Assignment of the human ferrochelatase gene (FECH) and a locus for protoporphyria to chromosome 18q22

We have mapped the human gene for ferrochelatase (FECH; ferroheme-protolyase, EC 4.99.1.1) to chromosome 18 by hybridization of cDNA to sorted chromosomes. The probe was obtained by PCR-directed amplification of a human marrow cDNA library in lambda gt 10. Subchromosomal localization of ferrochelatase to 18q22 was determined by chromosomal hybridization in situ using a human ferrochelatase genomic clone in lambda EMBL 3 that contained a 20-kb insert. Since ferrochelatase activity is deficient in patients with the inherited disease erythropoietic protoporphyria, a locus for this disease may be assigned to 18q22, one of few monogenic defects that have been mapped to this chromosome.

Biosynthesis of delta-aminolevulinic acid and the regulation of heme formation by immature erythroid cells in man

Heme formation in the erythron is subject to end product regulation by negative feedback, but the exact point of metabolic control in human erythroid cells is unknown. To investigate the mode of action of heme on its own formation, the effects of micromolar concentrations of hemin on de novo synthesis of protoporphyrin IX and delta-aminolevulinate (delta-ALA) by intact human reticulocytes were examined in the presence of 1 mM alpha,alpha'-bipyridyl and 200 microM 4,6-dioxoheptanoate to block their further conversion by ferrochelatase or delta-ALA dehydrase, respectively. At final concentrations (25-40 microM), hemin, which is known to reduce incorporation of [2-14C]glycine into cellular heme, significantly inhibited formation of protoporphyrin IX and total delta-aminolevulinate in situ by these cells. Since synthesis of the first committed precursor, delta-aminolevulinate, as well as protoporphyrin (which is derived from it) were diminished, the effects of hemin on delta-aminolevulinate synthase (EC 2.3.1.37) were studied. Hemin, at concentrations up to 40 microM, had no direct effect on enzymatic activity, as measured with [5-14C] alpha-ketoglutarate (in hypotonically lysed cells) or [1,4-14C]succinyl coenzyme A (in deoxycholate lysates), even after preincubation. However, when intact human reticulocytes were incubated with hemin before assay for delta-ALA synthase, there was a rapid, concentration-dependent reduction in enzymatic activity (mean 42 and 23% inhibition after 60 min for these two substrates, respectively). Hemin had no effect on steady-state levels of delta-ALA synthase mRNA, as determined by Northern blot hybridization using an erythroid-specific human cDNA probe. Thus, a mechanism for inducing feedback inhibition of the tetrapyrrole pathway exists in human erythroid cells. It controls formation of the first committed precursor of protoporphyrin IX, delta-aminolevulinate, and hence regulates heme biosynthesis by limiting the availability of the porphyrin, rather than the metal substrate for the ferrochelatase reaction. Hemin interacts with constituents of the intact reticulocyte significantly to reduce delta-aminolevulinic acid synthase activity by an indirect cellular process that does not influence the abundance of erythroid-specific synthase mRNA but may either inhibit its ribosomal translation in an unknown manner or promote degradation of the enzyme itself by specific proteolysis.

Aldolase B mutations in Italian families affected by hereditary fructose intolerance

Hereditary fructose intolerance (HFI) is an inborn error of metabolism caused by aldolase B deficiency. The aldolase B gene has been cloned and the following mutations causing HFI have been identified: A149P (a G----C transversion in exon 5), A174D (a C----A transversion in exon 5), L288 delta C (a base pair deletion in exon 8), and N334K (a G----C transversion in exon 9). We have investigated the occurrence of these mutations in 11 Italian patients affected by HFI using PCR and hybridisation to specific oligomers. We found that four patients were homozygous for the A149P mutation, two patients were homozygous for the A174D mutation, three patients were compound heterozygotes for both the A149P and A174D mutations, one patient was homozygous for the N334K mutation, and one patient did not show any of the reported mutations (HFI diagnosis carried out by aldolase B assay). The L288 delta C mutation has not been found in this survey.

