae". The ability of galactose-containing epitopes on the surface of IgG model immune complexes (IC) to interact with a high mol. wt ligand in solution has been elucidated. Different types of IgG model IC with pre-determined molecular mass were mixed with Ricinus Agglutinin (RCI), which is known to bind specifically to galactose-containing oligosaccharides. The relative light-scattering increases (delta I) in the reaction mixture were measured as a function of time. The galactose-associated epitopes of the IgG model IC were accessible for binding with RC1. The rate of the interaction between IgG model IC and RC1 was dependent on the molecular mass of the complexes; the larger the model IC molecular mass, the faster the rate of interaction. The binding of RC1 to IgG model IC was highly specific because it was completely abolished in the presence of lactose. The galactose-containing epitopes of monomeric IgG were also able to interact with RC1 but the kinetics of the interaction was much slower. We suggest than an increase in the density of the epitopes on the surface of the model IC, by close attachment of the IgG molecules, mainly determines the ability of galactose-containing epitopes to be recognized by RC1. The data presented support the importance of IgG glycans in recognition events of IgG by biologically active molecules.

Antigen-Antibody Complex↗
PubMed1990

Expression and functional properties of genetically engineered ricin B chain lacking galactose-binding activity.

Ricin is a potent plant toxin consisting of two disulfide-bonded subunits. The A chain of ricin is an N-glycosidase which inactivates 28 S RNA and inhibits protein synthesis. The B chain is a galactose-specific lectin with two galactose-binding sites. The genes encoding preproricin and its A and B chains have been cloned and expressed. In addition, X-ray crystallographic studies have identified the galactose-contact residues in both the high- and low-affinity galactose-binding sites of the B chain. In this study, the high-affinity galactose-contact residue of the B chain was changed from Asn-255 to Ala-255 by oligonucleotide-directed mutagenesis. The resulting mutant was sequenced to confirm the presence of a single mutation and was expressed in Cos-M6 cells. Both wild-type and mutant recombinant B chain could be immunoprecipitated with a heterologous anti-B chain antibody and both could form A-B heterodimers. However, as compared to the wild-type, the mutant B chain lacked more than 99% of its lectin activity and cytotoxicity as an A-B dimer. In conclusion, altering the contact residue of the high-affinity galactose-binding site of ricin B chain from Asn-255 to Ala-255 abrogates more than 99% of its lectin activity and the cytotoxicity of the A-B heterodimer to ricin-sensitive cells.

Asialoglycoproteins↗
PubMed2006

Characterization of galactose-binding proteins in equine testis and spermatozoa.

Carbohydrate-binding proteins are thought to be involved in a myriad of sperm functions including sperm-oviductal and sperm-zona interactions. Recent studies in our laboratory have characterized galactose-binding proteins on equine spermatozoa as possible candidate molecules for sperm adhesion to oviduct epithelial cells. In the current study, equine sperm membrane proteins were subjected to galactose-affinity chromatography, and bound proteins were eluted with excess galactose in a calcium-free buffer. The eluted fraction recovered after galactose-affinity chromatography was used for generation of a polyclonal antibody which was immobilized on an affinity column to recover a purified protein from equine sperm extracts. Several protein bands of approximately 70, 25, and 20-18 kDa were detected with a major band at 25k Da on immunoblots which was subjected to N-terminal amino acid sequencing. These galactose binding proteins (GBP) were specific to sperm and testis and were absent in all the somatic tissues tested. Based upon immunocytochemistry, GBP were localized over the sperm head. In noncapacitated sperm, fluorescent labeling was observed over the rostral sperm head as well as the postacrosomal area; whereas in capacitated sperm, the labeling was localized primarily in the equatorial segment. Immunohistochemistry of equine testis demonstrated abundant staining in the adluminal region of the seminiferous tubules corresponding to round spermatids. In summary, this study demonstrates the presence of testis- and sperm-specific galactose binding proteins in the horse. The function of these proteins remains to be determined.

Animals↗
PubMed2005

The molecular relationship between deficient UDP-galactose uridyl transferase (GALT) and ceramide galactosyltransferase (CGT) enzyme function: a possible cause for poor long-term prognosis in classic galactosemia.

