The fundamental importance of human galactose metabolism: lessons from genetics and biochemistry

Dietary Galactose Increases the Expression of Mitochondrial OXPHOS Genes and Modulates the Carbohydrate Oxidation Pathways in Mouse Intestinal Mucosa

BACKGROUND

Prolonged lactation provides substantial health benefits, possibly because of galactose as part of milk sugar lactose. Isocaloric replacement of dietary glucose [16 energy%(en%)] with galactose within a normal diet (64en% carbohydrates) during a 3-wk postweaning period provided substantial benefits on short- and long-term physiologic and metabolic parameters at the whole-body level and liver in female mice, which might be attributable to intestinal function.

OBJECTIVES

This study aimed to investigate if partial dietary replacement of glucose with galactose alters intestinal metabolism underlying hepatic health effects.

METHODS

Proximal intestinal mucosa gene profiles in female mice were analyzed using RNAseq technology, validated, and correlated with hepatic health parameters.

RESULTS

Transcriptome analysis revealed that the presence of galactose primarily affected the pathways involved in energy metabolism. A consistently higher expression was observed in the subset of mitochondrial transcripts (78 of 80, all P.adj < 0.1). Oxidative phosphorylation (OXPHOS) represented the most upregulated process (all top 10 pathways) independent of the total mitochondrial mass (P = 0.75). Moreover, galactose consistently upregulated carbohydrate metabolism pathways, specifically glycolysis till acetyl-CoA production and fructose metabolism. Also, the expression of transcripts involved in these pathways was negatively correlated with circulating serum amyloid A3 protein, a marker of hepatic inflammation [R (-0.61, -0.5), P (0.002, 0.01)]. Accordingly, CD163+ cells were decreased in the liver. Additionally, the expression of key fructolytic enzymes in the small intestinal mucosa was negatively correlated with triglyceride accumulation in the liver [R (-0.45, -0.4), P (0.03, 0.05)].

CONCLUSIONS

To our knowledge, our results show for the first time the role of galactose as an OXPHOS activator in vivo. Moreover, the concept of intestinal cells acting as the body's metabolic gatekeeper is strongly supported, as they alter substrate availability and thereby contribute to the maintenance of metabolic homeostasis, protecting other organs, as evidenced by their potential ability to shield the liver from the potentially detrimental effects of fructose.

Integrating strategies of metabolomics, network pharmacology, and experiment validation to investigate the processing mechanism of Epimedium fried with suet oil to warm kidney and enhance yang

Introduction: Epimedium, a traditional Chinese medicine (TCM) commonly used in ancient and modern China, is one of the traditional Chinese medicines clinically used to treat kidney yang deficiency syndrome (KYDS). There are differences in the efficacy of Epimedium before and after processing, and the effect of warming the kidney and enhancing yang is significantly enhanced after heating with suet oil. However, the active compounds, corresponding targets, metabolic pathways, and synergistic mechanism of frying Epimedium in suet oil to promote yang, remain unclear. Methods: Herein, a strategy based on comprehensive GC-TOF/MS metabolomics and network pharmacology analysis was used to construct an "active compounds-targets-metabolic pathways" network to identify the active compounds, targets and metabolic pathways involved. Subsequently, the targets in kidney tissue were further validated by real-time quantitative polymerase chain reaction (RT-qPCR). Histopathological analysis with physical and biochemical parameters were performed. Results: Fifteen biomarkers from urine and plasma, involving five known metabolic pathways related to kidney yang deficiency were screened. The network pharmacology results showed 37 active compounds (13 from Epimedium and 24 from suet oil), 159 targets, and 267 pathways with significant correlation. Importantly, integrated metabolomics and network pharmacologic analysis revealed 13 active compounds (nine from Epimedium and four from suet oil), 7 corresponding targets (ALDH2, ARG2, GSTA3, GSTM1, GSTM2, HPGDS, and NOS2), two metabolic pathways (glutathione metabolism, arginine and proline metabolism), and two biomarkers (Ornithine and 5-Oxoproline) associated with improved kidney yang deficiency by Epimedium fried with suet oil. Discussion: These finds may elucidate the underlying mechanism of yang enhancement via kidney warming effects. Our study indicated that the mechanism of action mainly involved oxidative stress and amino acid metabolism. Here, we demonstrated the novel strategies of integrating metabolomics and network pharmacology in exploring of the mechanisms of traditional Chinese medicines.

Galactose Enhances Chondrogenic Differentiation of ATDC5 and Cartilage Matrix Formation by Chondrocytes

Galactose, an important carbohydrate nutrient, is involved in several types of cellular metabolism, participating in physiological activities such as glycosaminoglycan (GAG) synthesis, glycosylation, and intercellular recognition. The regulatory effects of galactose on osteoarthritis have attracted increased attention. In this study, in vitro cell models of ATDC5 and chondrocytes were prepared and cultured with different concentrations of galactose to evaluate its capacity on chondrogenesis and cartilage matrix formation. The cell proliferation assay demonstrated that galactose was nontoxic to both ATDC5 cells and chondrocytes. RT-PCR and immunofluorescence staining indicated that the gene expressions of cartilage matrix type II collagen and aggrecan were significantly upregulated with increasing galactose concentration and the expression and accumulation of the extracellular matrix (ECM) protein. Overall, these results indicated that a galactose concentration below 8 mM exhibited the best effect on promoting chondrogenesis, which entitles galactose as having considerable potential for cartilage repair and regeneration.

Association between host genetics of sheep and the rumen microbial composition

A synergy between the rumen microbiota and the host genetics has created a symbiotic relationship, beneficial to the host's health. In this study, the association between the host genetics and rumen microbiome of Damara and Meatmaster sheep was investigated. The composition of rumen microbiota was estimated through the analysis of the V3-V4 region of the 16S rRNA gene, while the sheep blood DNA was genotyped with Illumina OvineSNP50 BeadChip and the genome-wide association (GWA) was analyzed. Sixty significant SNPs dispersed in 21 regions across the Ovis aries genome were found to be associated with the relative abundance of seven genera: Acinetobacter, Bacillus, Clostridium, Flavobacterium, Prevotella, Pseudomonas, and Streptobacillus. A total of eighty-four candidate genes were identified, and their functional annotations were mainly associated with immunity responses and function, metabolism, and signal transduction. Our results propose that those candidate genes identified in the study may be modulating the composition of rumen microbiota and further indicating the significance of comprehending the interactions between the host and rumen microbiota to gain better insight into the health of sheep.