Regulation and expression of type V (tartrate-resistant) acid phosphatase in human mononuclear phagocytes

Human type V (tartrate-resistant) acid phosphatase belongs to a unique group of iron-binding proteins that includes uteroferrin and other purple phosphatases. The enzyme is normally restricted to osteoclasts and certain phagocytic cells but its rôle is unknown. We show that phosphatase mRNA is abundant in cells of monohistiocytic phenotype and that enzyme expression in cultured human monocyte-derived macrophages is depressed by gamma-interferon and bacterial lipopolysaccharide, agents that promote functional differentiation in these cells. In contrast, phorbol ester, which stimulates intracellular calcium-mediated events, greatly enhances type V phosphatase expression and mRNA abundance. Lymphokine and phorbol ester-modulated expression of type V acid phosphatase expression thus represents a model system for investigating proliferative responses that are specific to cells of the mononuclear macrophage system.

Molecular analysis of the human MHC class I region in hereditary haemochromatosis. A study by pulsed-field gel electrophoresis

The unknown allele that predisposes to the development of haemochromatosis in man has been localized to the HLA class I region on the short arm of chromosome 6. We have utilized pulsed-field gel electrophoresis in conjunction with probes that map within, or in the vicinity of, this region to search for structural lesions that may further define the disease locus. Using the enzyme Mlu I, fragments that associated specifically with the HLA-A23, A31 and B8 alleles were identified. However, in members of three pedigrees affected by haemochromatosis, and in six unrelated patients with the disorder, no disease-specific differences were detected in the DNA fragments with four restriction enzymes and eight probes when compared with healthy individuals. These data suggest that the lesion responsible for hereditary haemochromatosis lies beyond the resolution of this technique and does not involve large structural deletions or extensive re-arrangements in this highly polymorphic region of the genome.

Genetic heterogeneity in erythropoietic protoporphyria: a study of the enzymatic defect in nine affected families

Erythropoietic protoporphyria (EPP) is associated with a deficiency of protohaem ferrolyase. We have used a novel assay for this enzyme based on its ability to utilize zinc as a substrate to investigate the inheritance of EPP in nine affected families. Zinc chelatase activity was markedly reduced in peripheral blood mononuclear cells from 14 EPP patients (mean, 3.3 nmol Zn protohaem/h/mg protein; range, 0.3-8.0) when compared with 41 controls (16.8 +/- 3.6) p less than 0.01. In three families with parent-to-child transmission of disease, the asymptomatic parent had an enzymatic activity within the normal range. In three pedigrees where the parents were asymptomatic, enzymatic activities were below the 95% confidence limits in both. Zinc chelatase activity was below the mean control value in 17 of the 18 parents in nine affected pedigrees, and six of seven asymptomatic offspring of patients with protoporphyria. The findings suggest that EPP is not transmitted as a simple dominant trait and that inheritance of more than one gene may be required for disease expression.

Partial aldolase B gene deletions in hereditary fructose intolerance

Hereditary fructose intolerance (HFI) is an autosomal recessive condition caused by a deficiency of aldolase B. We have recently shown that three point mutations in this gene account for approximately 85% of HFI alleles in Europe and the United States and are thus of diagnostic importance. In this paper we define three new lesions in the aldolase B gene: two are large deletions, one of 1.65 kb and one of 1.4 kb; the third is a small deletion of 4 bp. We have determined the breakpoints of these deletions and have demonstrated that the presence of such lesions may complicate the genotyping of individuals for diagnosis of HFI.

Vitamin B-12 and folate deficiency presenting as leukaemia

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Type 5 acid phosphatase. Sequence, expression and chromosomal localization of a differentiation-associated protein of the human macrophage

The purple acid phosphatases and uteroferrin belong to a diverse multifunctional class of binuclear iron-containing proteins that includes haemerythrin and ribonucleotide reductase. In the pig, uteroferrin has been implicated in the delivery of iron to the foetus, but the role of the related human type 5 acid phosphatase that is principally found in resident tissue macrophages is not yet clear. To define further the function of this metalloenzyme, we have isolated and sequenced a cDNA clone for type 5 acid phosphatase and investigated expression of its gene in human tissues. The phosphatase clone contains an open reading frame of 975 bp and encodes a protein of 325 amino acids, including a signal peptide of 19 residues and two potential sites for N-glycosylation. The type 5 acid phosphatase gene mapped to the short arm of human chromosome 19 and was found to have a restriction fragment length polymorphism on digestion with XbaI. Expression of phosphatase mRNA was restricted to mononuclear phagocytes and the enzyme was induced greater than 20-fold on transformation of normal human monocytes to macrophages by culture in serum-supplemented medium. Type 5 acid phosphatase thus represents a tightly regulated system for the study of molecular events in the differentiation programme of the normal macrophage.

