Inborn errors of molybdenum metabolism: combined deficiencies of sulfite oxidase and xanthine dehydrogenase in a patient lacking the molybdenum cofactor

Molybdenum kinetics in men differ during molybdenum depletion and repletion

In this study we developed an expanded compartmental model of molybdenum (Mo) kinetics to determine rates of molybdenum distribution during molybdenum depletion and repletion. The model was based on a clinical study in which 4 men consumed a low-molybdenum diet of 22 microg/d (0.23 micromol/d) for 102 d, followed by a high molybdenum diet of 467 microg/d (4.9 micromol/d) for 18 d. Stable isotopes 100Mo and 97Mo were administered orally and intravenously, respectively, at several time points during the study, and serial samples of plasma, urine, and feces were analyzed for 100Mo, 97Mo, and total Mo. Based on plasma, urine, and fecal molybdenum levels, kinetic parameters of distribution and elimination were determined. The rates of molybdenum distribution and elimination were different during depletion and repletion. During high intake, urinary molybdenum excretion was greater than during low intake. In addition, fractional tissue storage of molybdenum was lower during high intake than during low intake. This suggests that low intake results in an adaptation to conserve body Mo, and that high intake results in an adaptation to eliminate Mo. The model also suggested that food-bound molybdenum was approximately 16% less bioavailable than purified Mo. Finally, under the conditions of this study, the model suggested that an intake of 43 microg/d (0.45 micromol/d) would be sufficient to maintain plasma molybdenum levels at steady state. This is a minimum estimate because subjects in this study were in a molybdenum-sparing state. These findings provide an understanding of the adaptations in molybdenum metabolism that take place during depletion and repletion.

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

Molybdenum cofactor biosynthesis and molybdenum enzymes

The molybdenum cofactor (Moco) forms the active site of all eukaryotic molybdenum (Mo) enzymes. Moco consists of molybdenum covalently bound to two sulfur atoms of a unique tricyclic pterin moiety referred to as molybdopterin. Moco is synthesized from GTP by an ancient and conserved biosynthetic pathway that can be divided into four steps involving the biosynthetic intermediates cyclic pyranopterin monophosphate, molybdopterin, and adenylated molybdopterin. In a fifth step, sulfuration or bond formation between Mo and a protein cysteine result in two different catalytic Mo centers. There are four Mo enzymes in plants: (1) nitrate reductase catalyzes the first and rate-limiting step in nitrate assimilation and is structurally similar to the recently identified, (2) peroxisomal sulfite oxidase that detoxifies excessive sulfite. (3) Aldehyde oxidase catalyzes the last step of abscisic acid biosynthesis, and (4) xanthine dehydrogenase is essential for purine degradation and stress response.

Synthesis of adenylated molybdopterin: an essential step for molybdenum insertion

The molybdenum cofactor (Moco) is part of the active site of all molybdenum (Mo)-dependent enzymes, except nitrogenase. Moco consists of molybdopterin (MPT), a phosphorylated pyranopterin with an enedithiolate coordinating Mo and it is synthesized by an evolutionary old multistep pathway. The plant protein Cnx1 from Arabidopsis thaliana catalyzes with its two domains (E and G) the terminal step of Moco biosynthesis, the insertion of Mo into MPT. Recently, the high-resolution MPT-bound structure of the Cnx1 G domain (Cnx1G) has been determined (Kuper, J., Llamas, A., Hecht, H. J., Mendel, R. R., and Schwarz, G. (2004) Nature 430, 803-806). Besides defining the MPT-binding site a novel and unexpected modification of MPT has been identified, adenylated MPT. Here we demonstrate that it is Cnx1G that catalyzes the adenylation of MPT. In vitro synthesized MPT was quantitatively transferred from Escherichia coli MPT synthase to Cnx1G. The subsequent adenylation reaction by Cnx1G was Mg(2+)- and ATP-dependent. Whereas Mn(2+) could partially replace Mg(2+), ATP was the only nucleotide accepted by Cnx1G. Consequently the formation of pyrophosphate was demonstrated, which was dependent on the ability of Cnx1G to bind MPT. Pyrophosphate, either formed in the reaction or added externally, inhibited the Cnx1G-catalyzed MPT adenylation reaction. Catalytically inactive Cnx1G mutant variants showed impaired MPT adenylation confirming that MPT-AMP is the reaction product of Cnx1G. Therefore Cnx1G is a MPT adenylyltransferase catalyzing the activation of MPT, a universal reaction in the Moco synthetic pathway because Cnx1G is able to reconstitute also bacterial and mammalian Moco biosynthesis.