Route of exposure influences pesticide body burden and the hepatic metabolome in post-metamorphic leopard frogs

Pesticides are being applied at a greater extent than in the past. Once pesticides enter the ecosystem, many environmental factors can influence their residence time. These interactions can result in processes such as translocation, environmental degradation, and metabolic activation facilitating exposure to target and non-target species. Most anurans start off their life cycle in aquatic environments and then transition into terrestrial habitats. Their time in the aquatic environment is generally short; however, many important developmental stages occur during this tenure. Post-metamorphosis, most species spend many years on land but migrate back to the aquatic environment for breeding. Due to the importance of both the aquatic and terrestrial environments to the life stages of amphibians, we investigated how the route of exposure (i.e., uptake from contaminated soils vs. uptake from contaminated surface water) influences pesticide bioavailability and body burden for four pesticides (bifenthrin (BIF), chlorpyrifos (CPF), glyphosate (GLY), and trifloxystrobin (TFS)) as well as the impact on the hepatic metabolome of adult leopard frogs (Gosner stage 46 with 60-90 days post-metamorphosis). Body burden concentrations for amphibians exposed in water were significantly higher (ANOVA p < 0.0001) compared to amphibians exposed to contaminated soil across all pesticides studied. Out of 80 metabolites that were putatively identified, the majority expressed a higher abundance in amphibians that were exposed in pesticide contaminated water compared to soil. Ultimately, this research will help fill regulatory data gaps, aid in the creation of more accurate amphibian dermal uptake models and inform continued ecological risk assessment efforts.

Tryptophan-galactosylamine conjugates inhibit and disaggregate amyloid fibrils of Aβ42 and hIAPP peptides while reducing their toxicity

Self-assembly of proteins into amyloid fibrils is a hallmark of various diseases, including Alzheimer's disease (AD) and Type-2 diabetes Mellitus (T2DM). Aggregation of specific peptides, like Aβ42 in AD and hIAPP in T2DM, causes cellular dysfunction resulting in the respective pathology. While these amyloidogenic proteins lack sequence homology, they all contain aromatic amino acids in their hydrophobic core that play a major role in their self-assembly. Targeting these aromatic residues by small molecules may be an attractive approach for inhibiting amyloid aggregation. Here, various biochemical and biophysical techniques revealed that a panel of tryptophan-galactosylamine conjugates significantly inhibit fibril formation of Aβ42 and hIAPP, and disassemble their pre-formed fibrils in a dose-dependent manner. They are also not toxic to mammalian cells and can reduce the cytotoxicity induced by Aβ42 and hIAPP aggregates. These tryptophan-galactosylamine conjugates can therefore serve as a scaffold for the development of therapeutics towards AD and T2DM.

Glycopolymer Brushes by Reversible Deactivation Radical Polymerization: Preparation, Applications, and Future Challenges

The cellular surface contains specific proteins, also known as lectins, that are carbohydrates receptors involved in different biological events, such as cell-cell adhesion, cell recognition and cell differentiation. The synthesis of well-defined polymers containing carbohydrate units, known as glycopolymers, by reversible deactivation radical polymerization (RDRP) methods allows the development of tailor-made materials with high affinity for lectins because of their multivalent interaction. These polymers are promising candidates for the biomedical field, namely as novel diagnostic disease markers, biosensors, or carriers for tumor-targeted therapy. Although linear glycopolymers are extensively studied for lectin recognition, branched glycopolymeric structures, such as polymer brushes can establish stronger interactions with lectins. This specific glycopolymer topology can be synthesized in a bottlebrush form or grafted to/from surfaces by using RDRP methods, allowing a precise control over molecular weight, grafting density, and brush thickness. Here, the preparation and application of glycopolymer brushes is critically discussed and future research directions on this topic are suggested.

Overexpression of UDP-Glucose 4-Epimerase Is Associated with Differentiation Grade of Gastric Cancer

The UDP-glucose 4-epimerase (GALE) is a glycosyltransferase, which acts on protein and lipid glycosylation in normal and neoplastic cells. This study is aimed at investigating the differential tissue expression of GALE and its possible association with clinical-pathological parameters and the outcome of gastric adenocarcinoma patients. Seventy-one patients were evaluated in relation to GALE expression by immunohistochemistry. Our results showed that 48 (67.6%) patients were GALE positive and 23 (32.4%) negative. Positive staining was present on well-differentiated and moderate-differentiated histological grade of gastric adenocarcinomas (p < 0.0001). There was no significant association with outcome parameters (p > 0.05). Besides that, our results corroborated with the validation cohort analysis, where the expression of GALE mRNA was also associated with the histological grade (p < 0.001). These results suggest a possible use of this enzyme as a biomarker for well and moderately differentiated tumors.

Association network analysis identifies enzymatic components of gut microbiota that significantly differ between colorectal cancer patients and healthy controls

The human gut microbiota plays a major role in maintaining human health and was recently recognized as a promising target for disease prevention and treatment. Many diseases are traceable to microbiota dysbiosis, implicating altered gut microbial ecosystems, or, in many cases, disrupted microbial enzymes carrying out essential physio-biochemical reactions. Thus, the changes of essential microbial enzyme levels may predict human disorders. With the rapid development of high-throughput sequencing technologies, metagenomics analysis has emerged as an important method to explore the microbial communities in the human body, as well as their functionalities. In this study, we analyzed 156 gut metagenomics samples from patients with colorectal cancer (CRC) and adenoma, as well as that from healthy controls. We estimated the abundance of microbial enzymes using the HMP Unified Metabolic Analysis Network method and identified the differentially abundant enzymes between CRCs and controls. We constructed enzymatic association networks using the extended local similarity analysis algorithm. We identified CRC-associated enzymic changes by analyzing the topological features of the enzymatic association networks, including the clustering coefficient, the betweenness centrality, and the closeness centrality of network nodes. The network topology of enzymatic association network exhibited a difference between the healthy and the CRC environments. The ABC (ATP binding cassette) transporter and small subunit ribosomal protein S19 enzymes, had the highest clustering coefficient in the healthy enzymatic networks. In contrast, the Adenosylhomocysteinase enzyme had the highest clustering coefficient in the CRC enzymatic networks. These enzymic and metabolic differences may serve as risk predictors for CRCs and are worthy of further research.

