Rapid determination of δ-aminolevulinate synthase activity by a specific fluorometric coupled enzyme assay

The rapid and specific determination of picomole quantities of δ-aminolevulinate has been accomplished by its specific enzymatic conversion to uroporphyrinogen I and fluorometric detection of the oxidized uroporphyrin I. The coupled enzyme assay was linear with time and protein concentration and required less than 3 h for 25 individual determinations. Under the standard assay conditions, 1 to 100 pmol of uroporphyrin I was reliably quantitated; these values corresponded to a range of ALA synthase activities from 0.15 to 15 nmol/h/ml of enzyme. The sensitivity of this method was comparable to the more time-consuming radiochemical determinations of ALA synthase. In addition, this method was at least 10 times more sensitive than the colorimetric assays for ALA synthase activity. The rapidity, specificity, and sensitivity of this new method make it useful for monitoring the purification of ALA synthase and for reliable determinations of low levels of ALA synthase activity in crude tissue or cultured cell homogenates.     

Alpha-galactosidase A gene rearrangements causing Fabry disease. Identification of short direct repeats at breakpoints in an Alu-rich gene

Fabry disease, an inborn error of glycosphingolipid catabolism, results from mutations in the X-linked gene encoding the lysosomal enzyme, alpha-galactosidase A (EC 3.2.1.22). Six alpha-galactosidase A gene rearrangements that cause Fabry disease were investigated to assess the role of Alu repetitive elements and short direct and/or inverted repeats in the generation of these germinal mutations. The breakpoints of five partial gene deletions and one partial gene duplication were determined by either cloning and sequencing the mutant gene from an affected hemizygote, or by polymerase chain reaction amplifying and sequencing the genomic region containing the novel junction. Although the alpha-galactosidase A gene contains 12 Alu repetitive elements (representing approximately 30% of the 12-kilobase (kb) gene or approximately 1 Alu/1.0 kb), only one deletion resulted from an Alu-Alu recombination. The remaining five rearrangements involved illegitimate recombinational events between short direct repeats of 2 to 6 base pairs (bp) at the deletion or duplication breakpoints. Of these rearrangements, one had a 3' short direct repeat within an Alu element, while another was unusual having two deletions of 1.7 kb and 14 bp separated by a 151-bp inverted sequence. These findings suggested that slipped mispairing or intrachromosomal exchanges involving short direct repeats were responsible for the generation of most of these gene rearrangements. There were no inverted repeat sequences or alternating purine-pyrimidine regions which may have predisposed the gene to these rearrangements. Intriguingly, the tetranucleotide CCAG and the trinucleotide CAG (or their respective complements, CTGG and CTG) occurred within or adjacent to the direct repeats at the 5' breakpoints in three and four of the five alpha-galactosidase A gene rearrangements, respectively, suggesting a possible functional role in these illegitimate recombinational events. These studies indicate that short direct repeats are important in the formation of gene rearrangements, even in human genes like alpha-galactosidase A that are rich in Alu repetitive elements.     

Identification and expression of five mutations in the human acid sphingomyelinase gene causing types A and B Niemann-Pick disease. Molecular evidence for genetic heterogeneity in the neuronopathic and non-neuronopathic forms.

The deficient activity of the human lysosomal hydrolase, acid sphingomyelinase (ASM, EC 3.1.4.12), results in the neuronopathic (Type A) and non-neuronopathic (Type B) forms of Niemann-Pick disease (NPD). To investigate the genetic basis of the phenotypic heterogeneity in NPD, the molecular lesions in the ASM gene were determined from three unrelated NPD patients and evaluated by transient expression in COS-1 cells. A Type A NPD patient of Asian Indian ancestry (proband 1) was homoallelic for a T to A transversion in exon 2 of the ASM gene which predicted a premature stop at codon 261 of the ASM polypeptide (designated L261X). In contrast, an unrelated Type A patient of European ancestry (proband 2) was heteroallelic for a two-base (TT) deletion in exon 2 which caused a frame-shift mutation at ASM codon 178 (designated fsL178), leading to a premature stop at codon 190, and a G to A transition in exon 3 which caused a methionine to isoleucine substitution at codon 382 (designated M382I). Transient expression of the fsL178, L261X, and M382I mutations in COS-1 cells demonstrated that these lesions did not produce catalytically active ASM, consistent with the severe neuronopathic Type A NPD phenotype. In contrast, an unrelated Type B patient of European descent (proband 3) was heteroallelic for two missense mutations, a G to A transition in exon 2 which predicted a glycine to arginine substitution at ASM codon 242 (designated G242R), and an A to G transition in exon 3 which resulted in an asparagine to serine substitution at codon 383 (designated N383S). Interestingly, the G242R allele produced ASM activity in COS-1 cells at levels about 40% of that expressed by the normal allele, thereby explaining the mild Type B phenotype of proband 3 and the high residual activity (i.e. approximately 15% of normal) in cultured lymphoblasts. In contrast, the N383S allele did not produce catalytically active enzyme. None of these five ASM mutations was detected in over 60 other unrelated NPD patients analyzed, nor were these mutations found in over 100 normal ASM alleles. Thus, small deletions or nonsense mutations which trunctated the ASM polypeptide, or missense mutations that rendered the enzyme noncatalytic, resulted in Type A NPD disease, whereas a missense mutation that produced a defective enzyme with residual catalytic activity caused the milder nonneuronopathic Type B phenotype. These findings have facilitated genotype/phenotype correlations for this lysosomal storage disease and provided insights into the functional organization of the ASM polypeptide.     