Structure of the molybdopterin-bound Cnx1G domain links molybdenum and copper metabolism

The molybdenum cofactor is part of the active site of all molybdenum-dependent enzymes, except nitrogenase. The molybdenum cofactor consists of molybdopterin, a phosphorylated pyranopterin, with an ene-dithiolate coordinating molybdenum. The same pyranopterin-based cofactor is involved in metal coordination of the homologous tungsten-containing enzymes found in archea. The molybdenum cofactor is synthesized by a highly conserved biosynthetic pathway. In plants, the multidomain protein Cnx1 catalyses the insertion of molybdenum into molybdopterin. The Cnx1 G domain (Cnx1G), whose crystal structure has been determined in its apo form, binds molybdopterin with high affinity and participates in the catalysis of molybdenum insertion. Here we present two high-resolution crystal structures of Cnx1G in complex with molybdopterin and with adenylated molybdopterin (molybdopterin-AMP), a mechanistically important intermediate. Molybdopterin-AMP is the reaction product of Cnx1G and is subsequently processed in a magnesium-dependent reaction by the amino-terminal E domain of Cnx1 to yield active molybdenum cofactor. The unexpected identification of copper bound to the molybdopterin dithiolate sulphurs in both structures, coupled with the observed copper inhibition of Cnx1G activity, provides a molecular link between molybdenum and copper metabolism.

Determination of several elements in duplicate meals from catering establishments using closed vessel microwave digestion with inductively coupled plasma mass spectrometry detection: estimation of daily dietary intake

An estimation of the dietary exposure of French consumers to 21 essential and non-essential mineral elements using duplicate meals (breakfast and lunch) purchased from catering establishments was investigated after digestion by a closed vessel microwave procedure and quantification by ICP-MS. Daily dietary exposure estimates for metals and minerals were compared with the Provisional Tolerable Weekly Intakes (PTWI), the Tolerable Daily Intake (TDI) or the Acceptable Daily Intakes (ADI), as established by the FAO/WHO to estimate the risk of toxicity, and the US Recommended Daily Allowances (US RDA) or the Estimate Safe & Adequate Daily Dietary Intakes (ESADDI). Moreover, comparisons were made with those from previous French studies as well as those from other countries. The estimated mean daily intakes were 11 microgram for lithium, 3.42 g for sodium, 192 mg for magnesium, 2.03 mg for aluminium, 3.64 g for potassium, 642 mg for calcium, 154 microgram for chromium, 12.3 mg for iron, 2.15 mg for manganese, 4 microgram for cobalt, 74 microgram for nickel, 925 microgram for copper, 10.2 mg for zinc, 147 microgram for arsenic, 66 microgram for selenium, 112 microgram for molybdenum, 3.6 microgram for cadmium, 2.32 mg for tin, 3 microgram for antimony, 9 microgram for mercury and 34 microgram for lead. For the non-essential (toxic) elements, aluminium, tin, antimony, cadmium, arsenic, mercury and lead, the daily intake estimates were far below tolerable limits; and similar or somewhat lower than their respective PTWI, ADI, TDI, ESADDI and US RDA for individual minerals and essential trace elements, with good agreement with other country studies. The performance of the multi-elemental ICP-MS technique was also evaluated.

Identification of genes downstream of Pax6 in the mouse lens using cDNA microarrays

Pax6 is a transcription factor that regulates the development of the visual, olfactory, and central nervous systems, pituitary, and pancreas. Pax6 is required for induction, growth, and maintenance of the lens; however, few direct Pax6 target genes are known. This study was designed to identify batteries of differentially expressed genes in three related systems: 8-week old Pax6 heterozygous lenses, 8-week old Pax6 heterozygous eyes, and transgenic lenses overexpressing PAX6(5a), using high throughput cDNA microarrays containing about 9700 genes. Initially, we obtained almost 400 differentially expressed genes in lenses from mice heterozygous for a Pax6 deletion, suggesting that Pax6 haploinsufficiency causes global changes in the lens transcriptome. Comparisons between the three sets of analyses revealed that paralemmin, molybdopterin synthase sulfurylase, Tel6 oncogene (ETV6), a cleavage-specific factor (Cpsf1) and tangerin A were abnormally expressed in all three experimental models. Semiquantitative reverse transcription (RT)-PCR analysis confirmed that all five of these genes were differentially expressed in Pax-6 heterozygous and Pax6(5a) transgenic lenses. Western blotting and immunohistochemistry demonstrated that paralemmin is found at high levels in the adult lens and confirmed its down-regulation in the Pax6(5a)-transgenic lenses. Collectively, our data provide insights into the genetic programs regulated by Pax6 in the lens.

Isolated sulfite oxidase deficiency: review of two cases in one family

OBJECTIVE

The authors describe two cases of isolated sulfite oxidase deficiency found in one family. This is a rare autosomal-recessive disorder presenting at birth with seizures, severe neurologic disease, and ectopia lentis. It can be easily missed with metabolic screening; however, the finding of lens subluxation stresses the importance of ophthalmic assessment in making the diagnosis.

DESIGN

Two observational case reports.

INTERVENTION/METHODS

Ophthalmic assessment, biochemical assay for specific urinary and plasma metabolites, magnetic resonance imaging, and gene sequencing were used to make the diagnosis of the disease in the proband. The diagnosis was subsequently recognized in a previously affected sibling after the postmortem neuropathology was reviewed. Mutation analysis was performed on cultured fibroblasts from the proband to identify and categorize the specific mutation responsible for the disease in the family. From this, future prenatal detection of sulfite oxidase deficiency is possible.

MAIN OUTCOME MEASURES

The diagnosis of sulfite oxidase deficiency was established in this family, enabling appropriate genetic counseling and recurrence risk estimation.

