Homology modeling studies on human galactose-1-phosphate uridylyltransferase and on its galactosemia-related mutant Q188R provide an explanation of molecular effects of the mutation on homo- and heterodimers

We have created theoretical models of the three-dimensional dimeric structure of human galactose-1-phosphate uridylyltransferase as well as of homo- and heterodimers carrying the Q188R mutation by using comparative modeling procedures. These mutants are associated to the most frequent form of the genetic disease galactosemia. We have analyzed the impact of this mutation both on enzyme-substrate interactions as well as on interchain interactions in the heterodimers and in the homodimer. We suggest a molecular explanation for the altered function, caused by different enzyme-substrate interactions, and for the partial dominant negative effect of the mutant allele that is present in heterozygotes for this gene, related to a substantial loss of interchain hydrogen bonds. These results can be considered a starting point for a more extensive characterization at the molecular level of the other mutations linked to this genetic disease.

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.

Normalization of lens protein kinase Cgamma in galactosemic dogs by a novel aldose reductase inhibitor

The purpose of this study was to determine the effects of a novel aldose reductase inhibitor on lens protein kinase Cgamma (PKCgamma) levels in galactosemic dogs. Six-month old Beagles (12 total; 6 male and 6 female) were made galactosemic by feeding a diet of 40% galactose for 6 weeks. Three dogs per group were fed either control, normal diet, 40% galactose diet, 40% galactose diet with aldose reductase inhibitor at 100 mg/kg body weight per day given orally, or a control diet with aldose reductase inhibitor alone (1-H,7-H-5alpha,6,8,9-tetrahydro-1-oxopyran[4,3-beta](1) benzopyran, referred to herein as HAR-1). Lenses were removed and analyzed for toxicity by pathological examination. Lens polyol concentrations were determined by GC/MS. PKCgamma levels were determined by Western blot and by reverse transcriptase-polymerase chain reaction (RT-PCR). No toxicity was observed from the aldose reductase inhibitor when given at 100 mg/kg body weight per day for 6 weeks. Galactosemic dogs showed deterioration of lens cells. Deterioration included vacuole formation in the lens, cell lysis, and loss of cell nuclei. Galactosemic dogs given the HAR-1 appeared identical to control dogs. Polyol concentrations in the lenses were reduced by 50% in dogs fed the 40% galactose diet with the aldose reductase inhibitor, HAR-1. PKCgamma protein levels were reduced in the galactosemic dog lenses, but synthesis of PKCgamma was not affected, as measured by RT-PCR. The PKCgamma protein levels were similar to controls in dogs given the aldose reductase inhibitor, HAR-1, even when polyol concentrations remained 50% elevated above control levels. HAR-1, when given to control dogs, caused a reduction in the synthesis of PKCgamma mRNA but not in total PKCgamma protein levels. This study demonstrates the use of a novel aldose reductase inhibitor to control changes in PKCgamma in dog lens, a PKC that is known to control gap junction activity.

Protective effect of ?-cysteine and glutathione on the modulated suckling rat brain Na+,K+-ATPase and Mg2+-ATPase activities induced by the in vitro galactosaemia

Galactosaemia is an inborn error of galactose (Gal) metabolism characterized by irreversible brain damage. The aim of this study was to evaluate whether the antioxidants L-cysteine (Cys) and the reduced glutathione (GSH) could reverse the alterations of brain total antioxidant status (TAS) and the modulated activities of the enzymes Na+,K+ -ATPase and Mg2+ -ATPase in in vitro galactosaemia. Mixture A (mix. A: galactose-1-phosphate (Gal-1-P, 2mM) plus galactitol (Galtol, 2mM) plus Gal (4mM) = classical galactosaemia) or Mixture B (mix. B: Galtol (2mM) plus Gal (1mM) = galactokinase deficiency galactosaemia) were preincubated in the presence or absence of Cys (0.83mM) or GSH (0.83 mM) with whole brain homogenates of suckling rats at 37 degrees C for 1h. TAS and the enzyme activities were determined spectrophotometrically. The preincubation of brain homogenates with mix. A or mix. B resulted in a decrease of TAS to 30% (P < 0.01), while the presence of Cys or GSH increased TAS to 20% (P < 0.01) and 60% ( P < 0.001), respectively. The antioxidants reversed the inhibited Na+,K+ -ATPase by mix. A or mix. B and the stimulated Mg2+ -ATPase by mix. B to control values, whereas no effect was observed on the enormously activated Mg2+ -ATPase by mix. A.