Cognitive, behavioral and metabolic effects of oral galactose treatment in the transgenic Tg2576 mice

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated with insulin resistance and glucose hypometabolism in the brain. Oral administration of galactose, a nutrient that provides an alternative source of energy, prevents and ameliorates early cognitive impairment in a streptozotocin-induced model (STZ-icv) of the sporadic AD (sAD). Here we explored the influence of 2-month oral galactose treatment (200 mg/kg/day) in the familial AD (fAD) by using 5- (5M) and 10- (10M) month-old transgenic Tg2576 mice mimicking the presymptomatic and the mild stage of fAD, and compared it to that observed in 7-month old STZ-icv rats mimicking mild-to-moderate sAD. Cognitive and behavioral performance was tested by Morris Water Maze, Open Field and Elevated Plus Maze tests, and metabolic status by intraperitoneal glucose tolerance test and fluorodeoxyglucose Positron-Emission Tomography scan. The level of insulin, glucagon-like peptide-1 (GLP-1) and soluble amyloid β1-42 (sAβ1-42) was measured by ELISA and the protein expression of insulin receptor (IR), glycogen synthase kinase-3β (GSK-3β), and pre-/post-synaptic markers by Western blot analysis. Although galactose normalized alterations in cerebral glucose metabolism in all Tg2576 mice (5M+2M; 10M+2M) and STZ-icv rats, it did not improve cognitive impairment in either model. Improvement of reduced grooming behavior and normalization in reduced plasma insulin levels were seen only in 5M+2M Tg2576 mice while in 10M+2M Tg2576 mice oral galactose induced metabolic exacerbation at the level of plasma insulin, GLP-1 homeostasis and glucose intolerance, and additionally increased hippocampal sAβ1-42 level, decreased IR expression and increased GSK-3β activity. The results indicate that therapeutic potential of oral galactose seems to depend on the stage and the type/model of AD and to differ in the absence and the presence of AD-like pathology.

Metabolomic analysis of short-term sulfamethazine exposure on marine medaka (Oryzias melastigma) by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

Toxicological effects of sulfamethazine (SM₂) have garnered increasing concern due to its wide applications in aquaculture and persistence in the aquatic environment. Most studies have main focused on freshwater fish (i.e. zebrafish), while information regarding effects of SM₂ on marine species is still scarce. Here, the hepatotoxicities in marine medaka (Oryzias melastigma) with an increasing SM2 concentration exposures (0.01 mg/L, 0.1 mg/L and 1 mg/L) were assessed by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOF/MS) based metabolomics. Significant metabolites belonging to different metabolites classes were identified by multivariate statistical analysis. The increases levels of amino acids including alanine, asparagine, ornithine, proline, threonine, glutamic acid, lysine, tyrosine and phenylalanine were found in at least two exposure levels. Pathway analysis revealed that amino acids played important biological roles during SM₂ exposure: up-regulation of high energy-related amino acids for energy alteration; immune function disorder, oxidative stress and corresponding toxicities defenses. The down regulations of sugar and fatty acid metabolism were observed with an increasing level of SM2 exposure, suggesting that extra energy for cellular defense and detoxification was demanded in terms of different stress request. This study provided an innovative perspective to explore possible SM2 induced hepatic damages at three exposure levels on a nontarget aquatic specie.

Common and divergent features of galactose-1-phosphate and fructose-1-phosphate toxicity in yeast

Metabolic dysregulation leading to sugar-phosphate accumulation is toxic in organisms ranging from bacteria to humans. By comparing two models of sugar-phosphate toxicity in Saccharomyces cerevisiae, we demonstrate that toxicity occurs, at least in part, through multiple, isomer-specific mechanisms, rather than a single general mechanism.

Effects of Qijian mixture on type 2 diabetes assessed by metabonomics, gut microbiota and network pharmacology

Qijian mixture, a new traditional Chinese medicine (TCM) formula comprising of Astragalus membranaceus, Ramulus euonymi, Coptis chinensis and Pueraria lobata, was designed to ameliorate the type 2 diabetes (T2D), and its safety and efficacy were evaluated in the research by metabonomics, gut microbiota and system pharmacology. To study the hypoglycemic effect of Qijian mixture, male KKay mice (28-30 g, 8-9 week) and C57/BL6 mice (18-19 g, 8-9 week) were used. Thirty KKay diabetic mice were randomly distributed into 5 groups, abbreviated as Model group (Model), Low Qijian Mixture group (QJM(L)), High Qijian Mixture group (QJM(H)), Chinese Medicine (Gegen Qinlian Decoction) Positive group (GGQL), and Western Medicine (Metformin hydrochloride) Positive group (Metformin). C57/BL6 was considered as the healthy control group (Control). Moreover, a system pharmacology approach was utilized to assess the physiological targets involved in the action of Qijian mixture. There was no adverse drug reaction of Qijian mixture in the acute toxicity study and HE result, and, compared with Model group, Qijian mixture could modulate blood glycemic level safely and effectively. Qijian Mixture was lesser effective than metformin hydrochloride; however, both showed similar hypoglycemic trend. Based on ¹H NMR based metabonomics study, the profoundly altered metabolites in Qijian mixture treatment group were identified. Qijian mixture-related 55 proteins and 4 signaling pathways, including galactose metabolism, valine, leucine and isoleucine degradation metabolism, aminoacyl-tRNA biosynthesis metabolism and alanine, aspartate and glutamate metabolism pathways, were explored. The PCoA analysis of gut microbiota revealed that Qijian mixture treatment profoundly enriched bacteroidetes. In addition, the system pharmacology paradigm revealed that Qijian mixture acted through TP53, AKT1 and PPARA proteins. It was concluded that Qijian mixture effectively alleviated T2D, and this effect was linked with the altered features of the metabolite profiles and the gut microbiota.

Modulation of cellular energetics by galactose and pioglitazone

The Warburg effect is ameliorated by culturing transformed cells in the presence of galactose instead of glucose as the primary carbon source. However, metabolic consequences may occur in addition to sensitizing the cells to mitochondrial toxins. The screening of pharmaceutical agents against transformed cells while using galactose must therefore be carefully evaluated. Pioglitazone is employed in clinical applications to treat type-2 diabetes but clearly has other off-target effects. Human hepatocellular carcinoma cells (HepG2) were cultured in glucose or galactose-containing medium to investigate the role of pioglitazone on cellular bioenergetics by calorimetry and respirometry. Compared with cells cultured in 10 mM glucose, HepG2 cells cultured in the presence of 10 mM galactose showed decreased metabolic activity as measured by cellular heat flow. Interestingly, cellular heat flow increased after the addition of pioglitazone for cells cultured in glucose, but not for cells cultured in galactose. Our calorimetric data indicated that a reduction in cellular capacity for glycolysis was the mechanism responsible for the increase in sensitivity to pioglitazone, and possibly to mitochondrial toxins in general, for cells cultured in galactose. Furthermore, oxygen consumption rates were decreased after the addition of pioglitazone to cells grown in glucose but remained unchanged for cells grown in the presence of galactose. We have demonstrated that pioglitazone induces a reduction in mitochondrial activity that is partially compensated via an increase in glycolysis in the presence of glucose.