RESULTS

Point mutations were found in both alleles of the sulfite oxidase gene in the proband. The first is a 623C-->A mutation, which predicts an A208D substitution, and the second is a 1109C-->A, which predicts an S370Y substitution. Both residues A208D and S370Y are critical for sulfite oxidase activity.

CONCLUSIONS

Isolated sulfite oxidase deficiency is a rare heritable disease for which mutation analysis can allow accurate prenatal screening. It often is difficult to diagnose by clinical presentation alone, but the critical finding of lens subluxation accompanying seizures and diffuse neurologic disease in an infant should alert the physician to the diagnosis.

Molybdenum

Molybdenum does not exist naturally in the pure metallic form and of the 5 oxidation states (2-6) the predominant species are Mo(IV) and Mo(VI). Molybdenum rapidly polymerizes to a wide variety of complex polymolybdate compounds in solution. The vast majority of molybdenum is used in metallurgical applications (stainless steel, cast-iron alloys). Ammonium tetrathiomolybdate is an experimental chelating agent for Wilson's disease. For the general population, the diet is the most important source of molybdenum and concentrations in water and air usually are negligible. The average daily dietary intake is about 0.1-0.5 mg m.o. Molybdenum is an essential element with relatively low toxicity. Enzymes containing molybdenum catalyze basic metabolic reactions in the carbon, sulfur, and nitrogen cycles. Elimination of molybdenum occurs via the kidney and usually is complete within several weeks. Molybdenosis (teart) is a form of molybdenum toxicity that produces a disease in ruminants similar to copper-deficiency. Little data are available on the human toxicity of molybdenum. A gout-like syndrome and pneumoconiosis have been associated with excessive concentrations of molybdenum, but the inadequate design of the studies prevents an adequate determination of the etiology of these effects.

Human molybdopterin synthase gene: genomic structure and mutations in molybdenum cofactor deficiency type B

Biosynthesis of the molybdenum cofactor (MoCo) can be divided into (1) the formation of a precursor and (2) the latter's subsequent conversion, by molybdopterin synthase, into the organic moiety of MoCo. These two steps are reflected by the complementation groups A and B and the two formally distinguished types of MoCo deficiency that have an identical phenotype. Both types of MoCo deficiency result in a pleiotropic loss of all molybdoenzyme activities and cause severe neurological damage. MOCS1 is defective in patients with group A deficiency and has been shown to encode two enzymes for early synthesis via a bicistronic transcript with two consecutive open reading frames (ORFs). MOCS2 encodes the small and large subunits of molybdopterin synthase via a single transcript with two overlapping reading frames. This gene was mapped to 5q and comprises seven exons. The coding sequence and all splice site-junction sequences were screened for mutations, in MoCo-deficient patients in whom a previous search for MOCS1 mutations had been negative. In seven of the eight patients whom we investigated, we identified MOCS2 mutations that, by their nature, are most likely responsible for the deficiency. Three different frameshift mutations were observed, with one of them found on 7 of 14 identified alleles. Furthermore, a start-codon mutation and a missense mutation of a highly conserved amino acid residue were found. The locations of the mutations confirm the functional role of both ORFs. One of the patients with identified MOCS2 mutations had been classified as type B, in complementation studies. These findings support the hypothetical mechanism, for both forms of MoCo deficiency, that formerly had been established by cell-culture experiments.

Localization of a gene for molybdenum cofactor deficiency, on the short arm of chromosome 6, by homozygosity mapping

Molybdenum cofactor deficiency (MoCoD) is a fatal disorder manifesting, shortly after birth, with profound neurological abnormalities, mental retardation, and severe seizures unresponsive to any therapy. The disease is a monogenic, autosomal recessive disorder, and the existence of at least two complementation groups suggests genetic heterogeneity. In humans, MoCoD leads to the combined deficient activities of sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. By using homozygosity mapping and two consanguineous affected kindreds of Israeli-Arab origin, including five patients, we demonstrated linkage of a MoCoD gene to an 8-cM region on chromosome 6p21.3, between markers D6S1641 and D6S1672. Linkage analysis generated the highest combined LOD-score value, 3.6, at a recombination fraction of 0, with marker D6S1575. These results now can be used to perform prenatal diagnosis with microsatellite markers. They also provide the only tool for carrier detection of this fatal disorder.

An HPLC assay for detection of elevated urinary S-sulphocysteine, a metabolic marker of sulphite oxidase deficiency

Sulphite oxidase deficiency occurs in man in two forms, as the isolated deficiency and as a syndrome of combined molybdoenzyme deficiency. This latter pleiotropic condition has as its underlying cause a defect in the synthesis of the molybdenum cofactor required for the activity of all molybdoenzymes in humans. Difficulties in diagnosis of sulphite oxidase deficiency are often encountered. A new method for detection of a key diagnostic metabolite, S-sulphocysteine, is outlined. The procedure is based on precolumn derivatization of urinary amino acids with dimethylaminoazobenzene sulphonyl chloride (Dabsyl-Cl) and resolution of the modified S-sulphocysteine by reversed-phase HPLC. A number of affected patients and control individuals with similar clinical symptoms have been studied, and a clear demarcation between the two groups has been noted.