Classic galactosemia is an autosomal recessive disorder that is caused by activity deficiency of the UDP-galactose uridyl transferase (GALT). The clinical spectrum of classic galactosemia differs according to the type and number of mutations in the GALT gene. Short-term clinical symptoms such as jaundice, hepatomegaly, splenomegaly and E. coli sepsis are typically associated with classic galactosemia. These symptoms are often severe but quickly ameliorate with dietary restriction of galactose. However, long-term symptoms such as mental retardation and primary ovarian failure do not resolve irrespective of dietary intervention or the period of initial dietary intervention. There seem to be an association between deficient galactosylation of cerebrosides and classic galactosemia. Galactocerebrosides and glucocerebrosides are the primary products of the enzyme UDP-galactose:cerebroside galactosyl transferase (CGT). There has been an observation of deficient galactosylation coupled with over glucosylation in the brain tissue specimens sampled from deceased classic galactosemia patients. The plausible mechanism with which the association between GALT and CGT had not been explained before. Yet, UDP-galactose serves as the product of GALT as well as a substrate for CGT. In classic galactosemia, there is a consistent deficiency in cerebroside galactosylation. We postulate that the molecular link between defective GALT enzyme, which result in classic galactosemia; and the cerebroside galactosyl transferase, which is responsible for galactosylation of cerebrosides is dependent on the cellular concentrations of UDP-galactose. We further hypothesize that a threshold concentration of UDP-galactose exist below which the integrity of cerebroside galactosylation suffers.

Galactosemias↗
PubMed2001

Evidence for a non-phosphorylated route of galactose breakdown in cell-free extracts of Aspergillus niger.

Aspergillus niger could utilize D-galactose as sole source of carbon. Cell-free extracts of D-galactose-grown mycelia were able to catalyze the oxidation of D-galactose to D-galactonic acid-gamma-lactone (GalA-gamma-lact) in the presence of NAD, followed by the appearance of 2-keto-3-deoxy-D-galactonate (KDGal), pyruvate and glyceraldehyde. From 10 &mgr;moles only 6.6 &mgr;moles of GalA-gamma-lact were disappeared after 60 min of reaction indicating the presence of GalA-gamma-lactonase. Identification of GalA-gamma-lact was achieved by ascending paper chromatography. KDGal, pyruvate and glyceraldehyde were also chromatographically identified in the reaction mixture containing D-galactonate which suggests that D-galactonate is degraded into pyruvate and glyceraldehyde via the intermediate formation of KDGal. Such reactions are supposed to be catalyzed by an inducible D-galactonate dehydratase and a constitutive KDGal aldolase. The amount of KDGal, pyruvate and glyceraldehyde were found to be almost equivalent and the equilibrium of the reaction being toward the formation of KDGal. The apparent equilibrium constant (K(eq)) was calculated and found to be 0.5 x 10(-3) M. Results also proved the reversibility of the reaction catalyzed by KDGal aldolase of A. niger. In the light of the findings obtained from the degradation of D-galactose by cell-free extracts of A. niger grown on D-galactose and D-galactonate a nonphosphorolytic pathway was suggested to be operative for the degradation of D-galactose in extracts of A. niger.

Journal Article↗
PubMed2001

Acid-base catalysis by UDP-galactose 4-epimerase: correlations of kinetically measured acid dissociation constants with thermodynamic values for tyrosine 149.

