Biomedical subjects

P Beyer

Publications and source records attributed to P Beyer.

At least 19 recordsLinked to original sources

PubMed2005

Nonlinear dynamics of transport barrier relaxations in tokamak edge plasmas.

A new mechanism for intermittent relaxations of transport barriers is found by using three dimensional fluid turbulence simulations. This mechanism is generic since it only requires a stationary E x B shear flow. It is found here that if the flow shear increases faster than linearly with heating power, the relaxation frequency decreases with power. An analytical study reveals that this nonlinear dynamics is governed by a time delay for effective velocity shear stabilization.

Journal Article↗

PubMed2004

Pheochromocytoma in childhood: implication for further diagnostic procedures.

UNLABELLED: We report on our experience with two patients with pheochromocytoma. One patient underwent surgery of pheochromocytoma at the age of 30 y; 18 y later, medullary thyroid carcinoma (MTC) was detected in his son. Subsequently, multiple endocrine neoplasia (MEN) type 2A was diagnosed by genetic examination in both father and son. Further diagnostic procedures also revealed an MTC in the father. The other patient suffered from bifocal pheochromocytoma of the left suprarenal gland. Diagnostic work-up revealed papillary thyroid carcinoma, which was also detected in the mother 8 mo later. Whereas a point mutation in SDHB gene was found in the son, no genetic abnormality was detected in the mother. CONCLUSION: Every pheochromocytoma in childhood warrants further diagnostic work-up, including genetic examination. In addition, clinical data of patients suffering from pheochromocytoma and papillary thyroid carcinoma should be collected by an international registry, and a joint effort should be undertaken in order to define possible underlying mutated genes in these patients.

Adolescent↗

PubMed2003

Transport reduction by rotation shear in tokamak-edge turbulence.

Effects of externally imposed and self-generated poloidal flows on turbulent transport in the edge region of a tokamak are investigated using 3D nonlinear global simulations of resistive pressure-gradient-driven turbulence. Transport reduction is found to be due to synergetic changes in the fluctuation amplitude and in the dephasing of the fluctuations. A scaling of the fluctuation level and turbulent diffusivity with E x B flow shear strength is deduced from these simulations. These scalings agree with recent experimental observations on edge biasing as well as with analytical models.

Journal Article↗

PubMed2001

A novel phytyltransferase from Synechocystis sp. PCC 6803 involved in tocopherol biosynthesis.

The deduced polypeptide sequence of open reading frame slr1736 reveals homology to chlorophyll synthase and 1,4-dihydroxy-2-naphthoic acid phytyltransferase in Synechocystis sp. strain PCC 6803. In tocopherol and plastoquinone biosynthesis, a condensation reaction mechanistically similar to that of these two enzymes is performed. To analyze the function of this novel prenyltransferase, a deletion mutant of slr1736 was generated by homologous recombination. The mutant showed a markedly decreased tocopherol content, while plastoquinone levels remained unchanged. Since the aromatic precursor homogentisic acid accumulated in the mutant, the function of the enzyme was proven to be a novel tocopherol phytyltransferase.

Alkyl and Aryl Transferases↗

PubMed2001

Biosynthesis of beta-carotene (provitamin A) in rice endosperm achieved by genetic engineering.

To obtain a functioning provitamin A (beta-carotene) biosynthetic pathway in rice endosperm, we introduced in a single, combined transformation effort the cDNAs coding for (1) phytoene synthase (psy) and (2) lycopene beta-cyclase (beta-lcy; both from Narcissus pseudonarcissus and both under control of the endosperm-specific glutelin promoter), with (3) a bacterial phytoene desaturase (crtI, from Erwinia uredovora under constitutive 35S promoter control). This combination covers the requirements for beta-carotene synthesis, and yellow, beta-carotene-bearing rice endosperm was obtained in the T0 generation. However, further experiments revealed that the presence of beta-lcy was not necessary, since psy and crtI alone were able to drive beta-carotene synthesis as well as the formation of further downstream xanthophylls. This finding could be explained if these downstream enzymes are either constitutively expressed in rice endosperm or are induced by the transformation, e.g. by products derived therefrom. Based on results in N. pseudonarcissus as a model system, a likely hypothesis can be developed that trans lycopene or a trans lycopene derivative acts as an inductor in a kind of feedback mechanism stimulating endogenous carotenogenic genes.