Metabolic relationship between the CO dehydrogenase molybdenum cofactor and the excretion of urothione by Hydrogenophaga pseudoflava

Urothione was isolated as an excretion product of Hydrogenophaga pseudoflava and other bacteria at amounts approaching 253 micrograms/l of culture corresponding to 44 micrograms/g bacterial dry mass. The compound was identified as urothione by co-chromatography with urothione isolated from human urine, its characteristic ultraviolet and visible absorption spectra, oxidation to pterin-6-carboxylic-7-sulfonic acid by alkaline permanganate, 1H-NMR spectroscopy, double-quantum-filtered Fourier-transform 1H correlated spectroscopy, circular-dichroism spectroscopy and mass spectroscopy. A metabolic relationship between urothione and the carbon monoxide dehydrogenase molybdenum cofactor was suggested by a 1.1:0.5 molar ratio between urothione excreted and degradation of carbon monoxide dehydrogenase, a coincidence of urothione excretion and induction of carbon monoxide dehydrogenase with different species of carboxidotrophic bacteria, a structural relationship between molybdopterin cytosine dinucleotide of the carbon monoxide dehydrogenase molybdenum cofactor and urothione, and the demonstrated conversion of the carbon monoxide dehydrogenase molybdenum cofactor to urothione in vitro. A pathway for the conversion of the H. pseudoflava carbon monoxide dehydrogenase molybdenum cofactor to urothione has been proposed which involves molybdopterin cytosine dinucleotide, molybdopterin, phospho-norurothione and norurothione.

Molybdenum(VI) salts convert the xanthine oxidoreductase apoprotein into the active enzyme in mouse L929 fibroblastic cells

The mouse L929 fibroblastic cell line presents low, but detectable, levels of the mRNA encoding xanthine oxidoreductase under basal conditions, and it responds to type I and type II interferons by inducing the expression of the transcript [Falciani, Ghezzi, Terao, Cazzaniga, and Garattini (1992) Biochem. J. 285, 1001-1008]. This cell line, however, does not show any detectable amount of xanthine oxidoreductase enzymic activity, either before or after treatment with the cytokines. Molybdenum(VI) salts, in the millimolar range, are capable of activating xanthine oxidoreductase in L929 cells both under basal conditions and after treatment with interferon-alpha. The increase is observed in mouse L929 as well as in clones derived from it, but not in many other human and mouse cell lines. The induction observed in L929 cells is post-translational in nature and it is insensitive to cycloheximide, indicating that the molybdenum ion converts a pool of inactive xanthine oxidoreductase apoenzyme into its holoenzymic form. When grown in the absence of sodium molybdate, the L929 cell line has undetectable intracellular levels of the molybdenum cofactor, since the cell extracts are unable to complement the nitrate reductase defect of the nit-1 mutant of Neurospora crassa. L929 cells grown in the presence of millimolar concentrations of sodium molybdate, however, become competent to complement the nit-1 defect. L929 cells accumulate molybdenum ion inside the intracellular compartment as efficiently as TEnd cells, a mouse endothelial cell line that expresses xanthine oxidoreductase activity both under basal conditions and after treatment with interferon-gamma, suggesting that L929 cells have a defect in one or more of the metabolic steps leading to the synthesis of the molybdenum cofactor.

Molybdenum: an essential trace element

Molybdenum is found in most foods, with legumes, dairy products, and meats being the richest sources. This metal is considered essential because it is part of a complex called molybdenum cofactor that is required for the three mammalian enzymes xanthine oxidase (XO), aldehyde oxidase (AO), and sulfite oxidase (SO). XO participates in the metabolism of purines, AO catalyzes the conversion of aldehydes to acids, and SO is involved in the metabolism of sulfur-containing amino acids. Molybdenum deficiency is not found in free-living humans, but deficiency is reported in a patient receiving prolonged total parenteral nutrition with clinical signs characterized by tachycardia, headache, mental disturbances, and coma. The biochemical abnormalities in this acquired molybdenum deficiency include very low levels of uric acid in serum and urine (low XO activity) and low inorganic sulfate levels in urine (low SO activity). Inborn errors of isolated deficiencies of XO, SO, and molybdenum cofactor are described. Although XO deficiency is relatively benign, patients with isolated deficiencies of SO or molybdenum cofactor exhibit mental retardation, neurologic problems, and ocular lens dislocation. These abnormalities seem to be caused by the toxicity of sulfite and/or inadequate amounts of inorganic sulfate available for the formation of sulfated compounds present in the brain. XO and AO may also participate in the inactivation of some toxic substances, inasmuch as studies suggest that molybdenum deficiency is a factor in the higher incidence of esophageal cancer in populations consuming food grown in molybdenum-poor soil.

