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R M Brown

Publications and source records attributed to R M Brown.

At least 55 records · Page 3Linked to original sources
PubMed1995

Attenuation of isoproterenol-mediated vasodilatation in blacks.

BACKGROUND: The mechanism of enhanced vascular reactivity in young blacks, which may play a part in the development of hypertension, has not been defined. To determine the contribution of blunted vasodilatation mediated by beta 2-adrenergic receptors to this phenomenon, we compared forearm blood-flow responses to isoproterenol in young black and white normotensive men. METHODS: We used venous-occlusion plethysmography to measure the responses of blood flow in the forearm to the intraarterial administration of isoproterenol (10 to 400 ng per minute) in 9 normotensive black men (mean [+/- SD] age, 31.3 +/- 8.0 years) and 13 normotensive white men (mean age, 32.9 +/- 5.6 years). Sympathetic activity in the forearm was measured simultaneously by isotope-dilution techniques. RESULTS: Base-line blood flow in the forearm was similar in blacks and whites, but the degree of vasodilatation in response to isoproterenol was markedly lower in blacks. Isoproterenol at an infusion rate of 400 ng per minute produced a 9-fold increase in blood flow in whites but only a 3.5-fold increase in blacks (P < 0.001). The base-line rate of norepinephrine spillover in the forearm was higher in blacks (2.0 +/- 1.3 ng per minute [11.8 +/- 7.7 nmol per minute]) than in whites (0.6 +/- 0.5 ng per minute [3.5 +/- 3.0 nmol per minute], P = 0.002), but there was no difference between the groups after isoproterenol stimulation. CONCLUSIONS: Forearm blood-flow responses to isoproterenol were markedly attenuated in normotensive blacks, indicating a blunting of vasodilatation mediated by beta 2-adrenergic receptors. Sympathetic activity in the forearm was greater in blacks than in whites, but isoproterenol-stimulated presynaptic beta 2-adrenergic responses (which facilitated norepinephrine release) did not differ significantly between blacks and whites. Our findings suggest that the mechanisms responsible for blunted vasodilatation in response to the administration of isoproterenol may contribute to enhanced vascular reactivity in blacks and may play a part in the pathogenesis of hypertension in blacks.

Adult↗
PubMed1995

Intracellular heteroplasmy for disease-associated point mutations in mtDNA: implications for disease expression and evidence for mitotic segregation of heteroplasmic units of mtDNA.

Studies in vitro have shown that a respiratory-deficient phenotype is expressed by cells when the proportion of mtDNA with a disease-associated mutation exceeds a threshold level, but analysis of tissues from patients with mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) have failed to show a consistent relationship between the degree of heteroplasmy and biochemical expression of the defect. One possible explanation for this phenomenon is that there is variation of heteroplasmy between individual cells that is not adequately reflected by the mean heteroplasmy for a tissue. We have confirmed this by study of fibroblast clones from subjects heteroplasmic for the MELAS 3243 (A-->G) mtDNA mutation. Similar observations were made with fibroblast clones derived from two subjects heteroplasmic for the 11778 (G-->A) mtDNA mutation of Leber's hereditary optic neuropathy. For the MELAS 3243 mutation, the distribution of mutant mtDNA between different cells was not randomly distributed about the mean, suggesting that selection against cells with high proportions of mutant mtDNA had occurred. To explore the way in which heteroplasmic mtDNA segregates in mitosis we followed the distribution of heteroplasmy between clones over approximately 15 generations. There was either no change or a decrease in the variance of intercellular heteroplasmy for the MELAS 3243 mutation, which is most consistent with segregation of heteroplasmic units of multiple mtDNA molecules in mitosis. After mitochondria from one of the MELAS 3243 fibroblast cultures were transferred to a mitochondrial DNA-free (rho0) cell line derived from osteosarcoma cells by cytoplast fusion, the mean level and intercellular distribution of heteroplasmy was unchanged. We interpret this as evidence that somatic segregation (rather than nuclear background or cell differentiation state) is the primary determinant of the level of heteroplasmy.

Adolescent↗
PubMed1995

Identification of a second cellulose synthase gene (acsAII) in Acetobacter xylinum.