Ongoing resolution of duplicate gene functions shapes the diversification of a metabolic network

The evolutionary mechanisms leading to duplicate gene retention are well understood, but the long-term impacts of paralog differentiation on the regulation of metabolism remain underappreciated. Here we experimentally dissect the functions of two pairs of ancient paralogs of the GALactose sugar utilization network in two yeast species. We show that the Saccharomyces uvarum network is more active, even as over-induction is prevented by a second co-repressor that the model yeast Saccharomyces cerevisiae lacks. Surprisingly, removal of this repression system leads to a strong growth arrest, likely due to overly rapid galactose catabolism and metabolic overload. Alternative sugars, such as fructose, circumvent metabolic control systems and exacerbate this phenotype. We further show that S. cerevisiae experiences homologous metabolic constraints that are subtler due to how the paralogs have diversified. These results show how the functional differentiation of paralogs continues to shape regulatory network architectures and metabolic strategies long after initial preservation.

Epigenetic and SP1-mediated regulation is involved in the repression of galactokinase 1 gene in the liver of neonatal piglets born to betaine-supplemented sows

PURPOSE

In this study, we sought to investigate the effects of maternal betaine supplementation on the expression and regulation of GALK1 gene in the liver of neonatal piglets.

METHODS

Sixteen sows of two groups were fed control or betaine-supplemented diets (3 g/kg), respectively, throughout the pregnancy. Newborn piglets were individually weighed immediately after birth, and one male piglet close to mean body weight from the same litter was selected and killed before suckling. Serum samples of newborn piglets were analyzed for biochemical indexes, hormone and amino acid levels. Liver samples were analyzed for GALK1 expression by real-time PCR and western blotting, while GALK1 regulational mechanism was analyzed by methylated DNA immunoprecipitation, chromatin immunoprecipitation and microRNAs expression.

RESULTS

Betaine-exposed neonatal piglets had lower serum concentration of galactose, which was associated with significantly down-regulated hepatic GALK1 expression. The repression of GALK1 mRNA expression was associated with DNA hypermethylation and more enriched repression histone mark H3K27me3 on its promoter. Binding sites of SP1, GR and STAT3 were predicted on GALK1 promoter, and decreased SP1 protein content and lower SP1 binding to GALK1 promoter were detected in the liver of betaine-exposed piglets. Furthermore, the expression of miRNA-149 targeting GALK1 was up-regulated in the liver of betaine-exposed piglets, along with elevated miRNAs-processing enzymes Dicer and Ago2.

CONCLUSIONS

Our results suggest that maternal dietary betaine supplementation during gestation suppresses GALK1 expression in the liver of neonatal piglets, which involves complex gene regulation mechanisms including DNA methylation, histone modification, miRNAs expression and SP1-mediated transcriptional modulation.

The structural basis of substrate promiscuity in UDP-hexose 4-epimerase from the hyperthermophilic Eubacterium Thermotoga maritima

UDP-galactose 4-epimerase (GalE) catalyzes the interconversion of UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal), which is a pivotal step in the Leloir pathway for d-galactose metabolism. Although GalE is widely distributed in prokaryotes and eukaryotes, little information is available regarding hyperthermophilic GalE. We overexpressed the TM0509 gene, encoding a putative GalE from Thermotoga maritima (TMGalE), in Escherichia coli and characterized the encoded protein. To further investigate the molecular basis of this enzyme's catalytic function, we determined the crystal structures of TMGalE and TMGalE bound to UDP-Glc at resolutions of 1.9 Å and 2.0 Å, respectively. The enzyme was determined to be a homodimer with a molecular mass of 70 kDa. The enzyme could reversibly catalyze the epimerization of UDP-GalNAc/UDP-GlcNAc as well as UDP-Gal/UDP-Glc at elevated temperatures, with an apparent optimal temperature and pH of 80 °C and 7.0, respectively. Our data showed that TM0509 is a UDP-galactosugar 4-epimerase involved in d-galactose metabolism; consequently, this study provides the first detailed characterization of a hyperthermophilic GalE. Moreover, the promiscuous substrate specificity of TMGalE, which is more similar to human GalE than E. coli GalE, supports the notion that TMGalE might exhibit the earliest form of sugar-epimerizing enzymes in the evolution of galactose metabolism.

Pathomechanisms in coenzyme q10-deficient human fibroblasts

Primary coenzyme Q10 (CoQ10) deficiency is a rare mitochondrial disorder associated with 5 major clinical phenotypes: (1) encephalomyopathy, (2) severe infantile multisystemic disease, (3) cerebellar ataxia, (4) isolated myopathy, and (5) steroid-resistant nephrotic syndrome. Growth retardation, deafness and hearing loss have also been described in CoQ10-deficient patients. This heterogeneity in the clinical presentations suggests that multiple pathomechanisms may exist. To investigate the biochemical and molecular consequences of CoQ10 deficiency, different laboratories have studied cultures of skin fibroblasts from patients with CoQ10 deficiency. In this review, we summarize the results obtained in these studies over the last decade.

Role of Arg228 in the phosphorylation of galactokinase: the mechanism of GHMP kinases by quantum mechanics/molecular mechanics studies

GHMP kinases are a group of structurally related small molecule kinases. They have been found in all kingdoms of life and are mostly responsible for catalyzing the ATP-dependent phosphorylation of intermediary metabolites. Although the GHMP kinases are of clinical, pharmaceutical, and biotechnological importance, the mechanism of GHMP kinases is controversial. A catalytic base mechanism was suggested for mevalonate kinase that has a structural feature of the γ-phosphate of ATP close to an aspartate residue; however, for one GHMP family member, homoserine kinase, where the residue acting as general base is absent, a direct phosphorylation mechanism was suggested. Furthermore, it was proposed by some authors that all the GHMP kinases function by a similar mechanism. This controversy in mechanism has limited our ability to exploit these enzymes as drug targets and in biotechnology. Here the phosphorylation reaction mechanism of the human galactokinase, a member of the GHMP kinase family, was investigated using molecular dynamics simulations and density functional theory-based quantum mechanics/molecular mechanics calculations (B3LYP-D/AMBER99). The reaction coordinates were localized by potential energy scan using an adiabatic mapping method. Our results indicate that a highly conserved Glu174 captures Arg105 in the proximity of the α-phosphate of ATP, forming a H-bond network; therefore, the mobility of ATP in the large oxyanion hole is restricted. Arg228 functions to stabilize the negative charge developed at the β,γ-bridging oxygen of the ATP during bond cleavage. The reaction occurs via a direct phosphorylation mechanism, and the Asp186 in the proximity of ATP does not directly participate in the reaction pathway. Because Arg228 is not conserved among GHMP kinases, reagents which form interactions with Arg228, and therefore can interrupt its function in phosphorylation, may be developed into potential selective inhibitors for galactokinase.

