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Biomedical subjects

G Hübner

Publications and source records attributed to G Hübner.

At least 37 records · Page 2Linked to original sources
PubMed1999

Hepatic arterial flow volume and reserve in patients with cirrhosis: use of intra-arterial Doppler and adenosine infusion.

BACKGROUND & AIMS: In cirrhosis, liver blood flow becomes increasingly dependent on the hepatic artery. The aim of this study was to investigate hepatic arterial blood flow volume and resistance and hepatic arterial flow reserve in relation to liver function and systemic hemodynamic alterations in patients with cirrhosis. METHODS: In 38 patients with cirrhosis, liver function, cardiac output, and systemic vascular resistance were studied, and hepatic arterial blood flow velocity, flow volume, and pulsatility index at baseline and during intra-arterial administration of adenosine (2-40 microg. min-1. kg body wt-1) were assessed by angiography combined with intravascular Doppler flowmetry. RESULTS: Hepatic arterial flow velocity was 21 +/- 11, 31 +/- 17, and 41 +/- 27 cm/s; flow volume was 266 +/- 246, 342 +/- 289, and 417 +/- 220 mL/min; and pulsatility index was 2.2 +/- 0.7, 1.7 +/- 0.6, and 1.5 +/- 0.5 in Child-Pugh classes A, B, and C, respectively (differences not statistically significant). Adenosine-induced changes in these parameters were more marked in Child-Pugh class A (68 +/- 15 cm/s, 1246 +/- 486 mL/min, and -1.14 +/- 0.5) than in class C (45 +/- 23, P < 0.05; 704 +/- 492, P = 0.02; and -0.58 +/- 0.38, P < 0.05). Using analysis of variance, cardiac index, systemic vascular resistance, and ascites, but not Child-Pugh class, were related to baseline values and adenosine-induced changes. CONCLUSIONS: Adenosine is a potent dilator of the hepatic artery in humans. The data suggest that hepatic arterial blood flow and adenosine-dependent flow reserve in patients with cirrhosis are under systemic hemodynamic or neurohormonal control.

Adenosine↗
PubMed1999

Activin: a novel player in tissue repair processes.

Recent studies have demonstrated a strong expression of activin in repair processes of various tissues and organs, including the skin, the lung, the intestine, the cardiovascular system, and even the brain. Although little is as yet known about the function of activin in tissue repair, first results suggest a role of activin in epithelial differentiation, fibroblast proliferation and expression of matrix molecules by these cells, and also in neuroprotection. Whereas a transient overexpression of activin after tissue injury might be beneficial for the repair process, sustained expression of activin could lead to fibrotic processes. Therefore, the modulation of the availability or biological activity of activin could be of particular importance for the treatment of impaired tissue repair on the one hand and tissue fibrosis on the other hand.

Activins↗
PubMed1998

Activation of thiamine diphosphate in pyruvate decarboxylase from Zymomonas mobilis.

Replacement of tryptophan 392 located in the active site cavity of pyruvate decarboxylase (PDC; EC 4.1.1.1) from Zymomonas mobilis by methionine or glutamine yields enzymes with smaller catalytic constants of 8.5 s(-1) and 3.6 s(-1) at 4 degrees C, compared to that of the wild-type enzyme (17 s(-1)). The rate constants of the H/D exchange at the C2 of the coenzyme thiamine diphosphate have been determined to be 130 s(-1) for the wild-type enzyme, 56 s(-1) for the methionine and 30 s(-1) for the glutamine mutant, respectively. A group with a pKa of about 5 has been identified to be essential for C2 deprotonation of the enzyme-bound thiamine diphosphate from the pH dependence of the H/D exchange.

Hydrogen-Ion Concentration↗
PubMed1998

31Phosphorus magnetic resonance spectroscopy of the dorsolateral prefrontal region in schizophrenics--a study including 50 patients and 36 controls.

BACKGROUND: In a preliminary study we found decreased phosphodiester (PDE)% values and an increased phosphomonoester (PME)/phosphodiester ratio in the dorsolateral prefrontal region (DLPFR) of 13 chronic schizophrenics vs. 14 controls using 31phosphorus magnetic resonance spectroscopy (31P-MRS). Since these results are in contrast to the findings of other groups, we increased our study group to a total of 50 chronic schizophrenics on stable neuroleptic medication and 36 controls to minimize the possibility of a chance result due to small sample size. METHODS: An image-selected in vivo 31P-MRS method on a Philips Gyroscan ACS II scanner working at 1.5 T was used. RESULTS: We could confirm our earlier findings of decreased PDE% levels in schizophrenics. Additionally, we found phosphocreatine (PCr)% and PCr/adenosine triphosphate (ATP) to be increased in the schizophrenics. While no association between PME% and PDE% with neuroleptic medication was found, ATP% correlated positively and PCr/ATP negatively with the chlorpromazine equivalent dose. CONCLUSIONS: The decreased PDE% levels might be characteristic only for chronic, neuroleptic-treated patients. The finding of altered high-energy phosphate levels can be interpreted as an indication of decreased energy-demanding processes in the DLPFR of the investigated patients compared to controls.