A new aldolase B variant, N334K, is a common cause of hereditary fructose intolerance in Yugoslavia

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Hereditary fructose intolerance

Hereditary fructose intolerance (HFI) is an inborn error of carbohydrate metabolism that is inherited as an autosomal recessive condition. The disease is caused by a catalytic deficiency of aldolase B and is characterized by severe abdominal symptoms and hypoglycaemia which follow the ingestion of fructose, sucrose or sorbitol. The exact prevalence of HFI in different populations is unknown but studies from Switzerland suggest a disease frequency of about 1 in 20,000 live births, thus predicting a carrier frequency of greater than 1% and a gene prevalence that approaches polymorphic frequency. It is notable that many patients who endure severe symptoms during early infancy develop a marked aversion to harmful foodstuffs and thereby survive to adulthood. Although exposure to fructose may prove to be fatal in this disorder, institution of a strict exclusion diet is curative. HFI, when treated rigorously after diagnosis, is thus compatible with a long and healthy life. HFI vividly illustrates the interplay of dietary factors and heredity in the development of human disease. The recent identification of genetic lesions that cause this disorder further demonstrates the remarkable clinical benefits that may accrue from the study of the molecular basis of inherited diseases and its population genetics: it is now possible to detect asymptomatic disease carriers and diagnose the disorder in affected families by non-invasive analysis of small samples of genomic DNA. Moreover, the systematic investigation of natural mutations in the human gene for aldolase B has allowed regions that are critical for catalytic function of this enzyme to be identified as part of an extended study of its molecular biology.

Life threatening thrombocytopenia in sarcoidosis

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Molecular analysis of aldolase B genes in the diagnosis of hereditary fructose intolerance in the United Kingdom

To investigate the molecular basis of hereditary fructose intolerance, we have studied 12 British patients, all of whom were found to carry a single mutation in the gene coding for aldolase B. We have estimated the frequency of this lesion, termed A149P, amongst affected individuals in the population and predict that a diagnosis may be made non-invasively in more than 83 per cent of cases by demonstrating the presence of this allele. Genetic diagnosis and detection of asymptomatic carriers of the disease may be achieved by the specific amplification of DNA derived from mouthwash samples followed by hybridization to allele-specific oligonucleotides.

Campylobacter pylori, duodenal ulcer disease, and gastrin

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Imaging amyloidosis in Still's disease

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An immunoassay of human band 5 ("tartrate-resistant") acid phosphatase that involves the use of anti-porcine uteroferrin antibodies

We describe an immunoassay for human band-5 acid phosphatase in which antibodies to porcine uteroferrin, immobilized on Sepharose particles, are used. Band-5 acid phosphatase is the tartrate-resistant isoenzyme normally expressed in tissue macrophages such as osteoclasts and alveolar macrophages. The immunoassay is similar in reproducibility and sensitivity to assays based on inhibition by d-tartrate. However, compared with the latter, the greater specificity of the immunoassay makes it markedly less susceptible to errors arising from the presence of non-band-5 acid phosphatases, e.g., from prostate.

An immunoassay of human band 5 ("tartrate-resistant") acid phosphatase that involves the use of anti-porcine uteroferrin antibodies

We describe an immunoassay for human band-5 acid phosphatase in which antibodies to porcine uteroferrin, immobilized on Sepharose particles, are used. Band-5 acid phosphatase is the tartrate-resistant isoenzyme normally expressed in tissue macrophages such as osteoclasts and alveolar macrophages. The immunoassay is similar in reproducibility and sensitivity to assays based on inhibition by d-tartrate. However, compared with the latter, the greater specificity of the immunoassay makes it markedly less susceptible to errors arising from the presence of non-band-5 acid phosphatases, e.g., from prostate.

A potent inhibitor of Taq polymerase copurifies with human genomic DNA

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Zinc chelatase in human lymphocytes: detection of the enzymatic defect in erythropoietic protoporphyria

We describe a fluorometric assay for heme synthetase, the enzyme that is genetically deficient in erythropoietic protoporphyria. The method, which can readily detect activity in 1 microliter of packed human lymphocytes, is based on the formation of zinc protoheme from protoporphyrin IX. That zinc chelatase and ferrochelatase activities reside in the same enzyme was shown by the competitive action of ferrous ions and the inhibitory effects of N-methyl protoporphyrin (a specific inhibitor of heme synthetase) on zinc chelatase. The Km for zinc was 11 micrograms and that for protoporphyrin IX was 6 microM. The Ki fro ferrous ions was 14 microM. Zinc chelatase was reduced to 15.3% of the mean control activity in lymphocytes obtained from patients with protoporphyria, thus confirming the defect of heme biosynthesis in this disorder. The assay should prove to be useful for determining heme synthetase in tissues with low specific activity and to investigate further the enzymatic defect in protoporphyria.