Crystal structure of Ruminococcus albus cellobiose 2-epimerase: structural insights into epimerization of unmodified sugar

Enzymatic epimerization is an important modification for carbohydrates to acquire diverse functions attributable to their stereoisomers. Cellobiose 2-epimerase (CE) catalyzes interconversion between d-glucose and d-mannose residues at the reducing end of β-1,4-linked oligosaccharides. Here, we solved the structure of Ruminococcus albus CE (RaCE). The structure of RaCE showed strong similarity to those of N-acetyl-D-glucosamine 2-epimerase and aldose-ketose isomerase YihS with a high degree of conservation of residues around the catalytic center, although sequence identity between them is low. Based on structural comparison, we found that His184 is required for RaCE activity as the third histidine added to two essential histidines in other sugar epimerases/isomerases. This finding was confirmed by mutagenesis, suggesting a new catalytic mechanism for CE involving three histidines.

Development of an amperometric screen-printed galactose biosensor for serum analysis

The development of a disposable amperometric biosensor for the measurement of circulating galactose in serum is described. The biosensor comprises a screen-printed carbon electrode (SPCE), incorporating the electrocatalyst cobalt phthalocyanine (CoPC), which is covered by a permselective cellulose acetate (CA) membrane and a layer of immobilized galactose oxidase (GALOX). The optimal response of the biosensor, designated as GALOX-CA-CoPC-SPCE, was obtained by systematically examining the effects of enzyme loading, temperature, pH, and buffer strength. The optimal performance of the biosensor occurred with 2U of GALOX, at 35°C, using 50mM phosphate buffer solution (pH 7.0). The sensitivity was 7.00μAmM(-1)cm(-2) and the linear range from 0.1 to 25mM with a calculated limit of detection (LOD) of 0.02mM; this concentration range and LOD are appropriate to diagnose galactosemia, i.e., concentrations >1.1mM in infants. When the biosensor was used in conjunction with amperometry in stirred solution for the analysis of serum, the precision values obtained on unspiked (endogenous level of 0.153mM) and spiked serum (1mM added) (n=6) were 1.10% and 0.11%, respectively, with a calculated recovery of 99.9%.

Mechanistic studies on human N-acetylgalactosamine kinase

N-Acetylgalactosamine kinase (GALK2) is a small molecule kinase from the GHMP family which phosphorylates N-acetylgalactosamine at the expense of ATP. Recombinant GALK2 expressed in, and purified from, Escherichia coli was shown to be active with the following kinetic parameters: Michaelis constant for ATP, 14 +/- 3 microM; Michaelis constant for N-acetylgalactosamine, 40 +/- 14 microM; and turnover number, 1.0 +/- 0.1 s(-1). The combination of substrate inhibition by N-acetylgalactosamine and alpha-methylgalactopyranoside acting as an uncompetitive inhibitor with respect to ATP suggested that the enzyme has an ordered ternary complex mechanism in which ATP is the first substrate to bind. The effects of pH on the kinetic parameters provided evidence for ionizable residues playing a role in substrate binding and catalysis. These results are discussed in the context of the mechanisms of the GHMP kinases.

Molecular and biochemical characterization of the GALT gene in Korean patients with galactose-1-phosphate uridyltransferase deficiency

BACKGROUND

Three different types of galactosemia have been described, and the most common form occurs due to a deficiency in the galactose-1-phosphate uridyltransferase (GALT) enzyme activity.

METHODS

To investigate the molecular defects of the GALT gene, PCR-direct sequencing was performed with genomic DNA from 18 Korean patients with reduced GALT activity.

RESULTS

Of the 18 patients tested, 13 (72.2%) had previously reported variants: Duarte variant (12 patients), p.R201H (1 patient), and g.A1962G. In addition, we identified six novel sequence variations by PCR-direct sequencing: five sequence variations in coding regions (p.H31R, p.L116I, p.Q169H, p.H186P and p.R333R), and one in an intron (g.2621A>G). Of 100 normal individuals tested, 4 were heterozygous for the Duarte variant, which indicates a Duarte allele frequency of 2%. Biochemical characteristics of the novel genetic alterations were determined: enzyme activity for exonic alterations and splicing for intron.

CONCLUSION

The genetic constitution of the GALT gene is responsible for galactosemia in the Korean population.

Multiplex Enzyme Assay for Galactosemia Using Ultraperformance Liquid Chromatography–Tandem Mass Spectrometry

Background: Galactosemia is one of the most important inherited disorders detected by newborn screening tests. Abnormal results in screening tests should be confirmed by enzyme activity assays, but existing methods are time and labor intensive. We developed a novel multiplex enzyme assay for galactosemia using ultraperformance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS).

Methods: [13C6]-galactose, [13C2]-galactose-1-phosphate, and UDP-glucose were used as substrates for 3 galactose-metabolizing enzymes. The end products from the combined reaction mixtures, [13C6]-galactose-1-phosphate, UDP-[13C2]-galactose, and UDP-galactose, were simultaneously measured using UPLC-MS/MS. Linearity, imprecision, ion suppression, and the effects of substrate were evaluated to determine assay performance. Enzyme activities from 35 healthy individuals, 8 patients with enzyme deficiency, and 18 mutant cells were analyzed.

Results: Substrates, products, and internal standards from the mixture of 3 enzyme reactions were clearly separated by using UPLC-MS/MS, with an injection cycle time of 10 min. Ion suppression was 0.1%–2.5%, the interassay imprecision of UPLC-MS/MS was 3.3%–10.6% CV, and the linearity of each system was good (R2 = 0.994–0.999). Patient samples and mutated cells showed consistently low enzyme activities compared with those of normal individuals and wild-type cells.

Conclusions: This method allows for a high-throughput and reproducible multiplex enzyme assay for galactosemia in erythrocytes.

Galactose toxicity in animals

In most organisms, productive utilization of galactose requires the highly conserved Leloir pathway of galactose metabolism. Yet, if this metabolic pathway is perturbed due to congenital deficiencies of the three associated enzymes, or an overwhelming presence of galactose, this monosaccharide which is abundantly present in milk and many non-dairy foodstuffs, will become highly toxic to humans and animals. Despite more than four decades of intense research, little is known about the molecular mechanisms of galactose toxicity in human patients and animal models. In this contemporary review, we take a unique approach to present an overview of galactose toxicity resulting from the three known congenital disorders of galactose metabolism and from experimental hypergalactosemia. Additionally, we update the reader about research progress on animal models, as well as advances in clinical management and therapies of these disorders.