Sulfite sensitivity and sulfite oxidase activity in Drosophila melanogaster

The relationship between sulfite oxidase (SO) and sulfite sensitivity in Drosophila melanogaster is addressed. Significant improvements to the SO assay have provided an investigative tool which can be applied to further studies of this molybdoenzyme. Using the second-instar larval stage of D, melanogaster, we have shown a direct relationship between measured levels of sulfite oxidase activity and the organism's ability to withstand a sulfite challenge. Implementation of a sulfite-testing procedure confirmed the documented instability of sulfite in solution and may explain some of the conflicting results reported in the SO literature. Results of the tungstate-addition experiments confirm that Drosophila SO is a molybdoenzyme and its activity was shown to be governed by three of the four loci known to affect more than one molybdoenzyme. The ability of D. melanogaster to withstand the application of exogenous sulfites is shown to be dependent on sulfite oxidase activity.

Molybdenum cofactor deficiency in two siblings: diagnostic difficulties

Two siblings with molybdenum cofactor deficiency are presented. They showed clinical, biochemical and neuroradiological features very similar to those of the few previously described cases. Difficulties in diagnosis are emphasised.

Protective mechanism of sodium molybdate against the acute toxicity of cadmium in rats. II. Prevention of cytoplasmic acidification

In order to clarify the protective mechanism of sodium molybdate against the acute toxicity of cadmium chloride in rat, the effect of in vivo sodium molybdate pretreatment on the cytotoxic action of cadmium in isolated hepatocytes was studied. The cytosolic pH of hepatocytes isolated from untreated rats immediately decreased with incubation in either neutral Hank's balanced salt solution (HBS), pH 7.4, containing 5 microM cadmium chloride minimum or acidic HBS (pH 7.1, 6.8, 6.5, and 6.2). The presence of 5 microM cadmium in HBS adjusted to pH 7.1 aggravated cytosolic acidification induced by the acidic medium alone. Cell viability of hepatocytes incubated in HBS at pH 6.2 was significantly reduced as compared to that of control cells in HBS at pH 7.4, but the presence of cadmium in the acidic HBS had no aggravating action against such a toxic action of the acidic medium although cellular uptake of the metal in the medium increased, as compared to that in HBS at pH 7.4. Molybdenum pretreatment alleviated cytoplasmic acidification induced by the treatment with HBS at pH 7.4 or 7.1 containing cadmium or by extracellular acid load without cadmium. This pretreatment also prevented the loss of cell viability induced by the treatment with HBS at pH 6.2 but could not attenuate that when cadmium was present in the medium. These facts suggest that molybdenum pretreatment alleviated the acute toxicity of cadmium in rat by preventing cytoplasmic acidification caused by the harmful metal.

Prenatal diagnosis of molybdenum cofactor deficiency by assay of sulphite oxidase activity in chorionic villus samples

Molybdenum cofactor deficiency is characterized by the absence of sulphite oxidase, xanthine dehydrogenase and aldehyde oxidase, the three known enzymes in man that require the cofactor for their activity. Prenatal diagnosis of the deficiency may be performed by assay of sulphite oxidase activity in cultured amniocytes. However, the activity in amniocytes is low and large numbers of cells are required for reliable assessment. We show that sulphite oxidase is present at high levels in chorionic villi obtained at 10-14 weeks gestation and can be assayed directly in the biopsy sample without cell culture. This assay has been applied to two pregnancies at risk for molybdenum cofactor deficiency with successful diagnoses of an unaffected and an affected fetus.

Hepatic, placental, and fetal trace elements following molybdenum supplementation during gestation

The effect of dietary Mo (Na2Mo(4)2H2O) added to drinking water at levels of 0, 5, 10, 50, or 100 mg on hepatic (gestating dams), placental, and fetal Mo, Cu, Zn, and Fe contents of Sprague-Dawley rats was studied. These elements were determined by a polarographic catalytic procedure for Mo and by atomic absorption spectrophotometry for Cu, Fe, and Zn. Hepatic Mo increased two to sixfold (5-100 mg Mo). There was a 1.5-fold increase in hepatic Cu, significant only at the 50 to 100 mg Mo/L treatment levels. Although the hepatic Fe content of the gestating rats significantly increased with Mo supplementation, the extent of the increase appeared to be influenced by the litter size, fetal weights, and the degree of fetal resorption. Zinc values did not differ at any of the treatment levels. Placental Mo increased 3-76-fold, Cu one to threefold. No differences were observed in placenta Fe or Zn. Fetal Mo increased two to six-fold (10-100 mg/L) and Cu increased one to fivefold. There were no differences in the Fe and Zn content although both of these elements appeared to decline as the level of supplemental Mo increased. Significant correlations were also observed between hepatic, placental, and fetal Mo, Cu, Fe, and Zn. These results suggest that changes in trace mineral status in gestation, owing to high Mo intake, do occur and such occurrences are also reflected in the fetus.