CONCLUSIONS

(a) Gal and its derivatives may produce free radicals in the suckling rat brain, reported for first time, (b) Na+,K+ -ATPase inhibition and Mg2+ -ATPase activation are probably due to the oxidative stress from the above compounds, (c) Cys or GSH could play a protective role reversing the inhibited Na+,K+ -ATPase toward normal in in vitro galactosaemia and (d) the addition of the above antioxidants may reduce the consequences of brain Mg2+ -ATPase activation by Gal and Galtol in galactokinase deficiency galactosaemia.

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.

[From gene to disease; galactosemia and galactose-1-phosphate uridyltransferase deficiency]

Classical galactosaemia (Mendelian Inheritance in Man, no 230400) is an autosomal recessive disorder of galactose metabolism caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase (GALT). The GALT enzyme is responsible for the conversion of galactose-1-phosphate with UDP glucose to glucose-1-phosphate and UDP galactose. The gene encoding for GALT is located on chromosome 9p13. Patients present with hepatomegaly, liver failure, food intolerance, hypoglycaemia, muscle hypotonia, sepsis and cataract. Treatment involving the total restriction of lactose-containing foods is life-saving but many patients develop late complications such as problems of mental development, disorders of motor function, disorders of speech and hypergonadotrophic hypogonadism.

Biochemical characterization of two GALK1 mutations in patients with galactokinase deficiency

Galactokinase (GALK1) deficiency is an autosomal recessive disorder, which causes cataract formation in children not maintained on a lactose-free diet. Galactokinase deficiency results from mutation in the GALK1 gene mapped on 17q24. Since GK1 cDNA was cloned about 20 mutations (prevalently deletions and missense) have been reported to date. Most of these reported mutations are confined to single families, and only one of them, P28T, has been referred as the founder Romani mutation. In this paper we report two novel missense mutations in GALK1 gene, identified in two unrelated patients with galactokinase deficiency. One mutation, g.575G>A, substitutes a valine for a methionine at amino acid 32 (p.V32M), while the other mutation, g.2839G>A, results in the arginine to glutamine substitution p.R239Q (GenBank sequence L76927). Biochemical studies demonstrate that these mutations led to a drastic modification in GALK activity when individual mutant cDNAs were expressed in an E. coli system. These findings indicate the pathogeneticity of these mutations causing GALK deficiency.

Insights into the pathogenesis of galactosemia

In humans, the absence of galactose-1-phosphate uridyltransferase (GALT) leads to significant neonatal morbidity and mortality which are dependent on galactose ingestion, as well as long-term complications of primary ovarian failure and cognitive dysfunction, which are diet independent. The creation of a knockout mouse model for GALT deficiency was aimed at providing an organism in which metabolic challenges and gene manipulation could address the enigmatic pathophysiologic questions raised by humans with galactosemia. Instead, the mouse represents a biochemical phenotype without evidence of clinical morbidity. The similarities and differences between mice and humans with galactosemia are explored from metabolite, enzyme, and process points of view. The mouse both produces and oxidizes galactose in a manner similar to humans. It differs in brain accumulation of galactitol. Future directions for exploration of this enigmatic condition are discussed.

Transgenic mice overexpressing aldose reductase in Schwann cells show more severe nerve conduction velocity deficit and oxidative stress under hyperglycemic stress

To further understand the role of aldose reductase (AR) in the etiology of diabetic neuropathy, we generated transgenic mice that overexpress AR specifically in the Schwann cells under the control of the rat myelin protein zero (P0) promoter. One of the transgenic mouse lines, which has overexpression of AR mRNA in the Schwann cell only and higher AR activity in the sciatic nerve, was used to examine the relationship between increased AR activity and motor nerve conduction velocity (MNCV) deficit under diabetic and galactosemic conditions. Under these conditions, nontransgenic mice showed a slight reduction in MNCV compared to those of controls. However, transgenic mice exhibited a significantly greater reduction in MNCV under these conditions, particularly under galactosemic condition, indicating that a Schwann cell-specific increase in aldose reductase activity is sufficient to produce the phenotype. Interestingly, under galactosemic condition where the difference in MNCV deficit between transgenic and nontransgenic mice was most pronounced, there was no significant difference in accumulated galactitol levels in the sciatic nerve between these mice. These results indicate that increase in AR activity leads to greater reduction of MNCV under galactosemic and diabetic conditions, but galactitol and sorbitol levels may not be good indicators of the severity of neuropathy. On the other hand, the level of reduced glutathione (GSH) in the sciatic nerve was found to be correlated with the severity of MNCV deficit under the diabetic condition. Diabetic AR transgenic mice showed significant reduction of GSH in their sciatic nerve, whereas the diabetic nontransgenic mice showed no reduction in GSH level compared to the nondiabetic control, suggesting that AR is a key contributor to oxidative stress under diabetic condition.