A second cellulose synthase gene (acsAII) coding for a 175-kDa polypeptide that is similar in size and sequence to the acsAB gene product has been identified in Acetobacter xylinum AY201. Evidence for the presence of this gene was obtained during analysis of A. xylinum mutants in which the acsAB gene was disrupted (I.M. Saxena, K. Kudlicka, K. Okuda, and R.M. Brown, Jr., J. Bacteriol. 176:5735-5752, 1994). Although these mutants produced no detectable cellulose, they exhibited significant cellulose synthase activity in vitro. The acsAII gene was isolated by using an acsAB gene fragment as a probe. The acsAII gene coded for cellulose synthase activity as determined from sequence analysis and study of mutants in which this gene was disrupted. A mutant in which only the acsAII gene was disrupted showed no significant differences in either the in vivo cellulose production or the in vitro cellulose synthase activity compared with wild-type cells. Mutants in which both the acsAII and acsAB genes were disrupted produced no cellulose in vivo and exhibited negligible cellulose synthase activity in vitro, thus confirming that the cellulose synthase activity observed in the acsAB mutants was coded by the acsAII gene. These results establish the presence of an additional gene for cellulose synthase expressed in cells of A. xylinum, yet this gene is not required for cellulose production when cells are grown under laboratory conditions.

Amino Acid Sequence↗
PubMed1995

Stimulus-response compatibility and videogame performance.

We investigated the role of stimulus-response compatibility in influencing manual reactions to a moving visual target in a videogame (pong). 40 right-handed university men were assigned randomly to one of two experimental conditions, the normal game condition or a reverse control condition in which the response device on the right controls the left game paddle and the device on the left controls the right paddle. Subjects in the normal condition performed marginally better playing pong when seated on the right when seated on the left, consistent with earlier findings. However, subjects in the reverse control condition showed the reverse effect, a left-side advantage. These findings suggest that compatibility between location of the moving target (the ball) and either handedness or hand used to respond may have been responsible for the right-side advantage observed in the normal-game condition.

Adult↗
PubMed1994

Assembly of synthetic cellulose I.

Cellulose microfibrils with an electron diffraction pattern characteristic of crystalline native cellulose I have been assembled abiotically by means of a cellulase-catalyzed polymerization of beta-cellobiosyl fluoride substrate monomer in acetonitrile/acetate buffer. Substantial purification of the Trichoderma viride cellulase enzyme was found to be essential for the formation of the synthetic cellulose I allomorph. Assembly of synthetic cellulose I appears to be a result of a micellar aggregation of the partially purified enzyme and the substrate in an organic/aqueous solvent system favoring the alignment of glucan chains with the same polarity and extended chain conformation, resulting in crystallization to form the metastable cellulose I allomorph.

Acetates↗
PubMed1994

Pyruvate dehydrogenase deficiency: molecular basis for intrafamilial heterogeneity.

Two half-brothers and their mother had symptomatic pyruvate dehydrogenase complex deficiency. The infants had severe congenital lactic acidosis, seizures, and apneic spells and died at the ages 3 and 4 months. The mother was less symptomatic with mental retardation, truncal ataxia, and dysarthria. The residual pyruvate dehydrogenase activities in cultured skin fibroblasts from the 2 infants and their mother were 7, 15, and 10% of control values. Immunoblot analysis showed negligible amounts of E1 alpha and E1 beta subunits of the complex. Northern blot analysis for the E1 alpha subunit showed normal results. In the 2 sons, complementary DNA sequence analysis revealed a cytosine to thymine mutation in exon 4, resulting in a change of arginine 127 to tryptophan in the E1 alpha subunit. Restriction enzyme analysis of the polymerase chain reaction product representing exon 4 of the E1 alpha gene revealed that the mother was a heterozygotes. Complementary DNA restriction analysis and methylation analysis of the X chromosome DXS255 loci revealed skewed activation of the mutant allele, consistent with the deficient pyruvate dehydrogenase activity in the mother's fibroblasts. The milder maternal phenotype is consistent with variable X-inactivation patterns in different organs of female heterozygotes.

Base Sequence↗
PubMed1994

Prenatal diagnosis of pyruvate dehydrogenase E1 alpha subunit deficiency.

Pyruvate dehydrogenase (PDH) E1 alpha subunit deficiency is an X-linked inborn error of metabolism affecting males and females with equal frequency. The diagnosis is usually based on determination of enzyme activity, although this may present difficulties in some females because of X-inactivation patterns favouring expression of the normal X chromosome. This is a particular problem for prenatal diagnosis using chorionic villus cells where normal enzyme assay results do not necessarily exclude the diagnosis and confirmatory X-inactivation analysis may be complicated by variable methylation of active and inactive X chromosomes. We describe prenatal diagnosis in two pregnancies in a family following diagnosis of a PDH E1 alpha deficient male. The first prenatal diagnosis was performed by enzyme assay, but by the time of the subsequent pregnancy, the underlying mutation in the affected male had been identified and direct gene analysis was possible. This study highlights the limitations of diagnosis of PDH E1 alpha deficiency based on measurement of the gene product and illustrates the need for mutation analysis in affected individuals.