Clonal rearrangement of the T-cell receptor gamma gene associated with a bizarre lymphoproliferative syndrome

Investigations in an unusual patient with prolonged fever, fat necrosis and haemorrhagic tendency showed widespread histiocytic phagocytosis in fat and liver biopsy specimens. Lymph nodes were also infiltrated by morphologically normal T lymphocytes predominantly expressing the OKT4 antigen. Analysis of lymph node DNA using immunoglobulin and T-cell receptor (TCR) probes showed rearrangement restricted to the gamma TCR gene combined with polyclonal beta TCR gene rearrangements, thus providing evidence of a unique lymphoproliferative disorder of T cells. The patient displayed a striking response to cytotoxic chemotherapy.

Catalytic deficiency of human aldolase B in hereditary fructose intolerance caused by a common missense mutation

Hereditary fructose intolerance (HFI) is a human autosomal recessive disease caused by a deficiency of aldolase B that results in an inability to metabolize fructose and related sugars. We report here the first identification of a molecular lesion in the aldolase B gene of an affected individual whose defective protein has previously been characterized. The mutation is a G----C transversion in exon 5 that creates a new recognition site for the restriction enzyme Ahall and results in an amino acid substitution (Ala----Pro) at position 149 of the protein within a region critical for substrate binding. Utilizing this novel restriction site and the polymerase chain reaction, the patient was shown to be homozygous for the mutation. Three other HFI patients from pedigrees unrelated to this individual were found to have the same mutation: two were homozygous and one was heterozygous. We suggest that this genetic lesion is a prevailing cause of hereditary fructose intolerance.

Biosynthesis of heme in immature erythroid cells. The regulatory step for heme formation in the human erythron

Heme formation in reticulocytes from rabbits and rodents is subject to end product negative feedback regulation: intracellular "free" heme has been shown to control acquisition of transferrin iron for heme synthesis. To identify the site of control of heme biosynthesis in the human erythron, immature erythroid cells were obtained from peripheral blood and aspirated bone marrow. After incubation with human 59Fe transferrin, 2-[14C]glycine, or 4-[14C]delta-aminolevulinate, isotopic incorporation into extracted heme was determined. Addition of cycloheximide to increase endogenous free heme, reduced incorporation of labeled glycine and iron but not delta-aminolevulinate into cell heme. Incorporation of glycine and iron was also sensitive to inhibition by exogenous hematin (Ki, 30 and 45 microM, respectively) i.e. at concentrations in the range which affect cell-free protein synthesis in reticulocyte lysates. Hematin treatment rapidly diminished incorporation of intracellular 59Fe into heme by human erythroid cells but assimilation of 4-[14C]delta-aminolevulinate into heme was insensitive to inhibition by hematin (Ki greater than 100 microM). In human reticulocytes (unlike those from rabbits), addition of ferric salicylaldehyde isonicotinoylhydrazone, to increase the pre-heme iron pool independently of the transferrin cycle, failed to promote heme synthesis or modify feedback inhibition induced by hematin. In human erythroid cells (but not rabbit reticulocytes) pre-incubation with unlabeled delta-aminolevulinate or protoporphyrin IX greatly stimulated utilization of cell 59Fe for heme synthesis and also attenuated end product inhibition. In human erythroid cells heme biosynthesis is thus primarily regulated by feedback inhibition at one or more steps which lead to delta-aminolevulinate formation. Hence in man the regulatory process affects generation of the first committed precursor of porphyrin biosynthesis by delta-aminolevulinate synthetase, whereas in the rabbit separate regulatory mechanisms exist which control the incorporation of iron into protoporphyrin IX.

Cyclophosphamide-induced liver necrosis: a possible interaction with azathioprine

Cyclophosphamide is a valuable drug for the treatment of vasculitis and hepatic toxicity resulting from its use has been reported rarely. We describe four patients in whom cyclophosphamide was associated with liver injury when its administration had been preceded by azathioprine. Microscopy of hepatic biopsy material in three of these cases showed liver cell necrosis. In two of the patients, cyclophosphamide had been given previously without earlier azathioprine and hepatic damage had not been noted. An apparent interaction of cyclophosphamide with azathioprine to cause liver cell necrosis has important implications for the treatment of patients with vasculitis and related disorders.