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.

Analysis of common mutations in the galactose-1-phosphate uridyl transferase gene: new assays to increase the sensitivity and specificity of newborn screening for galactosemia

Classical galactosemia is a genetic disease caused by mutations in the galactose-1-phosphate uridyl transferase (GALT) gene. Prospective newborn screening for galactosemia is routine and utilizes the universally collected newborn dried blood specimen on filter paper. Screening for galactosemia is achieved through analysis of total galactose (galactose and galactose-1-phosphate) and/or determining the activity of the GALT enzyme. While this approach is effective, environmental factors and the high frequency of the Duarte D2 mutation (N314D) does lead to false positive results. Using DNA derived from the original newborn dried blood specimen and Light Cycler technology a panel of five assays able to detect the four most frequently encountered classical galactosemia alleles (Q188R, S135L, K285N, and L195P) and the N314D Duarte variant mutation are described. The five assays are performed simultaneously using common conditions. Including DNA preparation, set-up, amplification, and analysis the genotype data for all five loci is obtained in less than 2 hours. The assays are easily interpreted and amenable to high-throughput newborn screening. Mutational analysis is useful to reduce false positive results, differentiate D/G mixed heterozygotes from classical galactosemia, and to clearly identify a very high percentage of those affected by classical galactosemia.

Pyruvate modulates antioxidant status of cultured human lens epithelial cells under hypergalactosemic conditions

Lens epithelial cells are the metabolic unit of the lens and antioxidant enzymes are mainly concentrated here. The purpose of this study was to maintain human lens epithelial cells (HLEC) in culture and examine the status of antioxidant enzymes (glutathione peroxidase (GSHPx), catalase (CAT), glutathione-S-transferase (GST)), lipid peroxidation product malondialdehyde (MDA) and glutathione (GSH) levels in these cells under normal as well as hypergalactosemic (30 mM galactose) conditions. Further, effect of pyruvate, a physiological antioxidant has also been evaluated on these parameters. For conducting experiments, anterior capsule specimens obtained from fresh cadaver eyes from eye bank were cultured in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 20% fetal calf serum. Upon confluency, the cells were subcultured in three separate flasks containing DMEM alone (normal group), DMEM + 30 mM D-galactose (control group), DMEM + 30 mM D-galactose + 5 mM pyruvate (test group) and incubated for 24 or 72 h. These cells were observed under the phase contrast microscope for any morphological changes and harvested for the estimation of various antioxidant parameters. Our results show significant weakened antioxidant defense in HLEC when incubated in the presence of galactose as compared to normal. Addition of pyruvate significantly modulated levels of GSH, MDA, GSHPx, CAT and GST.

Impact of patient mutations on heterodimer formation and function in human galactose-1-P uridylyltransferase

Impairment of the human enzyme galactose-1-P uridylyltransferase (hGALT) results in the potentially lethal disorder, galactosemia. One of the fundamental questions with regard to this dimeric enzyme involves the possible influence of patient mutations on heterodimer formation and activity. Indeed, considering that many if not most galactosemia patients are compound heterozygotes, this is an issue of clinical as well as basic science interest. We have utilized a yeast expression system for the human enzyme to test whether each of a small number of mutations in hGALT (S135L, F171S, F171W, Q188R, N314D, and R333W) impact either heterodimer formation or function. Our results clearly demonstrate that while a majority of the alleles tested show precisely random patterns of subunit assortment, two deviate slightly but significantly from this pattern. Similarly, while some heterodimers exhibit apparent independence of subunit activity, others do not. These data not only demonstrate that common patient mutations in hGALT can influence both heterodimer formation and function in heterozygotes, they further raise the question of whether such interactions may also occur between different mutant alleles in compound heterozygotes (i.e., patients). Indeed, such influences may underlie some of the biochemical and clinical heterogeneity observed in the galactosemia patient population.

[Abnormal antioxidant system in inborn errors of intermediary metabolism]

INTRODUCTION

Oxidative stress may be implied in the pathogenic mechanisms of inborn errors of intermediary metabolism (IEIM).