Cells, Cultured↗
PubMed1994

Cerebral dysgenesis and lactic acidemia: an MRI/MRS phenotype associated with pyruvate dehydrogenase deficiency.

Pyruvate dehydrogenase complex (PDHC) is an intramitochondrial multienzyme complex essential for the aerobic oxidation of glucose. The majority of patients with PDHC deficiency have abnormalities in the major catalytic and regulatory subunit, E1 alpha, which is encoded on the X chromosome. The clinical spectrum of PDHC deficiency is heterogeneous, particularly in heterozygous females, and diagnosis may be difficult. Three affected infant girls with PDHC deficiency were investigated. All had dysmorphic features, microcephaly with profound global developmental delay, and hypotonia. Systemic acidosis was absent, although serum lactate and pyruvate were abnormally elevated. Magnetic resonance imaging revealed hypoplasia of the corpus callosum in all patients. Proton magnetic resonance spectroscopy of brain revealed large increases in relative signal intensities for lactic acid and decreases in the relative signal intensities of N-acetylaspartate, a marker of neuronal damage or less. Phosphorus MRS of muscle revealed abnormally low phosphorylation potentials for all these patients, although the degree of abnormality was variable and not directly correlated with the amount of brain lactate. It is proposed that cerebral dysgenesis and cerebral lactic acidemia as shown by magnetic resonance imaging and proton magnetic resonance spectroscopy are useful diagnostic clues to PDHC deficiency, particularly in females in whom variable patterns of X-inactivation reduce sensitivity of laboratory diagnosis based on the biochemical studies of peripheral tissues. In addition, muscle bioenergetic abnormalities in conjunction with CNS dysfunction may contribute to profound hypotonia in this disorder.

Acidosis, Lactic↗
PubMed1994

Clinical diversity of pyruvate dehydrogenase deficiency.

Clinical features, magnetic resonance, and biochemical studies are reported in 7 children with pyruvate dehydrogenase (PDH) deficiency. These findings confirm the diverse clinical presentation of this condition, although neurological abnormalities are consistent features. Imaging results are also varied. Six of the children were investigated with proton magnetic resonance spectroscopy and lactate was demonstrated in brain in all patients. Regional variation in the lactate signal was observed in those patients in whom 2 regions were examined. Advances in molecular genetics have provided some explanations for the clinical variation in pyruvate dehydrogenase deficiency.

Brain↗
PubMed1994

Pyruvate dehydrogenase deficiency. Clinical presentation and molecular genetic characterization of five new patients.

Fibroblast cultures from five patients with early onset severe encephalopathy and lactic acidosis were studied for evidence of pyruvate dehydrogenase (PDH) deficiency. Three males had significantly reduced activity (0.29-0.45 nmol/mg protein/min versus normal controls 0.7-1.1 nmol/mg protein/min); two females had PDH activity within the normal range. However, as the majority of cases of PDH deficiency result from defects in the X-linked E1 alpha subunit and both females had biased patterns of X-inactivation (making it impossible to rule out the possibility that they were heterozygous for an E1 alpha gene defect) molecular genetic studies were performed. cDNA from the male patients was sequenced and mis-sense mutations found: Y243N (T-->A) in exon 7, D315A (G-->A) in exon 10 and R378H (G-->A) in exon 11. Single-strand conformation polymorphism analysis of amplified genomic DNA fragments and sequencing revealed a mis-sense mutation M282L (A-->C) in one female and a frameshift mutation caused by insertion of T (R288ins) in the other. Adding to recent descriptions of new mutations, this report emphasizes the allelic heterogeneity of the condition. The identification of mutations in females with a suggestive clinical phenotype, even when peripheral fibroblasts do not show deficient PDH activity, illustrates the importance of molecular analysis of this disease.

Base Sequence↗
PubMed1994

Characterization of genes in the cellulose-synthesizing operon (acs operon) of Acetobacter xylinum: implications for cellulose crystallization.