A metabolomics approach using juvenile cystic mice to identify urinary biomarkers and altered pathways in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease and affects 1 in 1,000 individuals. Ultrasound is most often used to diagnose ADPKD; such a modality is only useful late in the disease after macroscopic cysts are present. There is accumulating evidence suggesting that there are common cellular and molecular mechanisms responsible for cystogenesis in human and murine PKD regardless of the genes mutated, and, in the case of complex metabolomic analysis, the use of a mouse model has distinct advantages for proof of principle over a human study. Therefore, in this study we utilized a urinary metabolomics-based investigation using gas chromatography-time of flight mass spectrometry to demonstrate that the cystic mouse can be discriminated from its wild-type counterpart by urine analysis alone. At day 26 of life, before there is serological evidence of kidney dysfunction, affected mice are distinguishable by urine metabolomic analysis; this finding persists through 45 days until 64 days, at which time body weight differences confound the results. Using functional score analysis and the KEGG pathway database, we identify several biologically relevant metabolic pathways which are altered very early in this disease, the most highly represented being the purine and galactose metabolism pathways. In addition, we identify several specific candidate biomarkers, including allantoic acid and adenosine, which are augmented in the urine of young cystic mice. These markers and pathway components, once extended to human disease, may prove useful as a noninvasive means of diagnosing cystic kidney diseases and to suggest novel therapeutic approaches. Thus, urine metabolomics has great diagnostic potential for cystic renal disorders and deserves further study.

Structure-based Functional Annotation

Despite the generation of a large amount of sequence information over the last decade, more than 40% of well characterized enzymatic functions still lack associated protein sequences. Assigning protein sequences to documented biochemical functions is an interesting challenge. We illustrate here that structural genomics may be a reasonable approach in addressing these questions. We present the crystal structure of the Saccharomyces cerevisiae YMR099cp, a protein of unknown function. YMR099cp adopts the same fold as galactose mutarotase and shares the same catalytic machinery necessary for the interconversion of the alpha and beta anomers of galactose. The structure revealed the presence in the active site of a sulfate ion attached by an arginine clamp made by the side chain from two strictly conserved arginine residues. This sulfate is ideally positioned to mimic the phosphate group of hexose 6-phosphate. We have subsequently successfully demonstrated that YMR099cp is a hexose-6-phosphate mutarotase with broad substrate specificity. We solved high resolution structures of some substrate enzyme complexes, further confirming our functional hypothesis. The metabolic role of a hexose-6-phosphate mutarotase is discussed. This work illustrates that structural information has been crucial to assign YMR099cp to the orphan EC activity: hexose-phosphate mutarotase.

The first study of galactose-1-phosphate uridyl transferase mutations in Iranian galactosemia patients

OBJECTIVES

Classical galactosemia (McKusick 230400) is an autosomal recessive disorder caused by mutations in the galactose-1-phosphate uridyl transferase (GALT;EC 2.7.7.10) gene.

DESIGN AND METHODS

In the present study, we report molecular analysis of 14 unrelated Iranian galactosemia children with reduced or without GALT activity using PCR-RFLP and SSCP-Sequencing methods.

RESULTS

Q188R mutation was the most observed mutation with the allelic frequency of 57.1%. The allelic frequencies for S135L, Y209S, A320T, and K285N were found to be 7.1%, 7.1%, 7.1%, and 3.57% respectively.

CONCLUSIONS

Our results show that galactosemia is a heterogeneous disorder at the molecular level among the Iranian population.

Functional analysis of disease-causing mutations in human UDP-galactose 4-epimerase

UDP-galactose 4-epimerase (GALE, EC 5.1.3.2) catalyses the interconversion of UDP-glucose and UDP-galactose. Point mutations in this enzyme are associated with the genetic disease, type III galactosemia, which exists in two forms - a milder, or peripheral, form and a more severe, or generalized, form. Recombinant wild-type GALE, and nine disease-causing mutations, have all been expressed in, and purified from, Escherichia coli in soluble, active forms. Two of the mutations (N34S and G319E) display essentially wild-type kinetics. The remainder (G90E, V94M, D103G, L183P, K257R, L313M and R335H) are all impaired in turnover number (k cat) and specificity constant (k cat/Km), with G90E and V94M (which is associated with the generalized form of galactosemia) being the most affected. None of the mutations results in a greater than threefold change in the Michaelis constant (Km). Protein-protein crosslinking suggests that none of the mutants are impaired in homodimer formation. The L183P mutation suffers from severe proteolytic degradation during expression and purification. N34S, G90E and D103G all show increased susceptibility to digestion in limited proteolysis experiments. Therefore, it is suggested that reduced catalytic efficiency and increased proteolytic susceptibility of GALE are causative factors in type III galactosemia. Furthermore, there is an approximate correlation between the severity of these defects in the protein structure and function, and the symptoms observed in patients.

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.

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.

Molecular structure of human galactokinase: implications for type II galactosemia

Galactokinase functions in the Leloir pathway for galactose metabolism by catalyzing the MgATP-dependent phosphorylation of the C-1 hydroxyl group of alpha-D-galactose. The enzyme is known to belong to the GHMP superfamily of small molecule kinases and has attracted significant research attention for well over 40 years. Approximately 20 mutations have now been identified in human galactokinase, which result in the diseased state referred to as Type II galactosemia. Here we report the three-dimensional architecture of human galactokinase with bound alpha-D-galactose and Mg-AMPPNP. The overall fold of the molecule can be described in terms of two domains with the active site wedged between them. The N-terminal domain is dominated by a six-stranded mixed beta-sheet whereas the C-terminal motif contains six alpha-helices and two layers of anti-parallel beta-sheet. Those residues specifically involved in sugar binding include Arg37, Glu43, His44, Asp46, Gly183, Asp186, and Tyr236. The C-1 hydroxyl group of alpha-D-galactose sits within 3.3 A of the gamma-phosphorus of the nucleotide and 3.4 A of the guanidinium group of Arg37. The carboxylate side chain of Asp186 lies within approximately 3.2 A of the C-2 hydroxyl group of alpha-D-galactose and the guanidinium group of Arg37. Both Arg37 and Asp186 are strictly conserved among both prokaryotic and eukaryotic galactokinases. In addition to providing molecular insight into the active site geometry of the enzyme, the model also provides a structural framework upon which to more fully understand the consequences of the those mutations known to give rise to Type II galactosemia.