The steady-state kinetic parameters for epimerization of UDP-galactose by UDP-galactose 4-epimerase from Escherichia coli (GalE), Y149F-GalE, and S124A-GalE have been measured as a function of pH. The deuterium kinetic isotope effects for epimerization of UDP-galactose-C-d(7) by these enzymes have also been measured. The results show that the activity of wild-type GalE is pH-independent in the pH range of 5.5-9.3, and there is no significant deuterium kinetic isotope effect in the reaction of UDP-galactose-C-d(7). It is concluded that the rate-limiting step for epimerization by wild-type GalE is not hydride transfer and must be either a diffusional process or a conformational change. Epimerization of UDP-galactose-C-d(7) by Y149F-GalE proceeds with a pH-dependent deuterium kinetic isotope effect on k(cat) of 2.2 +/- 0.4 at pH 6.2 and 1.1 +/- 0.5 at pH 8.3. Moreover, the plot of log k(cat)/K(m) breaks downward on the acid side with a fitted value of 7.1 for the pK(a). It is concluded that the break in the pH-rate profile arises from a change in the rate-limiting step from hydride transfer at low pH to a conformational change at high pH. Epimerization of UDP-galactose-C-d(7) by S124A-GalE proceeds with a pH-independent deuterium kinetic isotope effect on k(cat) of 2.0 +/- 0.2 between pH 6 and 9. Both plots of log k(cat) and log k(cat)/K(m) display pH dependence. The plot of log k(cat) versus pH breaks downward with a pK(a) of 6.35 +/- 0.10. The plot of log k(cat)/K(m) versus pH is bell-shaped, with fitted pK(a) values of 6.76 +/- 0.09 and 9.32 +/- 0.21. It is concluded that hydride transfer is rate-limiting, and the pK(a) of 6.7 for free S124A-GalE is assigned to Tyr 149, which displays the same value of pK(a) when measured spectrophotometrically in this variant. Acid-base catalysis by Y149F-GalE is attributed to Ser 124, which is postulated to rescue catalysis of proton transfer in the absence of Tyr 149. The kinetic pK(a) of 7.1 for free Y149F-GalE is lower than that expected for Ser 124, as proven by the pH-dependent kinetic isotope effect. Epimerization by the doubly mutated Y149F/S124A-GalE proceeds at a k(cat) that is lower by a factor of 10(7) than that of wild-type GalE. This low rate is attributed to the synergistic actions of Tyr 149 and Ser 124 in wild-type GalE and to the absence of any internal catalysis of hydride transfer in the doubly mutated enzyme.

Alanine↗
PubMed1997

Expression of alpha-1,3-galactose and other type 2 oligosaccharide structures in a porcine endothelial cell line transfected with human alpha-1,2-fucosyltransferase cDNA.

The binding of xenoreactive natural antibodies to the Galalpha1-3Galbeta1-4GlcNAc (alpha-galactose) oligosaccharide epitope on pig cells activates the recipient's complement system in pig to primate xenotransplantation. Expression of human alpha-1, 2-fucosyltransferase in pigs has been proposed as a strategy for reducing the expression level of the alpha-galactose epitope, thereby rendering the pig organs more suitable for transplantation into humans. The aim of this study was to examine how the cell surface expression of alpha-galactose, H, and related fucosylated and sialylated structures on a pig liver endothelial cell line is affected by transfection of human alpha-1,2-fucosyltransferase cDNA. Nontransfected and mock-transfected cells expressed alpha-galactose, alpha-2,3-sialylated, and alpha-2,6-sialylated epitopes strongly, with low level expression of type 2 H and LewisX. By contrast, expression of the H epitope was increased 5-8-fold in transfected cells with a 40% reduction in the expression of alpha-galactose epitope and a 50% decrease in sialylation, as measured by binding of Maackia amurensis and Sambuccus nigra agglutinins. LewisX expression was reduced to background levels, while the LewisY neoepitope was induced in human alpha-1,2-fucosyltransferase-expressing pig cells. The activities of endogenous alpha-1,3-galactosyltransferase, alpha-1,3-fucosyltransferases, and alpha-2,3- and alpha-2, 6-sialyltransferases acting on lactosamine were unaffected. Our results show that a reduction in alpha-galactose epitope expression in porcine endothelial cells transfected with human alpha-1, 2-fucosyltransferase cDNA may be achieved but at the expense of considerable distortion of the overall cell surface glycosylation profile, including the appearance of carbohydrate epitopes that are absent from the parent cells.

ABO Blood-Group System↗
PubMed2000

Specificity and mechanism of metal ion activation in UDP-galactose:beta -galactoside-alpha -1,3-galactosyltransferase.