Antibodies to porcine uteroferrin used in measurement of human tartrate-resistant acid phosphatase

The immunological similarity between human tartrate-resistant acid phosphatase (EC 3.1.3.2) and porcine uteroferrin previously reported for the isoenzyme from spleens of patients with leukemic reticuloendotheliosis (Ketcham et al., J Biol Chem 1985;260:5768-76) has been confirmed for partly purified acid phosphatase found in the spleen of a patient with Gaucher's disease, and for the corresponding isoenzyme in other tissues and serum. Anti-uteroferrin antibodies raised in rabbits have been used to demonstrate the feasibility of their application in an immunoassay for tartrate-resistant acid phosphatase in serum.

Antibodies to porcine uteroferrin used in measurement of human tartrate-resistant acid phosphatase

The immunological similarity between human tartrate-resistant acid phosphatase (EC 3.1.3.2) and porcine uteroferrin previously reported for the isoenzyme from spleens of patients with leukemic reticuloendotheliosis (Ketcham et al., J Biol Chem 1985;260:5768-76) has been confirmed for partly purified acid phosphatase found in the spleen of a patient with Gaucher's disease, and for the corresponding isoenzyme in other tissues and serum. Anti-uteroferrin antibodies raised in rabbits have been used to demonstrate the feasibility of their application in an immunoassay for tartrate-resistant acid phosphatase in serum.

Molecular forms of porphobilinogen deaminase in acute intermittent porphyria. A study by Western immunoblotting

Acute intermittent porphyria is an inborn error of haem synthesis which is transmitted as a dominant character with variable phenotypic expression. The disorder is caused by a partial deficiency of porphobilinogen deaminase in all tissues so far studied. The nature of the enzymatic deficiency of porphobilinogen deaminase in haemolysates from patients with acute intermittent porphyria was examined by the use of monospecific antibody probes. In affected heterozygotes from three British pedigrees of diverse ancestry, the catalytic deficiency of porphobilinogen deaminase was accompanied by diminished enzyme protein, as determined by radial immunodiffusion. No evidence of functionally attenuated enzyme was demonstrable by kinetic studies. The molecular forms of the residual enzyme were investigated in red cell extracts and in lysed preparations of reticulocytes by a sensitive Western blotting procedure. This revealed the presence of reduced amounts of porphobilinogen deaminase polypeptide co-migrating with wild type enzyme (Mr approximately 40,000), and no evidence of variant forms in situ. The studies show that porphobilinogen deaminase deficiency in acute intermittent porphyria is commonly associated with a CRM-phenotype. The residual activity under these circumstances is thus related to expression of a single normal allele, since sensitive techniques detected neither aberrant nor degraded forms of the enzyme in erythroid tissues.

Acute myopericarditis in influenza A infection

Heart disease is a recognised complication of influenza. We report a unique case in which myopericarditis and collapse due to acute influenza A infection was associated with pericardial effusion and tamponade. In addition, the patient had myositis and pleurisy. Emergency pericardiocentesis and inotropic drugs were needed but recovery was complete.

Biosynthesis of the transferrin receptor in rabbit reticulocytes

These studies were performed to determine whether the reticulocyte can synthesize its own transferrin receptor and, if so, whether synthesis is subject to translational control by intracellular heme. Reticulocytosis (20-35%) was produced by bleeding rabbits and the washed cells were incubated for 1-4 h at 37 degrees C in buffered nutritional medium containing L-[35S]methionine. After washing and detergent lysis in the presence of protease inhibitors, supernatant reticulocyte extracts were analyzed for transferrin receptors by immunoprecipitation with specific ovine receptor antibody raised against denatured rabbit transferrin receptor. Immunoprecipitates were analyzed by SDS-gel electrophoresis and fluorography. Antibody, but not preimmune sheep immunoglobin, consistently precipitated a 35S-labeled protein with an Mr of 90,000 (reduced), coincident with bona fide receptor subunits purified by ligand-affinity chromatography. Incorporation of radioactive methionine was exclusively associated with receptor in reticulocyte stroma, and nascent receptor was not detected on free polyribosomes. Incorporation of radioactivity in the receptor moiety accounted for 0.1-0.2% of total incorporation into TCA insoluble cell protein. Treatment of the cells with 40 micrograms/ml cycloheximide markedly inhibited amino acid incorporation into the receptor, thus indicating de novo synthesis of receptor protein. On treatment of reticulocytes with 4,6 dioxoheptanoate to induce heme deficiency by diminishing the formation of intracellular heme, synthesis of the receptor was inhibited by greater than 50%; synthesis was restored to control rates on addition of 50 microM exogenous hemin. These findings indicate that the reticulocyte retains receptor mRNA and that synthesis of the receptor in erythroid cells is subject to translational regulation by intracellular heme.