Molecular Structure of Human Galactose Mutarotase

Galactose mutarotase catalyzes the conversion of beta-d-galactose to alpha-d-galactose during normal galactose metabolism. The enzyme has been isolated from bacteria, plants, and animals and is present in the cytoplasm of most cells. Here we report the x-ray crystallographic analysis of human galactose mutarotase both in the apoform and complexed with its substrate, beta-d-galactose. The polypeptide chain folds into an intricate array of 29 beta-strands, 25 classical reverse turns, and 2 small alpha-helices. There are two cis-peptide bonds at Arg-78 and Pro-103. The sugar ligand sits in a shallow cleft and is surrounded by Asn-81, Arg-82, His-107, His-176, Asp-243, Gln-279, and Glu-307. Both the side chains of Glu-307 and His-176 are in the proper location to act as a catalytic base and a catalytic acid, respectively. These residues are absolutely conserved among galactose mutarotases. To date, x-ray models for three mutarotases have now been reported, namely that described here and those from Lactococcus lactis and Caenorhabditis elegans. The molecular architectures of these enzymes differ primarily in the loop regions connecting the first two beta-strands. In the human protein, there are six extra residues in the loop compared with the bacterial protein for an approximate longer length of 9 A. In the C. elegans protein, the first 17 residues are missing, thereby reducing the total number of beta-strands by one.

Substrate specificity and mechanism from the structure of Pyrococcus furiosus galactokinase

Galactokinase (GalK) catalyses the first step of the Leloir pathway of galactose metabolism, the ATP-dependent phosphorylation of galactose to galactose-1-phosphate. In man, defects in galactose metabolism can result in disorders with severe clinical consequences, and deficiencies in galactokinase have been linked with the development of cataracts within the first few months of life. The crystal structure of GalK from Pyrococcus furiosus in complex with MgADP and galactose has been determined to 2.9 A resolution to provide insights into the substrate specificity and catalytic mechanism of the enzyme. The structure consists of two domains with the active site in a cleft at the domain interface. Inspection of the substrate binding pocket identifies the amino acid residues involved in galactose and nucleotide binding and points to both structural and mechanistic similarities with other enzymes of the GHMP kinase superfamily to which GalK belongs. Comparison of the sequence of the Gal3p inducer protein, which is related to GalK and which forms part of the transcriptional activation of the GAL gene cluster in the yeast Saccharomyces cerevisiae, has led to an understanding of the molecular basis of galactose and nucleotide recognition. Finally, the structure has enabled us to further our understanding on the functional consequences of mutations in human GalK which cause galactosemia.

Effects of a carbohydrate supplement upon resting brain activity

Glucose is a major energy source for the brain, and along with several monosaccharide derivatives as components of brain gangliosides, they play important roles in neurologic function. However, there is little information available on the role of glucose and other monosaccharides on resting brain activity. This study was designed to evaluate the effects of a single dose of a carbohydrate supplement containing glucose and several of its derivatives on resting brain activity in 20 healthy male college students. The supplement provided an insignificant amount of carbohydrate (3.9 g), protein (0.28 g), fat (0 g), and calories (14 kcal). The amount of glucose in the supplement was 0.5 g (1% the amount of glucose used in adult studies of cognitive functioning and memory). We hypothesized that the glyconutrient supplement would enhance brain activity associated with alertness and attention. The study design was double blind, with subjects randomly assigned to one of two orders, either carbohydrate supplement week one followed by placebo a week later, or the opposite. Electrical brain activity was monitored by 15 electrodes positioned at nine standard international 10-20 system locations, including three bilateral pairs at frontal, parietal, and occipital sites. Thirty minutes following ingestion of a placebo or carbohydrate supplement drink, EEG activity was recorded for 10-mins while subjects focused on a stationary visual target. Spectral power of resting brain activity was computed and analyzed contrasting the placebo and supplement groups. Relative to placebo, the carbohydrate supplement significantly enhanced power in three brain wave frequencies (theta, alpha, and beta) that are known to be associated with attention and arousal. Since changes were observed in the supplement but not placebo group, our study suggests that additional sugars in the glyconutritional supplement facilitate enhancement of brain electrical activity. Whether the apparent enhancement of arousal in baseline recordings is associated with improved task performance remains to be determined.

Sugar recognition by human galactokinase

BACKGROUND

Galactokinase catalyses the first committed step of galactose catabolism in which the sugar is phosphorylated at the expense of MgATP. Recent structural studies suggest that the enzyme makes several contacts with galactose--five side chain and two main chain hydrogen bonds. Furthermore, it has been suggested that inhibition of galactokinase may help sufferers of the genetic disease classical galactosemia which is caused by defects in another enzyme of the pathway galactose-1-phosphate uridyl transferase. Galactokinases from different sources have a range of substrate specificities and a diversity of kinetic mechanisms. Therefore only studies on the human enzyme are likely to be of value in the design of therapeutically useful inhibitors.

RESULTS

Using recombinant human galactokinase expressed in and purified from E. coli we have investigated the sugar specificity of the enzyme and the kinetic consequences of mutating residues in the sugar-binding site in order to improve our understanding of substrate recognition by this enzyme. D-galactose and 2-deoxy-D-galactose are substrates for the enzyme, but N-acetyl-D-galactosamine, L-arabinose, D-fucose and D-glucose are all not phosphorylated. Mutation of glutamate-43 (which forms a hydrogen bond to the hydroxyl group attached to carbon 6 of galactose) to alanine results in only minor changes in the kinetic parameters of the enzyme. Mutation of this residue to glycine causes a ten-fold drop in the turnover number. In contrast, mutation of histidine 44 to either alanine or isoleucine results in insoluble protein following expression in E. coli. Alteration of the residue that makes hydrogen bonds to the hydroxyl attached to carbons 3 and 4 (aspartate 46) results in an enzyme that although soluble is essentially inactive.

CONCLUSIONS

The enzyme is tolerant to small changes at position 2 of the sugar ring, but not at positions 4 and 6. The results from site directed mutagenesis could not have been predicted from the crystal structure alone and needed to be determined experimentally.

Iron requirement for GAL gene induction in the yeast Saccharomyces cerevisiae

Iron is an essential nutrient. Its deficiency hinders the synthesis of ATP and DNA. We report that galactose metabolism is defective when iron availability is restricted. Our data support this connection because 1) galactose-mediated induction of GAL promoter-dependent gene expression was diminished by iron limitation, and 2) iron-deficient mutants grew slowly on galactose-containing medium. These two defects were immediately corrected by iron replacement. Inherited defects in human galactose metabolism are characteristic of the disease called galactosemia. Our findings suggest that iron-deficient galactosemic individuals might be more severely compromised than iron-replete individuals. This work shows that iron homeostasis and galactose metabolism are linked with one another.