UDP-galactose:beta-galactosyl-alpha1,3-galactosyltransferase (alpha3GT) catalyzes the synthesis of galactosyl-alpha-1,3-beta-galactosyl structures in mammalian glycoconjugates. In humans the gene for alpha3GT is inactivated, and its product, the alpha-Gal epitope, is the target of a large fraction of natural antibodies. alpha3GT is a member of a family of metal-dependent-retaining glycosyltransferases that includes the histo blood group A and B enzymes. Mn(2+) activates the catalytic domain of alpha3GT (alpha3GTcd), but the affinity reported for this ion is very low relative to physiological levels. Enzyme activity over a wide range of metal ion concentrations indicates a dependence on Mn(2+) binding to two sites. At physiological metal ion concentrations, Zn(2+) gives higher levels of activity and may be the natural cofactor. To determine the role of the cation, metal activation was perturbed by substituting Co(2+) and Zn(2+) for Mn(2+) and by mutagenesis of a conserved D(149)VD(151) sequence motif that is considered to act in cation binding in many glycosyltransferases. The aspartates of this motif were found to be essential for activity, and the kinetic properties of a Val(150) to Ala mutant with reduced activity were determined. The results indicate that the cofactor is involved in binding UDP-galactose and has a crucial influence on catalytic efficiency for galactose transfer and for the low endogenous UDP-galactose hydrolase activity. It may therefore interact with one or more phosphates of UDP-galactose in the Michaelis complex and in the transition state for cleavage of the UDP to galactose bond. The DXD motif conserved in many glycosyltransferases appears to have a key role in metal-mediated donor substrate binding and phosphate-sugar bond cleavage.

Animals↗
PubMed2005

The molecular architecture of galactose mutarotase/UDP-galactose 4-epimerase from Saccharomyces cerevisiae.

The metabolic pathway by which beta-D-galactose is converted to glucose 1-phosphate is known as the Leloir pathway and consists of four enzymes. In most organisms, these enzymes appear to exist as soluble entities in the cytoplasm. In yeast such as Saccharomyces cerevisiae, however, the first and last enzymes of the pathway, galactose mutarotase and UDP-galactose 4-epimerase, are contained within a single polypeptide chain referred to as Gal10p. Here we report the three-dimensional structure of Gal10p in complex with NAD(+), UDP-glucose, and beta-D-galactose determined to 1.85-A resolution. The enzyme is dimeric with dimensions of approximately 91 A x 135 A x 108 A and assumes an almost V-shaped appearance. The overall architecture of the individual subunits can be described in terms of two separate N- and C-terminal domains connected by a Type II turn formed by Leu-357 to Val-360. The first 356 residues of Gal10p fold into the classical bilobal topology observed for all other UDP-galactose 4-epimerases studied thus far. This N-terminal domain contains the binding sites for NAD(+) and UDP-glucose. The polypeptide chain extending from Glu-361 to Ser-699 adopts a beta-sandwich motif and harbors the binding site for beta-D-galactose. The two active sites of Gal10p are separated by over 50 A. This investigation represents the first structural analysis of a dual function enzyme in the Leloir pathway.

Binding Sites↗
PubMed1971

Growth Inhibition and Metabolite Pool Levels in Plant Tissues Fed d-Glucosamine and d-Galactose.

The growth of corn (Zea mays) roots and barley (Hordeum vulgare) coleoptiles is sensitive to the presence of external d-glucosamine and d-galactose. In order to investigate this effect, tissues were fed the radioactive monosaccharides at concentrations that ranged from those that were strongly inhibitory to those that had little influence on growth. At low concentrations, d-glucosamine is converted to uridine diphosphate-N-acetyl-d-glucosamine, phosphate esters of N-acetylglucosamine, and free N-acetylglucosamine. As the external concentrations were increased, the pool levels of each of these metabolites rose several fold; and, in corn roots, two unidentified compounds, which had not been detected previously, began to accumulate in the tissues. The major products of d-galactose metabolism were uridine diphosphate-d-galactose and d-galactose 1-phosphate at all the concentrations tested. Both these compounds showed a marked increase as the external galactose concentrations were raised to inhibitory levels. The experiments indicate that efficient pathways exist in plants for the metabolism of d-glucosamine and d-galactose. These pathways, however, do not appear to be under strict control, so that metabolites accumulate in unusually high amounts and presumably interfere competitively with normal carbohydrate metabolism.