Pancreatic exocrine and endocrine responses in chronic pancreatitis

To test the discriminatory potential of certain indices of pancreatic function we performed duodenal perfusion studies and measured trypsin, bicarbonate, and lactoferrin outputs, and plasma concentrations of pancreatic polypeptide and motilin in the basal state and during continuous intravenous stimulation with 100 ng kg-1h-1 Ceruletide and 1 CU kg-1h-1 secretin. The following groups were studied: 12 normal volunteers (NV), seven patients with chronic pancreatitis with steatorrhea (CPS), and seven without steatorrhea (CP). Stimulated trypsin outputs, after 45 min of stimulation, were the best discriminant among the groups (NV versus CPS, p less than 0.0005; NV versus CP, p less than 0.005; CP versus CPS, p less than 0.05). Basal trypsin outputs showed similar patterns but failed to discriminate between NV and CP. Bicarbonate outputs were less discriminatory than trypsin outputs. Lactoferrin outputs failed to discriminate, but transient high peak outputs occurred in the initial stimulation period in all four patients with calcific chronic pancreatitis, suggesting a washout phenomenon. Basal motilin levels were elevated in both groups of pancreatitis (p less than 0.05). Stimulated pancreatic polypeptide levels were lower in CPS (NV versus CPS, p less than 0.05) but higher in CP (NV versus CP, p less than 0.005). These differences were also apparent in the basal state. We conclude that the best discrimination among the three groups was achieved by measurement of trypsin outputs, after 45 min of stimulation. In addition, the pancreatic polypeptide response may be used as a marker of residual pancreatic function in chronic pancreatitis.

Allelic heterogeneity in adult hereditary fructose intolerance. Detection of structural mutations in the aldolase B molecule

Hereditary fructose intolerance (HFI) is a disorder of visceral carbohydrate metabolism which is transmitted as a recessive character of moderate to high gene prevalence. The condition is caused by enzymic deficiency of aldolase B and is associated with the synthesis of inactive enzyme protein. The molecular structure of aldolase B was examined in tissue samples from four adult patients who were the offspring of non-consanguineous unions. Titration of aldolase protein, by radioimmunoassay, showed that antibody recognition of the inactive enzyme was attenuated differently in two unrelated HFI patients. The existence of separate structural lesions was confirmed by protein blotting and immunodetection of enzyme subunits after sodium dodecyl sulphate/polyacrylamide electrophoresis. In one patient the subunit size was identical to wild type (Mr 38,000) and in the other, a single faint band (Mr 39,000) was identified. Radioimmunotitration studies, in two affected offspring of this latter patient by a proven HFI carrier, also revealed differences in antibody recognition. Segregation of different mutant alleles within this kindred demonstrates heterogeneity in HFI occurring at the same genetic locus. Variations in apparent immunoreactivity of aldolase B in HFI are thus related to overt modification of enzyme subunits and indicate that the disorder results principally from structural rather than regulatory mutations in the aldolase B gene.

Isolation and characterization of a mutant liver aldolase in adult hereditary fructose intolerance. Identification of the enzyme variant by radioassay in tissue biopsy specimens