Molecular structure of galactokinase

Galactokinase plays a key role in normal galactose metabolism by catalyzing the ATP-dependent phosphorylation of alpha-D-galactose to galactose 1-phosphate. In humans, mutations in the galactokinase gene can lead to the diseased state referred to as Type II galactosemia. Here we describe the three-dimensional structure of galactokinase from Lactococcus lactis determined to 2.1-A resolution. As expected from amino acid sequence alignments, galactokinase adopts a similar topology to that observed for members of the GHMP superfamily. The N-terminal domain is characterized by a five-stranded mixed beta-sheet while the C-terminal motif is dominated by two distinct four-stranded anti-parallel beta-sheets. The structure was solved in the presence of alpha-D-galactose and inorganic phosphate. These ligands are wedged between the N- and C-terminal domains. Amino acid side chains responsible for anchoring the sugar ligand to the protein include Arg36, Glu42, Asp45, Asp183, and Tyr233. Both Arg36 and Asp183 are strictly conserved in the amino acid sequences available in the literature thus far for galactokinases. Interestingly, the carboxylate side chain of Asp183 is positioned within 3.5 A of the C-1 hydroxyl group of galactose, whereas the guanidinium group of Arg36 is situated between both the C-1 hydroxyl group and the inorganic phosphate. Most likely these residues play key roles in catalysis. The structure of galactokinase described here serves as a model for understanding the functional consequences of point mutations known to result in Type II galactosemia in humans.

Identification and characterisation of human aldose 1-epimerase

Aldose 1-epimerase or mutarotase (EC 5.1.3.3) is a key enzyme of carbohydrate metabolism catalysing the interconversion of the alpha- and beta-anomers of hexose sugars such as glucose and galactose. We identified an open reading frame in the human genome (BC014916) which has high sequence similarity to previously identified bacterial aldose 1-epimerases. This sequence was cloned into a bacterial expression vector, and expressed and purified from this source. Enzyme assays show that the protein has aldose 1-epimerase activity and exhibits a preference for galactose over glucose. Site-directed mutagenesis confirmed the involvement of three residues involved in catalysis and substrate binding.

Functional analysis of disease-causing mutations in human galactokinase

Galactokinase (EC 2.7.1.6) catalyzes the first committed step in the catabolism of galactose. The sugar is phosphorylated at position 1 at the expense of ATP. Lack of fully functional galactokinase is one cause of the inherited disease galactosemia, the main clinical manifestation of which is early onset cataracts. Human galactokinase (GALK1) was expressed in and purified from Escherichia coli. The recombinant enzyme was both soluble and active. Product inhibition studies showed that the most likely kinetic mechanism of the enzyme was an ordered ternary complex one in which ATP is the first substrate to bind. The lack of a solvent kinetic isotope effect suggests that proton transfer is unlikely to be involved in the rate determining step of catalysis. Ten mutations that are known to cause galactosemia were constructed and expressed in E. coli. Of these, five (P28T, V32M, G36R, T288M and A384P) were insoluble following induction and could not be studied further. Four of the remainder (H44Y, R68C, G346S and G349S) were all less active than the wild-type enzyme. One mutant (A198V) had kinetic properties that were essentially wild-type. These results are discussed both in terms of galactokinase structure-function relationships and how these functional changes may relate to the causes of galactosemia.

[Premature ovarian failure in galactosaemia: pathophysiology and clinical management]

Classic galactosaemia is a rare aetiology of premature ovarian failure. It is caused by galactose-1-phosphate uridyltransferase deficiency and leads to a severe disease in the newborn. This acute toxic syndrome will completely regress under a galactose-free diet, but some long-term complications, particularly hypergonadotropic hypogonadism in female patients, are frequently observed. Ovarian toxicity could be due to intracellular accumulation of galactose metabolites or to deficient glycosylation reactions. Moreover, the tremendous follicular decrease in the galactosaemic ovary could also involve programmed cell death (apoptosis). As the exact mechanisms of this ovarian injury are still unknown, there is no prevention of follicular loss, thus clinical management especially includes hormonal replacement therapy in order to prevent bone loss and cardiovascular risks and sometimes to allow patients to become pregnant.

Galactose-1-phosphate is a regulator of inositol monophosphatase: a fact or a fiction?

Classic galactosemia is due to the deficiency of galactose-1-phosphate uridyl transferase and is transmitted as an autosomal recessive disorder. Patients suffering from classic galactosemia display acute symptoms such as poor growth, feeding difficulties, jaundice, hepatomegaly etc., which disappear when the individual is on galactose free diet. However, these patients continue to suffer from defects such as neurological disturbances and ovarian dysfunction, due to the accumulation of galactose-1-phosphate, which is a normal intermediate of galactose metabolism. The biochemical mechanism of galactose-1-phosphate mediated toxicity is still an enigma. Recent experiments strongly suggest that galactose-1-phosphate is also a substrate for inositol monophosphatase (IMPase). Phosphatidylinositol bisphosphate [PI(P)2] dependent signaling serves as a second messenger for several neurotransmitters in the brain. Therefore, the brain is critically dependent on IMPase for the supply of free inositol in order to sustain [PI(P)2] signaling. Circumstantial evidence strongly supports the possibility that being a substrate, galactose-1-phosphate could modulate IMPase function in vivo. The implication of this idea is discussed in relation to classic galactosemia as well as bipolar disorder, which has been thought to be due to the hyper-activation of [PI(P)2] mediated second messenger pathways(s).

Frequencies of Q188R and N314D mutations and IVS5-24g>A intron variation in the galactose-1-phosphate uridyl transferase (GALT) gene in the Slovenian population

Numerous mutations in the galactose-1-phosphate uridyl transferase (GALT) gene have been found to impair GALT activity to different extent, causing galactosemia. This disorder exhibits considerable allelic heterogeneity in different populations and ethnic groups. The Q188R mutation accounts for 60-70% of classical galactosemia alleles in the Caucasian population. Individuals homoallelic for Q188R have a severe phenotype with complete loss of enzyme activity. Another form of GALT deficiency is Duarte galactosemia with N314D mutation associated alleles (Duarte-2). Although heterozygotes for classical galactosemia are asymptomatic at birth and Duarte galactosemia appears to be quite benign, there are some indications that these disorders can increase the risk of developing certain diseases later in life. The aim of our study was to analyze a healthy Slovenian population for the frequencies of Q188R and N314D mutations, and for the Duarte-2 indicative intronic variation IVS5-24G>A. DNA samples from 174 healthy subjects were analyzed for all three mutations by polymerase chain reaction and digestion with restriction enzymes. Allele frequencies for Q188R and N314D mutations and IVS5-24G>A intron variation were found to be 0.29%, 8.0% and 5.7%, respectively. These results correlate well with those reported for most other healthy Caucasian populations.