Alkyl and Aryl Transferases↗

PubMed2000

Nondiffusive transport in tokamaks: three-dimensional structure of bursts and the role of zonal flows

Large scale transport events are studied in simulations of resistive ballooning turbulence in a tokamak plasma. The spatial structure of the turbulent flux is analyzed, indicating radially elongated structures (streamers) at the low field side which are distorted by magnetic shear at different toroidal positions. The interplay between self-generated zonal flows and transport events is investigated, resulting in significant modifications of the frequency and the amplitude of bursts. The propagation of bursts is studied in the presence of a transport barrier generated by a strong shear flow.

Journal Article↗

PubMed2000

Identification of a novel gene coding for neoxanthin synthase from Solanum tuberosum.

The polymerase chain reaction analysis of potato plants, transformed with capsanthin capsorubin synthase ccs, revealed the presence of a highly related gene. The cloned cDNA showed at the protein level 89.6% identity to CCS. This suggested that the novel enzyme catalyzes a mechanistically similar reaction. Such a reaction is represented by neoxanthin synthase (NXS), forming the xanthophyll neoxanthin, a direct substrate for abscisic acid formation. The function of the novel enzyme could be proven by transient expression in plant protoplasts and high performance liquid chromatography analysis. The cloned NXS was imported in vitro into plastids, the compartment of carotenoid biosynthesis.

Abscisic Acid↗

PubMed2000

Engineering the provitamin A (beta-carotene) biosynthetic pathway into (carotenoid-free) rice endosperm.

Rice (Oryza sativa), a major staple food, is usually milled to remove the oil-rich aleurone layer that turns rancid upon storage, especially in tropical areas. The remaining edible part of rice grains, the endosperm, lacks several essential nutrients, such as provitamin A. Thus, predominant rice consumption promotes vitamin A deficiency, a serious public health problem in at least 26 countries, including highly populated areas of Asia, Africa, and Latin America. Recombinant DNA technology was used to improve its nutritional value in this respect. A combination of transgenes enabled biosynthesis of provitamin A in the endosperm.

Alkyl and Aryl Transferases↗

PubMed2000

Regulation and activation of phytoene synthase, a key enzyme in carotenoid biosynthesis, during photomorphogenesis.

During photomorphogenesis in higher plants, a coordinated increase occurs in the chlorophyll and carotenoid contents. The carotenoid level is under phytochrome control, as reflected by the light regulation of the mRNA level of phytoene synthase (PSY), the first enzyme in the carotenoid biosynthetic pathway. We investigated PSY protein levels, enzymatic activity and topological localization during photomorphogenesis. The results revealed that PSY protein levels and enzymatic activity increase during de-etiolation and that the enzyme is localized at thylakoid membranes in mature chloroplasts. However, under certain light conditions (e.g., far-red light) the increases in PSY mRNA and protein levels are not accompanied by an increase in enzymatic activity. Under those conditions, PSY is localized in the prolamellar body fraction in a mostly enzymatically inactive form. Subsequent illumination of dark-grown and/or in far-red light grown seedlings with white light causes the decay of these structures and a topological relocalization of PSY to developing thylakolds which results in its enzymatic activation. This light-dependent mechanism of enzymatic activation of PSY in carotenoid biosynthesis shares common features with the regulation of the NADPH:protochlorophyllide oxidoreductase, the first light-regulated enzyme in chlorophyll biosynthesis. The mechanism of regulation described here may contribute to ensuring a spatially and temporally coordinated increase in both carotenoid and chlorophyll contents.