Human alpha-L-iduronidase. I. Purification and properties of the high uptake (higher molecular weight) and the low uptake (processed) forms

Two major forms of human alpha-L-iduronidase have been individually purified over 175,000-fold to apparent homogeneity by sequential anion exchange, lectin affinity, and gel filtration chromatography. The two forms, initially designated as soluble and membrane-associated, were extracted from human lung in approximately equal amounts. Optimal solubilization of the membrane-associated form was facilitated by use of a non-ionic detergent or mannose 6-phosphate and saponin. Following detergent homogenization, the two forms were separated by anion exchange chromatography and then individually purified. The more electronegative form was membrane-associated, had a pI of approximately 5.9, and was selectively taken up (high uptake) by cultured Hurler syndrome fibroblasts; the more electropositive soluble form had a pI of about 6.6 and was incorporated into Hurler fibroblasts at a markedly lower rate (low uptake). After treatment with alkaline phosphatase, the pI values of both enzymes were about 7.8. Using 4-methylumbelliferyl-alpha-L-iduronide as substrate, the low and high uptake forms were each purified in milligram quantities to specific activities of 284,000 and 202,000 units/mg, respectively, with a combined yield greater than 35%. Each purified enzyme form migrated as a single protein band which also stained for enzymatic activity when electrophoresed in 7% native polyacrylamide disc gels at pH 4.3. By gel filtration, the high uptake form had an Mr = 85,000 whereas the Mr for the low uptake form was 68,000. Molecular weight estimates by analytical polyacrylamide gel electrophoresis were 82,000 and 70,000 for the high and low uptake forms, respectively. Rabbit anti-human low uptake alpha-L-iduronidase antibodies cross-reacted with the high uptake form as demonstrated by both immunotitration and Ouchterlony double immunodiffusion. Amino acid analysis revealed that the high uptake (higher molecular weight) form contained more arginine, glycine, alanine, glutamate or glutamine, leucine, isoleucine, histidine, and proline residues per molecule than the low uptake (lower molecular weight) form. Automated Edman degradation determined that the NH2-terminal residues of both forms were blocked. Both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high performance liquid chromatography demonstrated that each purified form was composed of several components; each post-high performance liquid chromatographic component retained catalytic activity and was immunologically cross-reactive with antibodies against the low uptake form.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunological similarities between specific chloroplast ribosomal proteins from Chlamydomonas reinhardtii and ribosomal proteins from Escherichia coli

Polyclonal antibodies were elicited against seven of the 33 different proteins of the large subunit of the chloroplast ribosome from Chlamydomonas reinhardtii. Three of these proteins are synthesized in the chloroplast and four are made in the cytoplasm and imported. In western blots, six of the seven antisera are monospecific for their respective large subunit ribosomal proteins, and none of these antisera cross-reacted with any chloroplast small subunit proteins from C. reinhardtii. Antisera to the three chloroplast-synthesized ribosomal proteins cross-reacted with specific Escherichia coli large subunit proteins of comparable charge and molecular weight. Only one of the four antisera to the chloroplast ribosomal proteins synthesized in the cytoplasm cross-reacted with an E. coli large subunit protein. None of the antisera cross-reacted with any E. coli small subunit proteins. On the assumption of a procaryotic, endosymbiotic origin for the chloroplast, those chloroplast ribosomal proteins still synthesized within the organelle appear to have retained more antigenic sites in common with E. coli ribosomal proteins than have those which are now the products of cytoplasmic protein synthesis. Antisera to this cytoplasmically synthesized group of chloroplast ribosomal proteins did not recognize any antigenic sites among C. reinhardtii cytoplasmic ribosomal proteins, suggesting that the genes for the cytoplasmically synthesized chloroplast ribosomal proteins either are not derived from the cytoplasmic ribosomal protein genes or have evolved to a point where no antigenic similarities remain.      