Molybdenum cofactor biosynthesis in humans. Identification of two complementation groups of cofactor-deficient patients and preliminary characterization of a diffusible molybdopterin precursor

Molybdenum cofactor deficiency is a devastating disease with affected patients displaying the symptoms of a combined deficiency of sulfite oxidase and xanthine dehydrogenase. Because of the extreme lability of the isolated, functional molybdenum cofactor, direct cofactor replacement therapy is not feasible, and a search for stable biosynthetic intermediates was undertaken. From studies of cocultured fibroblasts from affected individuals, two complementation groups were identified. Coculture of group A and group B cells, without heterokaryon formation, led to the appearance of active sulfite oxidase. Use of conditioned media indicated that a relatively stable, diffusible precursor produced by group B cells could be used to repair sulfite oxidase in group A recipient cells. Although the extremely low levels of precursor produced by group B cells preclude its direct characterization, studies with a heterologous, in vitro reconstitution system suggest that the precursor that accumulates in group B cells is the same as a molybdopterin precursor identified in the Neurospora crassa molybdopterin mutant nit-1, and that a converting enzyme is present in group A cells which catalyzes an activation reaction analogous to that of a converting enzyme identified in the Escherichia coli molybdopterin mutant ChlA1.

Molecular and developmental genetics of the Punch locus, a pterin biosynthesis gene in Drosophila melanogaster

Punch (Pu), the gene encoding the pterin biosynthetic enzyme GTP cyclohydrolase in Drosophila, is a complex locus. Mutations fall into several complementation classes that correspond to classes of mutants with distinct morphological and protein phenotypes. Two of these classes are developmentally specific, with mutants in each having defects in discrete subsets of the known functions of the locus. Defined functions of the locus include a role in embryonic nuclear divisions using initially a maternal Pu product, the synthesis of pterin cofactors that are required for catecholamine biosynthesis beginning in late embryogenesis, and the production of pterin-screening pigments in the developing adult eye. Mutant phenotypes include an interruption in synchronous nuclear divisions in precellular blastoderm embryos, a segment pattern phenotype in late embryos, failure to pigment and cross-link embryonic cuticular structures and failure to synthesize red eye pigments. Molecular analysis reveals that the locus is large, a minimum of 29 kb as defined by Southern mapping of Pu mutants. This region is transcriptionally extremely active, encoding at least 16 developmentally regulated transcripts. One transcript has been shown to be responsible for the production of the adult eye GTP cyclohydrolase on the basis of developmental profile, location with respect to the mapping of eye-specific Pu mutants, absence in eye-specific mutants, and hybrid-selection in vitro translation experiments. Several other transcripts are candidates for Pu vital functions, as suggested by their pattern of expression and their derivation from regions to which lethal Pu mutations map.

Report on a new patient with combined deficiencies of sulphite oxidase and xanthine dehydrogenase due to molybdenum cofactor deficiency

A newborn infant exhibiting seizures and spastic tetraparesis at the age of 1 week was shown to excrete excessive quantities of sulphite, taurine, S-sulphocysteine and thiosulphate, characteristic of sulphite oxidase deficiency. In addition, increased renal excretion of xanthine and hypoxanthine combined with a low serum and urinary uric acid was consistent with xanthine dehydrogenase deficiency. Both deficiencies could be established at the enzyme level. The primary defect giving rise to the combined abnormalities is the absence of a molybdenum cofactor, a molybdenum-containing pterin being an essential component of both enzymes. The patient developed a severe neurological syndrome, brain atrophy and lens dislocation and died at the age of 22 months. Attempts at treatment, such as oral administration of ammonium molybdate, sodium sulphate, D-penicillamine, 2-mercaptoethane sulphonic acid, pyridoxine and thiamine did not influence the clinical course.

Analysis and clinical significance of pterins

This review briefly describes the biochemistry of pterins, their involvement in pathological processes and the use of pterin measurement in diagnosis and monitoring of disease. Chromatographic and other methods of pterin analysis are detailed with particular emphasis being placed on the need for correct sample collection and handling.

Hereditary xanthinuria in 2 Pakistani sisters: asymptomatic in one with beta-thalassemia but causing xanthine stone, obstructive uropathy and hypertension in the other

We describe a 3-year-old Pakistani girl who presented with recurrent urinary infections. She had a nonfunctioning hydronephrotic right kidney and hypertension. At operation a calculus was impacted in the right ureter with dilatation of the pelviocaliceal system. Nephrectomy was performed. Histology revealed end stage pyelonephritis. The calculus consisted of pure xanthine. Further investigations demonstrated low serum uric acid and absent urinary uric acid with increased excretion of xanthine. Eight months after nephrectomy blood pressure had decreased to normal. Her 5-year-old sister, who has beta-thalassemia, also has a low serum uric acid concentration and xanthinuria. The treatment of choice is to increase fluid intake so that the urine xanthine concentration remains below the level at which xanthine crystallizes. This may require adjustment of the urine pH.

Mendelian etiologies of stroke

There are many genetic disorders associated with an increased risk for stroke that may easily be overlooked in the evaluation of both adult and pediatric acute stroke victims. The recognition of a genetic disorder as the cause of a stroke has important implications not only for the immediate care of the stroke victim, but often also for others in the patient's family who may be at risk for the same disease and for whom preventive measures sometimes can be taken. We present here a comprehensive review of genetic disorders associated with stroke in the nongeriatric age groups for which a causative role in the evolution of stroke has been recognized or is likely. For each disorder, the major clinical and biochemical characteristics as well as the probable pathogenetic mechanisms of stroke are discussed, together with the appropriate testing required to screen for and confirm the diagnosis. The great variety of genetic disorders and mechanisms causing stroke underscores the increasing importance of understanding genetic disease for appropriate diagnosis and treatment of a common clinical problem affecting both children and adults.