Hereditary fructose intolerance (HFI) is a metabolic disorder caused by enzymic deficiency of aldolase B, a genetically distinct cytosolic isoenzyme expressed exclusively in liver, kidney, and intestine. The molecular basis of this enzyme defect has been investigated in three affected individuals from a nonconsanguineous kindred, in whom fructose-l-phosphate aldolase activities in liver or intestinal biopsy samples were reduced to 2-6% of mean control values. To identify a putative enzyme mutant in tissue extracts, aldolase B was purified from human liver by affinity chromatography and monospecific antibodies were prepared from antiserum raised in sheep. Immunodiffusion gels showed a single precipitin line common to pure enzyme and extracts of normal liver and intestine, but no reaction with extracts of brain, muscle, or HFI liver. However, weak positive staining for aldolase in hepatocyte and enterocyte cytosol was demonstrated by indirect immunofluorescence of HFI tissues. This was abolished by pretreatment with pure enzyme protein. Accordingly, a specific radioimmunoassay (detection limit 7.5 ng) was established to quantify immunoreactive aldolase B in human biopsy specimens. Extracts of tissue from affected patients gave 10-25% immunoreactive enzyme in control samples; immunoreactive aldolase in intestinal extracts from four heterozygotes was reduced (to 55%) when compared with seven samples from normal control subjects (P < 0.05). In extracts of HFI tissues, there was a sevenfold reduction in apparent absolute specific activity (1.02 vs. 8.82 U/mg) of immunoreactive fructose-l-phosphate aldolase B, but the apparent specific activity in heterozygotes (7.71 U/mg) was only slightly impaired. Displacement radioimmunotitration of aldolase B in liver supernatants showed a significant (P < 0.005) decrease in antibody avidity for immunoreactive protein in HFI tissue when compared with the pure enzyme or extract of normal control liver. Immunoaffinity chromatography on antialdolase B-Sepharose facilitated isolation and purification of enzyme from liver biopsy specimens. Active aldolase in normal liver, with substrate activity ratios and Michaelis constants identical to biochemically purified human enzyme, could be recovered from antibody columns. Chromatography on monospecific Fab' antialdolase B enabled pure enzyme protein to be retrieved quantitatively from normal control and HFI liver: direct chemical assay showed 1.88 and 1.15 mg aldolase protein/g of tissue, respectively. This confirmed that the catalytic properties of the HFI aldolase were profoundly impaired with specific activities of fructose-l-phosphate cleavage of 7.21 and 0.07 U/mg, respectively. Radioimmunoassay gave estimates of 7.66 and 1.18 U/mg, respectively. Sodium dodecyl sulfate-polyacrylamide electrophoresis indicated that immunopurified aldolase from HFI liver possessed a single subunit size similar to material from control liver extracts: M(r) 39,100 vs. 37,900+/-700 (SD) D, respectively. Electrofocusing under denaturing conditions of aldolase isolated in parallel from control and HFI liver revealed the same complement of subunits and, despite qualitative differences in distribution of bands during degradation, no additional charged species. Fructose phosphate aldolase deficiency in hereditary fructose intolerance is attended by the synthesis of an immunoreactive, but functionally and structurally modified enzyme variant that results from a restricted genetic mutation.

Studies on the control of iron uptake by rabbit small intestine

1. Whole-body retention in vivo and uptake of 59Fe-labelled ascorbate and nitrilotriacetate chelates by intestinal slices in vitro were determined in groups of normal control rabbits and rabbits with experimentally-induced Fe deficiency. 2. Over-all absorption as measured by retention of doses of either chelate was greatly increased in conditions of Fe deficiency. 3. Intestinal Fe uptake in vitro was inhibited up to 77% in the presence of 2,4-dinitrophenol and sodium fluoride. Initial rates showed saturation within the concentration range 18-450 mumol/l, suggesting that uptake was brought about by an active transport process. 4. When studied at chelate concentrations of 450 mumol/l, significant regional differences in uptake rates were observed. Uptake in duodenal slices was increased when compared with slices from jejunum and ileum. 5. Fe uptake from ferric and ferrous chelates was greatly enhanced in Fe deficiency. This was chiefly due to increases in uptake by slices from the duodenum, but uptake into slices of distal intestine was also stimulated. 6. Kinetic analysis of Fe uptake by duodenal slices from animals rendered Fe deficient by diet or repeated bleeding indicated in both groups an increased apparent maximum velocity (Vmax) for influx of Fe without significant changes in apparent affinity for Fe. 7. The experiments provide further insight into the nature and regional distribution of transport of Fe into the intestine and suggest, in the rabbit, that important control of Fe absorption may be exerted by an active process operating at this initial entry step.

Microvillar iron-binding glycoproteins isolated from the rabbit small intestine

Rabbit intestinal microvillus membranes possess high-affinity receptors for iron whose activity reflects homeostatic changes in mucosal iron transport. To isolate and characterize these membrane components, purified microvilli were radiolabelled with 59Fe(II) and solubilized in Triton X-100. 59Fe in 105000g supernatants co-eluted with a major broad protein peak (Mr approx. 100000) on gel-permeation chromatography and was rendered diffusible by Pronase digestion but not mild periodate degradation. Fluorescence studies with castor-bean lectin conjugates showed specific binding of this affinity probe exclusively to brush-border membranes in the intestinal epithelium. Affinity chromatography of solubilized membrane proteins showed binding to columns of immobilized lectin. Elution with D(+)-galactose released glycoprotein-bound 59Fe purified up to sevenfold over initial membrane extracts. The lectin bound up to 82% of protein-bound 59Fe. In contrast polyspecific antisera raised against rabbit microvilli in guinea-pigs precipitated less than 10% of solubilized radioactivity. Significantly more protein-bound 59Fe in detergent extracts of microvilli purified from bled animals interacted specifically with the lectin, suggesting that membrane glycoprotein receptors are involved in the homeostatic control of intestinal iron transport.