Journal Article↗
PubMed1987

Characterization of the Stimulation of Ethylene Production by Galactose in Tomato (Lycopersicon esculentum Mill.) Fruit.

We have characterized the stimulation of ethylene production by galactose in tomatoes (Lycopersicon esculentum Mill.). The effect of concentration was studied by infiltrating 0, 4, 40, 100, 200, 400, or 800 micrograms galactose for each gram of fresh fruit weight into mature green ;Rutgers' fruit. Both 400 and 800 micrograms per gram fresh weight consistently stimulated a transient increase in ethylene approximately 25 hours after infiltration; the lower concentrations did not. Carbon dioxide evolution of fruit infiltrated with 400 to 800 micrograms per gram fresh weight was greater than that of lower concentrations. The ripening mutants, rin and nor, also showed the transient increase in ethylene and elevated CO(2) evolution by 400 micrograms per gram fresh weight galactose. 1-Aminocyclopropane-1-carboxylic acid (ACC) content and ACC-synthase activity increased concurrently with ethylene production. However, galactose did not stimulate ACC-synthase activity in vitro. The infiltrated galactose in pericarp tissue was rapidly metabolized, decreasing to endogenous levels within 50 hours. Infiltrated galacturonic acid, dulcitol, and mannose stimulated transient increases in ethylene production similar to that of galactose. The following sugars produced no response: sucrose, fructose, glucose, rhamnose, arabinose, xylose, raffinose, lactose, and sorbitol.

Journal Article↗
PubMed1989

Inhibition of Acid-Enhanced Elongation of Zea mays Root Segments by Galactose.

The effect of sugars and metabolic inhibitors on the elongation of Zea mays root segments was analyzed by a rhizometer which records the elongation of each of 32 root segments at the same time. Galactose suppressed the acid-enhanced rapid elongation after a lag period of 1.5 hours, but it did not inhibit the slow elongation at pH 7. Mannose was less inhibitory than galactose. Arabinose, xylose, glucose, sucrose, mannitol, and sorbitol caused no inhibition. When galactose was removed after a 1-hour treatment, the elongation was partially recovered. Cycloheximide and 2-deoxyglucose suppressed acid-enhanced elongation when these were applied at the same time as acid treatments, whereas cordycepin (3'-deoxyadenosine) inhibited elongation only if it was applied prior to acid treatment. Over the 9-hour period of elongation studied, the inhibition by galactose was comparable to that of cycloheximide. Since galactose has been reported to suppress the sugar metabolism necessary for the cell wall synthesis, the later phase of acid-enhanced elongation of root segments may at least partially depend on the synthesis or metabolism of cell wall components. The inhibition of root growth by galactose may be partially ascribed to a direct effect on the elongation process in roots, an effect that is enhanced by the acidification of the cell walls.

Journal Article↗
PubMed1990

d-Galactose Uptake by Fenugreek Cotyledons : Effect of Water Stress.

The uptake of d-galactose was studied in detached fenugreek (Trigonella foenum-graecum L.) cotyledons. Uptake kinetics and treatment with p-chloromercury-benzenesulfonic acid indicated that at low concentrations d-galactose was taken up by a carrier. At higher concentrations a diffusion-like component existed. Proton flux and pH studies, treatment with alpha-naphthaleneacetic acid, and uptake experiments under water stress conditions suggested that d-galactose was not taken up via H(+) contransport. However, d-galactose uptake was under metabolic control. Uptake kinetics under water stress conditions suggested that moderate water stress either increased the K(m) of the carrier or decreased the V(max). However, prolonged stress transformed the carrier-mediated uptake into a diffusion uptake transport. The uptake of d-galactose by fenugreek cotyledons was very low before and just after germination, was maximum after 35 hours imbibition, and started decreasing thereafter. The different uptake rates of d-galactose with imbibition times were attributed to the operation of the carrier. At low uptake rates the carrier did not operate. Treatment with cycloheximide suggested that the carrier was synthesized de novo just after germination and stopped operating when all galactomannan hydrolysis was over. Results were discussed in the context of control of endosperm galactomannan hydrolysis by the cotyledons of fenugreek embryo.

Journal Article↗