OBJECTIVE

The evaluation of the antioxidant status in IEIM by the measurement of erythrocyte antioxidant enzyme activities, superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase and catalase.

PATIENTS AND METHODS

34 patients with IEIM: 1) eleven with organic acidurias on protein restricted diet; 2) nine without special diet; 3) five patients with aminoacidopathies on protein restriction; 4) three patients with galactosemia and six with aminoacidopathies on protein free diet. Erythrocyte antioxidant enzymes were measured by spectrometric procedures adapted to the Cobas Fara II analyser.

RESULTS

SOD activity was significantly higher in groups 2 and 4 (p= 0.009, p= 0.001, respectively), and significantly lower in group 3 (p= 0.001) compared with age matched controls. SOD activity was significantly higher in the patients with IEIM on protein free diet (groups 2 and 4) compared with those on protein restricted diet (groups 1 and 3; p= 0.002) or with controls (p= 0.003). GPx activity was found significantly lower in group 1 patients (p= 0.004), and higher in group 2 (p= 0.029) compared with controls.

CONCLUSIONS

35% of the patients with IEIM had SOD activity above the control range, most of them with organic acidurias or homocystinuria, suggesting an induction of enzyme protein synthesis owing to an excess of free radical generation. The lower activities observed in patients on natural protein restriction may likely be due to a deficient bioavailability of antioxidant cofactors.

[Bone alkaline phosphatase: characteristic and its clinical applications]

Bone alkaline phosphatase (BALP) is one of the most frequently used biochemical markers of bone formation. The presented paper describes the enzyme's specificity, physiological values during normal growth and development as well as its clinical applications in various diseases. The main interest concerns the ability of BALP to predict bone loss in primary (postmenopausal and senile osteoporosis) and secondary osteoporosis associated with metabolic diseases (galactosemia, cystic fibrosis, celiac disease), renal osteodystrophy, Paget disease and others. The determination of BALP activity seems to be also helpful in diagnosis of the diseases and in monitoring of antiresorptive therapy. Further studies on BALP are needed to elucidate whether this bone formation marker reflect the therapy outcome of individual patients with primary osseus tumours and metastases.

Caspase activation in retinas of diabetic and galactosemic mice and diabetic patients

Apoptosis of retinal capillary cells begins early in diabetes and likely contributes to the capillary obliteration that is an important feature of diabetic retinopathy. Caspases are proteolytic enzymes that are closely involved in the induction and execution phases of apoptosis, but their role in the development of diabetic retinopathy has not been studied previously. Our study focused on the measurement of activities of multiple caspases in retinas of mice at different durations of diabetes. Several caspases (including caspases-1, -2, -6, -8, and -9) were activated as early as 2 months of diabetes. The caspases activity pattern changed with increasing duration of disease, suggesting a slowly developing caspases cascade. Activities of executioner caspases (e.g., cas-6 and -3) became elevated after longer duration of diabetes, and the induction of cas-3 activity was associated with the duration of diabetes at which capillary cells begin to show evidence of undergoing apoptosis. Retinas from patients with type 2 diabetes likewise showed a significant increase in activities of cas-1, -3, -4, and -6. For comparison, retinal caspases were also measured in experimental galactosemia, another model that develops a diabetic-like retinopathy. The pattern of caspases activation differed between diabetes and galactosemia, but cas-1 activity became elevated soon after elevation of blood hexose concentration in both. Caspases offer new therapeutic targets to test the role of apoptosis in the development of diabetic retinopathy.

Molecular analysis in newborns from Texas affected with galactosemia

The spectrum of mutations in the Galactose-1-phosphate uridyl transferase (GALT) gene is described in 11 cases of classic galactosemia and 38 of Duarte-2 type identified by the Texas Newborn Screening Program. Blinded studies were done by automated DNA sequencing of all the 11 exons and the exon-intron boundaries of the GALT gene using genomic DNA isolated from dry blood spots. Fourteen different mutations (11 missense mutations, 2 nonsense mutations and 1 splicing mutation) were detected in 94 of the 98 mutant alleles (diagnostic efficiency of 96%). The prevalent mutations were N314D (41%), Q188R (37%) and K285N (4%). The other less frequent mutations were IVS2-2A>G and S135L (3% each), T138M (2%) and T23A, H184Q, Y251S, L195P, Q207X, L264X, Q344K, and A345D (1% each). Three novel mutations, T23A, Q207X, and A345D, were identified. Our study supports previous findings that N314D and Q188R are prevalent in Hispanics and Whites and K285N was only observed in Whites. The IVS2-2A->G mutation is probably ethnic specific because it was identified exclusively in Hispanics. S135L, a prevalent mutation in Blacks, was also present in 3 Hispanics. Two unusual genotypes were observed in 2 patients homozygous for the Duarte-2 N314D allele and heterozygous for a novel mutation (Q207X- N314D/N314D in a classic galactosemia and T23A- N314D/N314D in a Duarte-2 case). The detection of GALT gene mutations in newborns from Texas should focus first on N314D, Q188R, K285N, IVS2-2A>G, S135L and T138M. Other exons and exon-intron boundaries would have to be studied if either one or no mutations are found in the primary screening.