Alkyl and Aryl Transferases↗

PubMed2000

Proper orthogonal decomposition and galerkin projection for a three-dimensional plasma dynamical system

A general method by which to investigate nonlinear dynamical systems close to a stability threshold is presented. This method combines a proper orthogonal decomposition and a subsequent Galerkin projection. This technique is applied to three-dimensional resistive ballooning plasma fluctuations in a tokamak. The corresponding dynamical system belongs to a large family of convective fluid systems including Rayleigh-Benard convection. A proper orthogonal decomposition of the fluctuating signal obtained by numerical simulation shows that the relevant modes are close to the linear (global) modes. The Galerkin projection provides a low-dimensional system that allows the study of shear flow generation, its subsequent fluctuation reduction, and the evolution to oscillating states.

Journal Article↗

PubMed1999

Antireflux surgery in children suffering from reflux-associated respiratory diseases.

The aim of our study was to evaluate the success, complications, and morbidity following a modified Thal fundoplication in children with reflux-associated respiratory disease (RARD). We used a procedure consisting of retroesophageal hiatal plasty, wrapping the gastric fundus around the gastroesophageal junction 180 degrees, and fixation of the lesser curvature at the abdominal wall. Follow-up by questionnaire of 128 (77 male, 51 females) out of 196 antireflux procedures between 1992 and 1995 was achieved. Surgical therapy was considered justified whenever there was gastroesophageal reflux resulting in severe recurrent respiratory symptoms. Eleven percent of the children suffered from bronchiectasis. The diagnosis of RARD was based on a high index of suspicion, barium swallow with fluoroscopy, 24-hr two-level pH-monitoring, bronchoscopy, bronchoalveolar lavage and detection of lipid-laden alveolar macrophages, esophago-gastroscopy, and esophageal biopsy. Patients with bronchopulmonary diseases such as allergy, immunodeficiency, cystic fibrosis, primary ciliary dyskinesia, and malformation of the bronchial tree or vessels had been excluded. "Evident improvement" as a result of surgery was reported in 88%, "no change" in 10%, and a "change for the worse" in 2% of patients. Persistent mild difficulties in swallowing were observed in 11%. Paraesophageal hernia, gas-bloat syndrome, and dumping syndrome were not observed. Two children needed a second operation because of relapse. The use of emergency steroidal medication for acute respiratory distress decreased impressively (219 single doses/year before surgery vs. 30 single doses/year after surgery). The need for more than 4 times/year of antibiotic therapy before surgery was reduced from 52. 3% before to 14% after surgery. Most (90.6%) of the parents stated they would agree to have surgery done again if medically indicated. In conclusion, Thal fundoplication is sufficient, safe, and effective in the management of RARD. Complications of the procedure were minor and of little consequence to the patient.

Adolescent↗

PubMed1999

Two Arabidopsis thaliana carotene desaturases, phytoene desaturase and zeta-carotene desaturase, expressed in Escherichia coli, catalyze a poly-cis pathway to yield pro-lycopene.

We have expressed in Escerichia coli the enzymes geranylgeranyl diphosphate synthase and phytoene synthase, from the soil bacterium Erwinia stewartii, and the two carotene desaturases phytoene desaturase and carotene zeta-carotene desaturase from Arabidopsis thaliana. We show that pro-lycopene (7,9,7',9'-tetra-cis)-lycopene is the main end product of the plant desaturation pathway in these cells. In addition, light is required in this system. Whereas in the dark mainly zeta-carotene, the phytoene desaturase product, accumulates, illumination leads to activation of this intermediate caused by its photoisomerization. zeta-Carotene then meets the stereospecific requirements of zeta-carotene desaturase and pro-lycopene is formed. In contrast, a strain of E. coli carrying geranylgeranyl diphosphate synthase, phytoene desaturase and the bacterial carotene desaturase CrtI, which mediates lycopene formation from phytoene, does not require light, nor is a poly-cis-lycopene species formed. The stereoselectivity of the plant-type desaturation pathway expressed in E. coli is the same as previously shown with chromoplast membranes. As the phytoene desaturase and zeta-carotene desaturase used originate from a system not capable of developing chromoplasts, this indicates that the poly-cis pathway of carotene desaturation may have a wider occurrence than initially believed.