Adenosine Triphosphate↗
PubMed1998

Activation of thiamin diphosphate in enzymes.

Activation of the coenzyme ThDP was studied by measuring the kinetics of deprotonation at the C2 carbon of thiamin diphosphate in the enzymes pyruvate decarboxylase, transketolase, pyruvate dehydrogenase complex, pyruvate oxidase, in site-specific mutant enzymes and in enzyme complexes containing coenzyme analogues by proton/deuterium exchange detected by 1H-NMR spectroscopy. The respective deprotonation rate constant is above the catalytic constant in all enzymes investigated. The fast deprotonation requires the presence of an activator in pyruvate decarboxylase from yeast, showing the allosteric regulation of this enzyme to be accomplished by an increase in the C2-H dissociation rate of the enzyme-bound thiamin diphosphate. The data of the thiamin diphosphate analogues and of the mutant enzymes show the N1' atom and the 4'-NH2 group to be essential for the activation of the coenzyme and a conserved glutamate involved in the proton abstraction mechanism of the enzyme-bound thiamin diphosphate.

Allosteric Regulation↗
PubMed1998

Activation of thiamin diphosphate and FAD in the phosphatedependent pyruvate oxidase from Lactobacillus plantarum.

The phosphate- and oxygen-dependent pyruvate oxidase from Lactobacillus plantarum is a homotetrameric enzyme that binds 1 FAD and 1 thiamine diphosphate per subunit. A kinetic analysis of the partial reactions in the overall oxidative conversion of pyruvate to acetyl phosphate and CO2 shows an indirect activation of the thiamine diphosphate by FAD that is mediated by the protein moiety. The rate constant of the initial step, the deprotonation of C2-H of thiamine diphosphate, increases 10-fold in the binary apoenzyme-thiamine diphosphate complex to 10(-2) s-1. Acceleration of this step beyond the observed overall catalytic rate constant to 20 s-1 requires enzyme-bound FAD. FAD appears to bind in a two-step mechanism. The primarily bound form allows formation of hydroxyethylthiamine diphosphate but not the transfer of electrons from this intermediate to O2. This intermediate form can be mimicked using 5-deaza-FAD, which is inactive toward O2 but active in an assay using 2,6-dichlorophenolindophenol as electron acceptor. This analogue also promotes the rate constant of C2-H dissociation of thiamine diphosphate in pyruvate oxidase beyond the overall enzyme turnover. Formation of the catalytically competent FAD-thiamine-pyruvate oxidase ternary complex requires a second step, which was detected at low temperature.

Catalysis↗
PubMed1998

High-energy phosphates in the frontal lobe correlate with Wisconsin Card Sort Test performance in controls, not in schizophrenics: a 31phosphorus magnetic resonance spectroscopic and neuropsychological investigation.

In recent years, a number of 31phosphorus magnetic resonance spectroscopy (P-MRS) studies on the frontal lobe of schizophrenics have been performed, reporting alterations of phospholipids and high-energy phosphates. Deicken et al. (1994b) recently found positive correlations between left frontal phosphomonoester% (PME%) levels and the performance of a specific frontal lobe task, the Wisconsin Card Sorting Test (WCST), in schizophrenics. In the present paper, the correlations between phospholipids and high-energy phosphates in the frontal lobe of 26 schizophrenics and 23 controls measured with a volume-selective P-MRS method were investigated. Overall, we could not find any correlations between WCST results and phospholipid levels, but in controls phosphocreatine% (PCr%) and PCr/adenenosine triphosphate (ATP) ratios were negatively correlated with test performance. Since PCr behaves as a buffer of ATP, in the sense that when ATP is consumed by neuronal activity PCr is catalysed rapidly to ATP, increased PCr% values and, moreover, increased PCr/ATP ratios point to a decreased ATP consumption. Thus, the correlations found between PCr% and PCr/ATP and test performance in controls point to an association between reduced performance in a specific frontal lobe task and decreased energy demanding processes at rest. This association was not found in schizophrenics, possibly due to the influence of neuroleptic medication or the disease process per se.