Structure-function analyses of a common mutation in blacks with transferase-deficiency galactosemia

We previously identified a missense mutation at amino acid 135 of human galactose 1-phosphate uridyltransferase (hGALT) in which a leucine (TTG) was substituted for a serine (TCG), S135L. This mutation was common in black patients with galactosemia and homozygotes (S135L/S135L) had no GALT activity or protein in their erythrocytes or lymphoblasts. However, there was residual GALT activity and protein in their leukocytes, and they had near normal total body [13C]galactose oxidation to 13CO2 in breath. To evaluate the biochemical mechanism(s) producing these effects, we overexpressed hGALT proteins with site-directed mutations in this nonconserved amino acid in a GALT-minus Escherichia coli. Enzyme activities detected in bacterial lysates overexpressing either S135 (wild type), A135, C135, H135, L135, S132-H135, T135, or Y135 were 100, 4.7, 3.0, 4.0, 2.7, 0.7, 35.4, and 1.4%, respectively. Only the threonine substitution (S135T) had significant enzyme activity in these lysates. There was also decreased abundance of all mutant proteins in the lysates exposed to bacterial proteolysis during preparation and analysis. This added the variable of bio-instability to analysis of enzyme activities in lysates. To further characterize the catalytic role of serine at amino acid 135 and to differentiate bio-instability from impaired catalysis by the leucine substitution, we purified wild-type and L135-hGALT proteins to homogeneity and analyzed identical amounts of enzyme protein. We found that the apparent Vmax of the purified L135-hGALT protein was significantly reduced from 80 +/- 5.9 to 5.8 +/- 1.8 micromol glucose 1-phosphate released/min/mg hGALT protein with no increase in KM for galactose 1-phosphate for the second displacement. The first displacement reaction, although three orders of magnitude slower, was similar between the wild type and L135-hGALT. We conclude that a hydroxyl group on amino acid 135 is required for the catalysis of uridyl transfer from UDP-glucose to UDP-galactose in the presence of galactose 1-phosphate, and plays a role in the bio-stability of hGALT.

Synthesis and evaluation of novel aldose reductase inhibitors: Effects on lens protein kinase Cgamma

PURPOSE

To synthesize novel aldose reductase inhibitors (ARI) that will normalize losses in protein kinase Cgamma (PKCgamma) observed during diabetes and galactosemia.

METHODS

ARI were synthesized as tricyclic pyrones 1-6 (HAR-1 through HAR-6) from 3-methyl-1H,7H-5a,6,8,9-tetrahydro-1-oxopyrano[4,3-b][1]benzopyran and (5aS,7S)-7-isopropenyl-3-methyl-1H,7H-5a,6,8,9-tetrahydro-1-oxopyrano[4,3-b][1]benzopyran and were tested by inhibition of aldose reductase enzyme activity in vitro and by inhibition of polyol formation in lens epithelial cells in culture. Identified compounds were further tested in galactosemic rat lens in vivo for (a) normalized PKCgamma levels by Western blot, (b) reduction of phosphorylation of the gap junction protein Cx46 by analyses of co-immunoprecipitated proteins, and (c) by normalization of gap junction activity as measured by dye transfer.

RESULTS

HAR-1 (1H,7H-5a,6,8,9-tetrahydro-1-oxopyrano[4,3-b][1]benzopyran-3-acetic acid) was identified as an ARI with IC50 for aldose reductase inhibition at 2 nM. Polyol accumulation in lens epithelial cells was reduced by 80% at 10 microM. Rats fed 40% galactose for 9 days had an 80% reduction in PKCgamma levels which were normalized by HAR-1 at 100 mg/kg/day, fed orally. Phosphorylation of Cx46 was increased by 50% and this was normalized in HAR-1 treated rats (6 day treatment). Gap junction activity of galactosemic rats was reduced by 55% and this was normalized by HAR-1 in six day-treated rats.