Alkyl and Aryl Transferases↗

PubMed1998

Induced beta-carotene synthesis driven by triacylglycerol deposition in the unicellular alga dunaliella bardawil

Under stress conditions such as high light intensity or nutrient starvation, cells of the unicellular alga Dunaliella bardawil overproduce beta-carotene, which is accumulated in the plastids in newly formed triacylglycerol droplets. We report here that the formation of these sequestering structures and beta-carotene are interdependent. When the synthesis of triacylglycerol is blocked, the overproduction of beta-carotene is also inhibited. During overproduction of beta-carotene no up-regulation of phytoene synthase or phytoene desaturase is observed on the transcriptional or translational level, whereas at the same time acetyl-CoA carboxylase, the key regulatory enzyme of acyl lipid biosynthesis, is increased, at least in its enzymatic activity. We conclude that under normal conditions the carotenogenic pathway is not maximally active and may be appreciably stimulated in the presence of sequestering structures, creating a plastid-localized sink for the end product of the carotenoid biosynthetic pathway.

Journal Article↗

PubMed1997

Chloroplast import of four carotenoid biosynthetic enzymes in vitro reveals differential fates prior to membrane binding and oligomeric assembly.

The precursor proteins of the carotenogenic enzymes geranylgeranyl diphosphate synthase, phytoene synthase, phytoene desaturase and lycopene cyclase were imported into isolated pea chloroplasts. Geranylgeranyl diphosphate synthase remained soluble in the stroma in a free form and phytoene synthase associated to thylakoid membranes upon import, both as expected. Surprisingly, phytoene desaturase and lycopene cyclase, which strongly depend on membrane association for enzymatic activity, also remained soluble in the chloroplast stroma. The soluble forms of these enzymes were, however, still competent for membrane-association, e.g. with protein-free liposomal membranes. Indeed the soluble forms of phytoene synthase, phytoene desaturase and lycopene cyclase occurred as ATP- and cold-sensitive high-molecular-mass complexes. Gel-filtration experiments and blue native-PAGE plus autoradiography and western blot analysis indicated a participation of the chloroplast 60-kDa chaperonin (Cpn60) in the soluble high-molecular-mass complexes of imported carotenogenic enzymes. Finally, it was inferred that a membrane-bound regulatory factor plays a decisive role in membrane-binding.

Adenosine Triphosphate↗

PubMed1997

Molecular analysis of the Rhodobacter capsulatus chaperone dnaKJ operon: purification and characterization of DnaK.

In Rhodobacter capsulatus (Rbc), the participation of DnaK in the synthesis of light harvesting antenna complex I (LHI) has been recently inferred from the finding that the amount of LHI alpha- and beta-polypeptides synthesized in an in vitro translation system was strongly reduced when DnaK was depleted. In the present work, a DnaK protein was isolated from Rbc and biochemically characterized. The N-terminus of the protein was sequenced and a corresponding oligo was used as probe in order to clone the gene coding for DnaK. The dnaK gene was located in an operon (dnaKJ) with two open reading frames, which code for DnaK and DnaJ, respectively. A promoter element corresponding to the consensus sequence of the atypical heat shock (HS) promoter of several alpha-purple proteobacteria was identified. Northern blot analysis indicated that dnaK and dnaJ belong to the same transcriptional unit; there were two transcripts, one comprising both the dnaK and dnaJ genes and a second with only dnaK. Primer extension analysis revealed that under both chemotrophic and phototrophic conditions transcription was initiated from the same position before and after HS. The promoter activity was studied under different growth conditions with a dnaK-lacZ fusion under the control of the dnaKJ promoter. The present work opens up the possibility to study the specific role of DnaK in the assembly of photosynthetic apparatus proteins.

Amino Acid Sequence↗

PubMed1997

Transgenic rice (Oryza sativa) endosperm expressing daffodil (Narcissus pseudonarcissus) phytoene synthase accumulates phytoene, a key intermediate of provitamin A biosynthesis.