The synthesis of an extracellular ribbon of cellulose in the bacterium Acetobacter xylinum takes place from linearly arranged, membrane-localized, cellulose-synthesizing and extrusion complexes that direct the coupled steps of polymerization and crystallization. To identify the different components involved in this process, we isolated an Acetobacter cellulose-synthesizing (acs) operon from this bacterium. Analysis of DNA sequence shows the presence of three genes in the acs operon, in which the first gene (acsAB) codes for a polypeptide with a molecular mass of 168 kDa, which was identified as the cellulose synthase. A single base change in the previously reported DNA sequence of this gene, resulting in a frameshift and synthesis of a larger protein, is described in the present paper, along with the sequences of the other two genes (acsC and acsD). The requirement of the acs operon genes for cellulose production was determined using site-determined TnphoA/Kanr GenBlock insertion mutants. Mutant analysis showed that while the acsAB and acsC genes were essential for cellulose production in vivo, the acsD mutant produced reduced amounts of two cellulose allomorphs (cellulose I and cellulose II), suggesting that the acsD gene is involved in cellulose crystallization. The role of the acs operon genes in determining the linear array of intramembranous particles, which are believed to be sites of cellulose synthesis, was investigated for the different mutants; however, this arrangement was observed only in cells that actively produced cellulose microfibrils, suggesting that it may be influenced by the crystallization of the nascent glucan chains.

Amino Acid Sequence↗
PubMed1994

Comparison of the relative levels of the 3243 (A-->G) mtDNA mutation in heteroplasmic adult and fetal tissues.

In this report, levels of the 3243 A to G mtDNA mutation associated with the mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome were measured in different heteroplasmic tissues of subjects in a kindred including adults with variable clinical phenotypes and a fetus. The relative proportions of mutant mtDNA varied widely (0.03 to 0.67) between identical tissues of the six different subjects and between different tissues of the same subjects. In the one adult for whom sufficient data were available there was an apparent correlation between the distribution of mutant mtDNA and clinical presentation. A woman without neurological symptoms who died prematurely with a cardiomyopathy and lactic acidosis had higher proportions of mutant in heart (0.49, SD 0.02), skeletal muscle (0.56, SD 0.01), and liver (0.55, SD 0.12) than in other tissues studied (for example, kidney, 0.03, SD 0.01). A strikingly different result was found in a 24 week old fetus in whom there was little variation in heteroplasmy in different tissues (average proportion of mutant mtDNA in six tissues, 0.53, SD 0.02). These observations add cardiomyopathy to the growing list of presenting features of the 3243 mtDNA mutation. The unique results from the fetus suggest also that selection pressures acting on either wild type or 3243 mutant mtDNA (rather than variation from replicative segregation of the heteroplasmic mtDNA) may be responsible primarily for the variable levels of 3243 mutant mtDNA in different heteroplasmic tissues of adults.

Adult↗
PubMed1993

Molecular genetic characterization of an X-linked form of Leigh's syndrome.

We report a patient with necrotizing encephalomyelopathy (Leigh's syndrome) associated with a deficiency of pyruvate dehydrogenase complex activity. The underlying mutation is an A to C transversion in the pyruvate dehydrogenase complex E1 alpha subunit gene. As the E1 alpha subunit is encoded on the X chromosome, this observation confirms that some patients with Leigh's syndrome may potentially exhibit X-linked inheritance.

Amino Acid Sequence↗
PubMed1993

Tritium sampling and measurement.

Current methods for sampling and measuring tritium are described. Although the basic techniques have not changed significantly over the last 10 y, there have been several notable improvements in tritium measurement instrumentation. The design and quality of commercial ion-chamber-based and gas-flow-proportional-counter-based tritium monitors for tritium-in-air have improved, an indirect result of fusion-related research in the 1980s. For tritium-in-water analysis, commercial low-level liquid scintillation spectrometers capable of detecting tritium-in-water concentrations as low as 0.65 Bq L-1 for counting times of 500 min are available. The most sensitive method for tritium-in-water analysis is still 3He mass spectrometry. Concentrations as low as 0.35 mBq L-1 can be detected with current equipment. Passive tritium-oxide-in-air samplers are now being used for workplace monitoring and even in some environmental sampling applications. The reliability, convenience, and low cost of passive tritium-oxide-in-air samplers make them attractive options for many monitoring applications. Airflow proportional counters currently under development look promising for measuring tritium-in-air in the presence of high gamma and/or noble gas backgrounds. However, these detectors are currently limited by their poor performance in humidities over 30%.

Environmental Pollutants↗