CONCLUSIONS

HAR-1 is a novel ARI which normalized losses of PKCgamma, changes in Cx46 phosphorylation, and gap junction activity.

Galactosemia: deletion in the 5' upstream region of the GALT gene reduces promoter efficiency

Galactosemia is a metabolic disorder caused by a defect in the galactose-1-phosphate uridyltransferase (GALT) enzyme. In previous studies, we have shown that the presence of a deletion in the 5' upstream (promoter) region of the GALT gene is associated with the Duarte (D2) allele. In the present study, by using a promoter fusion assay we provide direct evidence that a GTCA deletion located in position -119/-116 of the GALT gene (considered in relation to the translational start site) decreases transcription of a reporter gene to about 55% compared with a normal "healthy" promoter transfected into human hepatocyte HepG2 cells. This result coincides well with previously published biochemical data showing 50% GALT-gene activity in Duarte (D2) galactosemia patients. By transfecting the same promoters (normal and deleted) into mouse NIH/3T3 cells, we show that the GTCA motif in the promoter region of the GALT gene was conserved throughout evolution. We conclude that the -119/-116delGTCA promoter mutation is a crucial factor in reduction of Duarte allele enzyme activity.

Relationship between genotype, activity, and galactose sensitivity in yeast expressing patient alleles of human galactose-1-phosphate uridylyltransferase

Impairment of the human enzyme galactose-1-phosphate uridylyltransferase (GALT) results in the potentially lethal disorder galactosemia; the biochemical basis of pathophysiology in galactosemia remains unknown. We have applied a yeast expression system for human GALT to test the hypothesis that genotype will correlate with GALT activity measured in vitro and with metabolite levels and galactose sensitivity measured in vivo. In particular, we have determined the relative degree of functional impairment associated with each of 16 patient-derived hGALT alleles; activities ranged from null to essentially normal. Next, we utilized strains expressing these alleles to demonstrate a clear inverse relationship between GALT activity and galactose sensitivity. Finally, we monitored accumulation of galactose-1-P, UDP-gal, and UDP-glc in yeast expressing a subset of these alleles. As reported for humans, yeast deficient in GALT, but not their wild type counterparts, demonstrated elevated levels of galactose 1-phosphate and diminished UDP-gal upon exposure to galactose. These results present the first clear evidence in a genetically and biochemically amenable model system of a relationship between GALT genotype, enzyme activity, sensitivity to galactose, and aberrant metabolite accumulation. As such, these data lay a foundation for future studies into the underlying mechanism(s) of galactose sensitivity in yeast and perhaps other eukaryotes, including humans.

Functional analysis of the human galactose-1-phosphate uridyltransferase promoter in Duarte and LA variant galactosemia

Human galactose-1-phosphate uridyltransferase (hGALT) is an evolutionarily conserved enzyme central to D-galactose metabolism. The impairment of hGALT causes galactosemia. One missense mutation, an aspartate to asparagine substitution at amino acid 314 (N314D), impairs 50% activity in the homozygous state in some patients but gives near normal activity in others. The former condition is called Duarte (D) and the latter, Los Angeles (LA). The D allele is linked to hGALT polymorphisms including a deletion 5'to the translation start site (-119 to -116delGTCA), g1391G --> A and g1105G --> C. The LA allele is linked to a g1721C --> T transition. To investigate possible mechanisms for differences in hGALT activity between the D and LA alleles, we sequenced 3951 nucleotides of genomic DNA 5' to the hGALT translation start site. Using a dual-luciferase reporter system to express deletion constructs of the hGALT promoter, we noted both positive and negative regulatory regions. Two putative positive regulatory domains overlap with the naturally occurring -119 to -116delGTCA linked to Duarte. One is an E-box motif (CACGTG) at -117 to -112 bp. The second is an AP-1 motif (TCAGTCAG) at -124 to -119 bp. The delGTCA mutation confers reduced luciferase activity to transfected cell lines derived from human ovarian and liver neoplasms. Additionally, human lymphoblasts derived from patients with the Duarte allele have reduced GALT mRNA. We conclude that the human GALT gene is regulated in the first -165 bp of its promoter region by positive regulators of GALT gene expression. The -119 to -116delGTCA reduces hGALT transcription resulting in reduced GALT activity in the Duarte allele.