Rice (Oryza sativa L.), the major food staple for more than two billion people, contains neither beta-carotene (provitamin A) nor C40 carotenoid precursors thereof in its endosperm. To improve the nutritional value of rice, genetic engineering was chosen as a means to introduce the ability to make beta-carotene into rice endosperm tissue. Investigation of the biochemical properties of immature rice endosperm using [14C]-labelled substrates revealed the presence of geranyl geranyl diphosphate, the C20 general isoprenoid precursor necessary for C40 carotenoid biosynthesis. Phytoene synthase, which condenses two molecules of geranyl geranyl diphosphate, is the first of four specific enzymes necessary for beta-carotene biosynthesis in plants. Therefore, the Japonica rice model variety Taipei 309 was transformed by microprojectile bombardment with a cDNA coding for phytoene synthase from daffodil (Narcissus pseudonarcissus) under the control of either a constitutive or an endosperm-specific promoter. In transgenic rice plants, the daffodil enzyme is active, as measured by the in vivo accumulation of phytoene in rice endosperm. Thus, it is demonstrated for the first time that it is in principle possible to engineer a critical step in provitamin A biosynthesis in a non-photosynthetic, carotenoid-lacking plant tissue. These results have important implications for long-term prospects of overcoming worldwide vitamin A deficiency.

Alkyl and Aryl Transferases↗

PubMed1996

Molecular analysis of the Rhodobacter capsulatus chaperonin (groESL) operon: purification and characterization of Cpn60.

The heat-shock protein Cpn60 (chaperonin, GroEL homologue) from the phototrophic bacterium Rhodobacter capsulatus B10 was purified to homogeneity and biochemically characterized. Native Cpn60 from R. capsulatus was shown to be a tetradecamer of 840 kDa similar to that of homologous chaperones characterized so far. Cpn60 possesses ATPase activity and promotes refolding of chaotropically denatured citrate synthase. The groESL operon of R. capsulatus was cloned using a degenerate oligonucleotide and sequenced. Two open reading frames (285 and 1,635 bp) were found; they encode Cpn10 and Cpn60, with corresponding deduced molecular masses of 10.6 and 57.6 kDa. The deduced amino acid sequences coincided perfectly with those of the amino terminus and of three tryptic peptides of purified Cpn60 from R. capsulatus. Strong evidence that R. capsulatus encodes only one copy of the groESL operon was obtained. Primer-extension analysis revealed that the groESL operon is transcribed by a -35/-10-type promoter, and that transcription was initiated from the same positions before and after heat-shock under both chemotrophic and phototrophic conditions. The major initiation site is immediately followed by the inverted repeat structure CIRCE, which has been found upstream of many bacterial heat-shock operons. A second minor transcript starts just after the CIRCE element. Although heat-shock induction of a groEL-lacZ fusion failed because of thermal inactivation of the fusion protein, Western blot analysis revealed a two- to threefold induction of cellular Cpn60 levels 45-75 min after shifting from 28 degrees C to 39 degrees C. Deletion mapping of the groESL promoter identified upstream of the promoter a 19-bp element that enhances groESL transcription eightfold and contains the AT-rich sequence dAAATTTTT, which is found at similar positions in heat-shock operons of other gram-negative bacteria.

Adenosine Triphosphatases↗

PubMed1996

Phytoene synthase from Narcissus pseudonarcissus: functional expression, galactolipid requirement, topological distribution in chromoplasts and induction during flowering.

A cDNA coding for the carotenoid biosynthetic enzyme phytoene synthase was cloned from a Narcissus pseudonarcissus flower cDNA library, and the corresponding protein was overexpressed in insect cells using the baculovirus lipofection system. The full-length overexpressed enzyme exhibited very reduced catalytic activity compared with an overexpressed N-truncated form, with its transit sequence removed by site-directed mutagenesis. The shortened form readily bound quantitatively to lipid bilayers. Although it was active with liposomes prepared from plastid lipids, with phospholipid liposomes it was not, even though association took place. In this latter case, free galactose was capable of substituting for galactolipids, resulting in enzymatic activity. It is concluded that galactolipids are involved in catalytic activity, but do not serve as a membrane anchor. Antibodies raised against the recombinant enzyme made it possible to distinguish between a membrane-bound and a soluble, protein-complexed inactive form of phytoene synthase, present in the chromoplast stroma. These findings and data on phytoene synthase mRNA and protein expression presented here are discussed in terms of a possible regulatory role in color formation during chromoplast (flower) development.

Alkyl and Aryl Transferases↗