Rapid loss of δ-aminolevulinic acid dehydratase activity in primary cultures of adult rat hepatocytes: A new model of zinc deficiency

Nonproliferating cultures of adult rat hepatocytes were found to lose 60–70% of cell-associated zinc during their first 24 h of incubation in standard, serum-free medium. The loss of zinc was accompanied by a profound loss (95%) in the activity of the zinc metalloenzyme, δ-aminolevulinic acid dehydratase, as well as a loss (>85%) in the cellular content of immunoreactive δ-aminolevulinic acid dehydratase protein. Restoration of cellular zinc content by the addition of zinc to the culture medium partially prevented the losses of both δ-aminolevulinic acid dehydratase activity and immunoreactive protein. Since the spontaneous, selective loss of cellular zinc appears to have specific effects on a relevant hepatic function, this culture system constitutes a novel $\text{in}$$\text{vitro}$ model of zinc deficiency in mature liver.     

Species-wide distribution of highly polymorphic minisatellite markers suggests past and present genetic exchanges among house mouse subspecies

Background  

Four hypervariable minisatellite loci were scored on a panel of 116 individuals of various geographical origins representing a large part of the diversity present in house mouse subspecies. Internal structures of alleles were determined by minisatellite variant repeat mapping PCR to produce maps of intermingled patterns of variant repeats along the repeat array. To reconstruct the genealogy of these arrays of variable length, the specifically designed software MS_Align was used to estimate molecular divergences, graphically represented as neighbor-joining trees.     

Infusion of recombinant human acid sphingomyelinase into niemann-pick disease mice leads to visceral, but not neurological, correction of the pathophysiology.

An inherited deficiency of acid sphingomyelinase (ASM) activity results in the Type A and B forms of Niemann-Pick disease (NPD). Using the ASM-deficient mouse model (ASMKO) of NPD, we evaluated the efficacy of enzyme replacement therapy (ERT) for the treatment of this disorder. Recombinant human ASM (rhASM) was purified from the media of overexpressing Chinese Hamster ovary cells and i.v. injected into 16 five-month-old ASMKO mice at doses of 0.3, 1, 3, or 10 mg/kg every other day for 14 days (7 injections). On day 16, the animals were killed and the tissues were analyzed for their sphingomyelin (SPM) content. Notably, the SPM levels were markedly reduced in the hearts, livers, and spleens of these animals, and to a lesser degree in the lungs. Little or no substrate depletion was found in the kidneys or brains. Based on these results, three additional 5-month-old ASMKO animals were injected every other day with 5 mg/kg for 8 days (4 injections) and killed on day 10 for histological analysis. Consistent with the biochemical results, marked histological improvements were observed in the livers, spleens, and lungs, indicating a reversal of the disease pathology. A group of 10 ASMKO mice were then i.v. injected once a week with 1 mg/kg rhASM for 15 wk, starting at 3 wk of age. Although anti-rhASM antibodies were produced in these mice, the antibodies were not neutralizing and no adverse effects were observed from this treatment. Weight gain and rota-rod performance were slightly improved in the treated animals as compared with ASMKO control animals, but significant neurological deficits were still observed and their life span was not extended by ERT. In contrast with these CNS results, striking histological and biochemical improvements were found in the reticuloendothelial system organs (livers, spleens, and lungs). These studies indicate that ERT should be an effective therapeutic approach for Type B NPD, but is unlikely to prevent the severe neurodegeneration associated with Type A NPD.     

MAP2 IHC detection: a marker of antigenicity in CNS tissues

Immunohistochemistry (IHC) is used to detect antibody-specific antigens in tissues; the results depend on the ability of the primary antibodies to bind to their antigens. Therefore, results depend on the quality of preservation of the specimen. Many investigators have overcome the deleterious effects of over-fixation on the binding of primary antibodies to specimen antigens using IHC, but if the specimen is under-fixed or fixation is delayed, false negative results could be obtained despite certified laboratory practices. Microtubule-associated protein 2 (MAP2) is an abundant microtubule-associate protein that participates in the outgrowth of neuronal processes and synaptic plasticity; it is localized primarily in cell bodies and dendrites of neurons. MAP2 immunolabeling has been reported to be absent in areas of the entorhinal cortex and hippocampus of Alzheimer’s disease brains that were co-localized with the dense-core type of amyloid plaques. It was hypothesized that the lack of MAP2 immunolabeling in these structures was due to the degradation of the MAP2 antigen by the neuronal proteases that were released as the neurons lysed leading to the formation of these plaques. Because MAP2 is sensitive to proteolysis, we hypothesized that changes in MAP2 immunolabeling may be correlated with the degree of fixation of central nervous system (CNS) tissues. We detected normal MAP2 immunolabeling in fixed rat brain tissues, but MAP2 immunolabeling was decreased or lost in unfixed and delayed-fixed rat brain tissues. By contrast, two ubiquitous CNS-specific markers, myelin basic protein and glial fibrillary acidic protein, were unaffected by the degree of fixation in the same tissues. Our observations suggest that preservation of various CNS-specific antigens differs with the degree of fixation and that the lack of MAP2 immunolabeling in the rat brain may indicate inadequate tissue fixation. We recommend applying MAP2 IHC for all CNS tissues as a pre-screen to assess the quality of the tissue preservation and to avoid potentially false negative IHC results.