Sulfite hypersensitivity. A critical review

Sulfiting agents (sulfur dioxide and the sodium and potassium salts of bisulfite, sulfite, and metabisulfite) are widely used as preservatives in foods, beverages, and pharmaceuticals. Within the past 5 years, there have been numerous reports of adverse reactions to sulfiting agents. This review presents a comprehensive compilation and discussion of reports describing reactions to ingested, inhaled, and parenterally administered sulfite. Sulfite hypersensitivity is usually, but not exclusively, found within the chronic asthmatic population. Although there is some disagreement on its prevalence, a number of studies have indicated that 5 to 10% of all chronic asthmatics are sulfite hypersensitive. This review also describes respiratory sulfur dioxide sensitivity which essentially all asthmatics experience. Possible mechanisms of sulfite hypersensitivity and sulfur dioxide sensitivity are discussed in detail. Sulfite metabolism and the role of sulfite oxidase in the detoxification of exogenous sulfite are reviewed in relationship to the etiology of sulfite hypersensitivity.

Epoxidation of (+/-)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene during (bi)sulfite autoxidation: activation of a procarcinogen by a cocarcinogen

The (bi)sulfite ion undergoes extensive autoxidation in neutral aqueous media with the formation of sulfur trioxide radical anion that is detected by ESR. The radical anion subsequently reacts with molecular oxygen to form a peroxyl radical. We find that when (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) is included in this autoxidation system, BP-7,8-diol is converted to diolepoxides, ultimate carcinogenic derivatives of benzo[a]pyrene. This epoxidation occurs with a stereoselectivity consistent with either a peroxyl radical or a peracid as the epoxidizing agent. The epoxidation is dependent on the concentration of both (bi)sulfite and oxygen. In the presence of 10 microM butylated hydroxyanisole, which abolishes (bi)sulfite autoxidation, no (bi)sulfite-dependent epoxidation occurs. These results are discussed in regard to the mechanism of (bi)sulfite autoxidation, and in relationship to the cocarcinogenicity of sulfur dioxide [anhydrous (bi)sulfite] for benzo[a]pyrene-induced pulmonary neoplasia.

Molybdenum hydroxylases in Drosophila. III. Further characterization of the low xanthine dehydrogenase gene

The biochemical effects of several newly induced low xanthine dehydrogenase (lxd) mutations in Drosophila melanogaster were investigated. When homozygous, all lxd alleles simultaneously interrupt each of the molybdoenzyme activities to approximately the same levels: xanthine dehydrogenase, 25%; aldehyde oxidase, 12%; pyridoxal oxidase, 0%; and sulfite oxidase, 2% as compared to the wild type. In order to evaluate potentially small complementation or dosage effects, mutant stains were made coisogenic for 3R. These enzymes require a molybdenum cofactor, and lxd cofactor levels are also reduced to less than 10% of the wild type. These low levels of molybdoenzyme activities and cofactor activity are maintained throughout development from late larval to adult stages. The lxd alleles exhibit a dosage-dependent effect on molybdoenzyme activities, indicating that these mutants are leaky for wild-type function. In addition, cofactor activity is dependent upon the number of lxd+ genes present. The lxd mutation results in the production of more thermolabile XDH and AO enzyme activities, but this thermolability is not transferred with the cofactor to a reconstituted Neurospora molybdoenzyme. The lxd gene is localized to salivary region 68A4-9, 0.1 map unit distal to the superoxide dismutase (Sod) gene.

Use of biological fluids for the rapid diagnosis of potentially lethal inherited disorders of human purine and pyrimidine metabolism

Inherited purine and pyrimidine disorders may be associated with serious, sometimes life-threatening consequences. Early and accurate diagnosis is essential. Difficulties encountered when using existing high pressure liquid chromatographic (HPLC) methods led to the development of an improved method based on prior fractionation of urine. The advantages are as follows. 1. Production of fingerprints demonstrating altered urinary excretion patterns characteristic of any one of ten different disorders, in 30 minutes. 2. Positive identification and quantification by comparison with established methods (using conventional chromatography, electrophoresis and UV spectrophotometry) in addition to specific retention times and characteristic UV absorbance ratios at two separate wavelengths (245 and 280 nm) by HPLC. 3. Direct analysis of all the purines and pyrimidines normally found in human body fluids as well as identification of abnormal compounds. 4. Short time between successive analyses while maintaining excellent resolution between compounds of interest and column longevity. 5. Improved separation of the different adenine-based compounds encountered in some disorders, plus demonstration of potential interference by dietary or drug metabolites. 6. Applicability to the monitoring of therapy involving a variety of different purine and pyrimidine analogues. Particular attention should be paid to sample preparation. Plasma profiles will confirm the diagnosis in some, but not all, of these disorders.