Studies on the binding of iron by rabbit intestinal microvillus membranes

1. 59Fe binding by microvillus membranes purified from rabbit intestine was studied by means of a microfiltration procedure. 2. Binding activity from ferrous ascorbate chelates was 100-fold greater than from ferric chelates of citrate and nitrilotriacetate. Dual-label experiments indicated dissociation of iron complexes before binding to the membranes. 3. Binding was inhibited at low incubation temperatures and was optimal at neutral pH. 4. Binding activity was reduced in ileal preparations when compared with membranes prepared from proximal intestine. 5. Initial binding velocity followed saturation kinetics over the range 45-450 microM-iron: it was weakly inhibited in the presence of excess Co2+ and V3+. 6. The data provide additional evidence for high-affinity iron-binding sites on the intestinal microvillus membrane and indicate properties that may reflect the functional significance of the binding step in the absorption pathway for iron.

In vitro studies of duodenal iron uptake in patients with primary and secondary iron storage disease

Initial rates of iron uptake into fresh specimens of human duodenal mucosa incubated with radio-iron in vitro were examined over a range of physiological iron concentrations. In normal control subjects and patients with iron overload, mucosal iron uptake showed saturation kinetics, thus providing evidence for a carrier-mediated transport process. In five patients with idiopathic haemochromatosis uptake at the lower concentrations of iron was increased up to 80 per cent when compared with 12 control subjects. Uptake at the highest medium iron concentration was not increased. These changes were related to a significant reduction in the apparent affinity constant (Kt) for iron transport and were independent of iron stores. In contrast, the uptake rates in six patients with secondary haemochromatosis were not different from controls except in a subgroup with severe iron-loading anaemias where uptake was modestly enhanced at the higher concentrations only. The studies point to a disorder of mucosal iron transport in idiopathic haemochromatosis which appears to be a primary abnormality rather than a consequence of the iron overload.

Cellular mechanisms in the regulation of iron absorption by the human intestine: studies in patients with iron deficiency before and after treatment

An in vitro method for measuring initial rates of iron uptake by mucosal biopsies of human duodenum was used to study control mechanisms for iron absorption. Within the physiological range of intraluminal concentrations (18--450 mumol/l) iron influx has many features of active, carrier-mediated transport. In biopsies form six patients with iron deficiency anaemia, uptake rates were increased 2--3-fold at the higher concentrations, when compared with normal controls (P less than 0.01) and overall were related inversely to serum transferrin saturation. Uptake was examined in four anaemic patients before and after therapy: the enhanced uptake fell to normal after repletion with iron, but was not reduced in two patients treated initially by red cell transfusion alone. Total mucosal iron in the anaemic patients was significantly lower at 58+/-7 nmol/mg protein, compared with 129+/-25 nmol/mg in normal subjects (P less than 0.05). In the serial studies, iron therapy for 6 weeks corrected the anaemia but did not restore mucosal iron levels to normal, even though uptake had fallen to control values. The experiments indicate that iron deficiency reversibly induces brush border iron carriers, and suggest that in man initial entry into the enterocyte rather than cellular retention of iron is a major regulatory step in the control of iron absorption.

Iron binding proteins and influx of iron across the duodenal brush border. Evidence for specific lactotransferrin receptors in the human intestine

The ability of a range of homologous transferrin-like proteins to donate iron to pieces of human duodenal mucosa, was examined with an in vitro incubation technique. In contrast to serum transferrin and ovotransferrin, only lactotransferrin was able to yield its iron to intestinal tissue, but in an autologous system this protein was unable to donate iron to human reticulocyte preparations. Studies with 125I-labelled lactotransferrin and lactotransferrin dual-labelled with 59Fe and 125I, indicated that the intact protein is excluded from entry into the enterocytes. The experiments suggest that iron may be transported across the brush border after delivery to specific protein binding sites at the cell surface.