Adenosine Triphosphate↗
PubMed1998

31P magnetic resonance spectroscopy in the frontal lobe of major depressed patients.

Most research with 31P-magnetic resonance spectroscopy (31P-MRS) in affective disorders has been done in the field of bipolar disturbances. Reduced frontal and temporal lobe phosphomonoester (PME) concentrations were measured in the euthymic state, whereas increased values were found in the depressed state. In bipolar-II patients reduced phosphocreatine (PCr) concentrations were reported in the euthymic, depressed, and manic state. The aim of the present study was to explore whether PME and PCr were also altered in the frontal lobe of major depressed, unipolar patients. Therefore, we used 31P-MRS to investigate the relative phospholipid and high-energy phosphate concentrations in the frontal lobe of 14 unipolar patients, mostly medicated, and 8 age-matched controls. We found increased PME and decreased ATP values. Other 31P-MRS parameters were not different in both groups. Phosphomonoester percentages correlated negatively with the degree of depression. Thus, the main alterations found in bipolar depressed patients could also be demonstrated in unipolar depressed patients. The results are discussed with regard to disturbed phospholipid and intracellular high-energy phosphate metabolism in depressed patients.

Adenosine Triphosphate↗
PubMed1997

Decreased energy demanding processes in the frontal lobes of schizophrenics due to neuroleptics? A 31P-magneto-resonance spectroscopic study.

In the present investigation on 31P-magneto-resonance spectroscopic parameters in the frontal lobe, we found phosphocreatine levels and the ratio phosphocreatine/adenosine triphosphate to be increased (12.62 +/- 1.98% resp. 0.31 +/- 0.06) in 50 neuroleptic-treated schizophrenics, whereas no differences were detected in 10 neuroleptic-free patients (11.66 +/- 2.57% resp. 0.29 +/- 0.08) compared to 36 controls (11.37 +/- 1.45 resp. 0.29 +/- 0.04). This result points to a major role of neuroleptics in the metabolism of high-energy phosphates.

Adenosine Triphosphate↗
PubMed1997

Molecular mechanism of regulation of the pyruvate dehydrogenase complex from E. coli.

The pyruvate dehydrogenase multienzyme complex from E. coli shows a sigmoidal dependency of the reaction rate on the substrate concentration when product formation is followed in the presence of physiological concentrations of the cofactor thiamin diphosphate. To elucidate the molecular mechanism of this regulation, the influence of the substrate pyruvate on the coenzyme-protein interaction has been investigated using several coenzyme analogues. The observed binding constants of all coenzymatically active analogues are increased in the presence of the substrate pyruvate, whereas those of all coenzymatically inactive analogues are not altered in the presence of pyruvate. This points to an increased binding affinity of a reaction-intermediate-coenzyme complex to the protein. Since cofactor binding and dissociation at physiological concentrations of thiamin diphosphate are slow compared to the catalytic reaction, a slow transition to the active state of the enzyme occurs. After lowering the pyruvate concentration, the opposite effect, a dissociation of the thiamin diphosphate from the enzyme is observed. This slow substrate dependent enhancement of cofactor binding enables efficient regulation of the pyruvate dehydrogenase complex by its substrate pyruvate.

Coenzymes↗
PubMed1997

A novel type of glutathione peroxidase: expression and regulation during wound repair.

We have previously identified and cloned a novel keratinocyte growth factor (KGF)-regulated gene in human keratinocytes that encodes the human homologue of a bovine non-selenium glutathione peroxidase (GPx). To gain insight into the regulation of this gene in vivo, we isolated the murine homologue from a mouse skin cDNA library. In vitro transcription/translation demonstrated that the cDNA encodes a 27 kDa protein. Furthermore, we amplified by PCR a partial cDNA that most likely corresponds to a related gene. RNase protection analysis revealed tissue-specific expression of both genes and the occurrence of alternative splicing or RNA editing of at least one of the primary transcripts. Similar to that of KGF, expression of GPx was strongly induced after cutaneous injury, and each isoform displayed unique kinetics of expression during the repair process. In situ hybridization studies demonstrated high levels of GPx mRNA in keratinocytes of the hyperproliferative epithelium at the wound edge. Since these cells express functional KGF receptors, induction of GPx expression by KGF might also occur in vivo. These data suggest a role for GPx in the protection of epithelial cells against oxidative stress, particularly during the inflammatory phase of wound repair.

Amino Acid Sequence↗
PubMed1997

Role of Glu51 for cofactor binding and catalytic activity in pyruvate decarboxylase from yeast studied by site-directed mutagenesis.