Of mice and men, metals and mutations

Several mutations affecting the transport of copper and zinc in humans and in mice have been discovered over the last 15 years, joining the long known disturbance of copper transport in Wilson's disease. Menkes' disease (classical and mild variant forms) and X linked Ehlers-Danlos syndrome (type IX, X linked cutis laxa) have features in common with one another and with the brindled (Mobr) and blotchy (Moblo) mouse mutants, respectively. There may be one allelic series of mutants in each species or two loci may be involved in each. The toxic milk mutant (tx) in the mouse may be homologous to Wilson's disease in man. The defect of intestinal absorption of zinc in acrodermatitis enteropathica has no homologue yet in the mouse. However, the lethal milk (lm) mutant in the mouse may be homologous to a condition of zinc deficiency described in a few breastfed, low birth weight infants. Many more genetic defects of transport of copper and of zinc may await discovery. Conversely, these mutants are valuable in elucidating the normal processes of copper and zinc transport.

Combined xanthine and sulphite oxidase defect due to a deficiency of molybdenum cofactor

Increased urinary excretion of xanthine, hypoxanthine, sulphite, thiosulphate and decreased serum uric acid were observed in an infant with profound failure to thrive. Other clinical findings included refractory seizures, spastic quadriplegia and profound psychomotor retardation. The patient died at 20 months of age. There were no detectable activities for xanthine oxidase and sulphite oxidase in the postmortem liver. Urothione, which is the metabolic excretory product of the molybdenum cofactor for molybdoenzymes was not present in the urine. A deficiency of the molybdenum cofactor which is common to both xanthine and sulphite oxidase is presumed to be the metabolic defect responsible for the absent activities of both enzymes.

Genetic trace metal disturbances

Genetic trace metal disturbances can be at three levels. Trace metals play an important role in the metabolism of genetic macromolecules and the information transfer system. Deficiency or excess of trace metals caused either by dietary or genetic factors will affect the normal functioning of the whole organism. The roles of trace metals in carcinogenesis/mutagenesis and ageing are typical of this category. The second level of genetic trace metal disturbances affect the metabolic pathway of the trace metal itself. Biochemical derangement resulting from genetic defects cause aberrant metabolism of the element and thus disease symptoms. Diseases caused by abnormal metabolism of copper, zinc, iron, and molybdenum are discussed. Trace metal disturbances can also be the result of other genetic diseases. This aspect of genetic trace metal disturbances is least investigated. However, information should be important for improving the existing treatment protocol for the more common inborn errors of metabolism, such as phenylketonuria.

Sulphiting agents in foods: some risk/benefit considerations

The current toxicological status of sulphiting agents is reviewed, including evidence of adverse reactions to sulphited foods by a sub-population of asthmatics. Against this background are assessed the applications and benefits of sulphiting agents in foods. It is concluded that further information is required to determine the magnitude of risk and that, in the interim, the controlled use of sulphiting agents is justifiable.

Quantitative transfer of the molybdenum cofactor from xanthine oxidase and from sulphite oxidase to the deficient enzyme of the nit-1 mutant of Neurospora crassa to yield active nitrate reductase

An assay method is described for measurement of absolute concentrations of the molybdenum cofactor, based on complementation of the defective nitrate reductase ('apo nitrate reductase') in extracts of the nit-1 mutant of Neurospora crassa. A number of alternative methods are described for preparing, anaerobically, molybdenum-cofactor-containing solutions from sulphite oxidase, xanthine oxidase and desulpho xanthine oxidase. For assay, these were mixed with an excess of extract of the nit-1 mutant, incubated for 24 h at 3.5 degrees C then assayed for NADPH:nitrate reductase activity. In all cases, the specific activity of the molybdenum cofactor, expressed as mumol of NO2-formed/min per ng-atom of Mo added from the denatured molybdoenzyme , was 25 +/- 4, a value that agrees with the known catalytic activity of the nitrate reductase of wild-type Neurospora crassa. This indicates that, under our conditions, there was quantitative transfer of the molybdenum cofactor from denatured molybdoenzyme to yield fully active nitrate reductase. Comparable cofactor assay methods of previous workers, apparently indicating transfer efficiencies of at best a few per cent, have never excluded satisfactorily the possibility that cofactor activity arose, not from stoichiometric constituents of the molybdoenzymes , but from contaminants. The following factors were investigated separately in developing the assay:the efficiency of extraction of the cofactor from the original enzyme, the efficiency of the complementation reaction between cofactor and apo nitrate reductase, and the assay of the resultant nitrate reductase, which must be carried out under non-inhibitory conditions. Though the cofactor is unstable in air (t1/2 about 15 min at 3.5 degrees C), it is stable when kept anaerobic in the presence of sodium dithionite, in aqueous solution or in dimethyl sulphoxide (activity lost at the rate of about 3%/24 h at 20-25 degrees C). Studies of stabilities, and investigations of the effect of added molybdate on the assay, permit conclusions to be drawn about the ligation of molybdenum to the cofactor and about steps in incorporation of the cofactor into the apoenzyme. Though the development of nitrate reductase activity is slow at 3.5 degrees C (t1/2 1.5-3 h) the complementation reaction may be carried out in high yield, aerobically. This is ascribed to rapid formation of an air-stable but catalytically inactive complex of the cofactor, as a precursor of the active nitrate reductase.(ABSTRACT TRUNCATED AT 400 WORDS)