We investigated the importance of the interaction between the Nl'-atom of the cofactor thiamine diphosphate and glutamic acid residue 51 in pyruvate decarboxylase (EC 4.1. 1.1). The yeast wild type gene PDCl and the respective mutant genes (E51Q and E51A) were expressed in Escherichia coli. The three enzymes were purified to homogeneity. They comigrated as a single band during silver-stained SDS/PAGE with a molecular mass of 60 000 Da. A molecular mass of 61 200 +/- 200 Da was determined by mass spectrometry for the subunit. The native enzyme is a homotetramer as demonstrated by gel filtration experiments. Near- and far-UV CD spectra showed no significant differences for the apoenzyme of the wild type and the mutants. Slight differences in the rate of thiamine diphosphate binding to the apoprotein component were observed between the wild type and the E51Q PDC by CD spectroscopy. Compared to the wild type enzyme, thiamine diphosphate binding at the E51A mutant apoprotein is very slow. Only 0.04% of the catalytic activity of the wild type enzyme was observed for the E51Q mutant; the E51A mutant has no detectable catalytic activity. The S0.5 value for the substrate pyruvate is increased 33-fold for the E51Q mutant. Substrate activation was observed for both the wild type and the E51Q mutant. The interaction between the N1'-atom of the coenzyme and glutamic acid 51 strongly influences the catalytic activity but only moderately the binding of the cofactor to the apoenzyme and the substrate activation rate.

Binding Sites↗
PubMed1997

How thiamine diphosphate is activated in enzymes.

The controversial question of how thiamine diphosphate, the biologically active form of vitamin B1, is activated in different enzymes has been addressed. Activation of the coenzyme was studied by measuring thermodynamics and kinetics of deprotonation at the carbon in the 2-position (C2) of thiamine diphosphate in the enzymes pyruvate decarboxylase and transketolase by use of nuclear magnetic resonance spectroscopy, proton/deuterium exchange, coenzyme analogs, and site-specific mutant enzymes. Interaction of a glutamate with the nitrogen in the 1'-position in the pyrimidine ring activated the 4'-amino group to act as an efficient proton acceptor for the C2 proton. The protein component accelerated the deprotonation of the C2 atom by several orders of magnitude, beyond the rate of the overall enzyme reaction. Therefore, the earlier proposed concerted mechanism or stabilization of a C2 carbanion can be excluded.

Allosteric Regulation↗
PubMed1997

Partial inhibition of protein synthesis by Pseudomonas exotoxin A deranges catecholamine sensitivity of cultured rat heart myocytes.

To elucidate cellular mechanisms of myocardial depression in Pseudomonas sepsis the effects of sublethal concentrations of P. aeruginosa exotoxin A--a main virulence factor--were studied in cultured neonatal rat cardiomyocytes. It is known that this toxin exerts its pathogenic effect by inhibition of protein synthesis via ADP-ribosylation and thereby inactivation of elongation factor 2 (EF-2). Within 48 72 h, half maximal inhibition of protein synthesis occurs at 4-10 ng/ml. The toxin prevents the beta-adrenoceptor(AR)-mediated myosin heavy chain isozyme shift (V3/V1), while the T3-induced myosin shift is not suppressed. While beta 1-AR-downregulation by excess of norepinephrine (NE) is not affected, protein synthesis-dependent receptor upregulation in the recover period after removal of NE is completely suppressed by P. aeruginosa exotoxin A. Thus, a non-lethal, partial inhibition of global cellular protein synthesis by P. aeruginosa exotoxin A: (1) completely prevents beta 1-AR-mediated myosin isozyme shift and beta-AR upregulation: (2) sustains the cardiomyocytes in a catecholamine-refractory contractile state in the recovery period after catecholamine desensitization: (3) suggests cellular mechanisms by which P. aeruginosa exotoxin A might impair heart function in Pseudomonas sepsis: and (4) may help reveal the possible influence of endogenous inhibitors of EF-2.

ADP Ribose Transferases↗
PubMed1997

Lack of pharmacokinetic interaction of meloxicam with methotrexate in patients with rheumatoid arthritis.

OBJECTIVE: To investigate the pharmacokinetic interaction of oral meloxicam with intravenous (i.v.) methotrexate (MTX) in patients with rheumatoid arthritis (RA). METHODS: Thirteen patients with RA received MTX 15 mg i.v. in the absence of nonsteroidal antiinflammatory drugs (NSAID) and after one week in the presence of steady state levels of meloxicam. Plasma concentrations of MTX and meloxicam were determined using validated high performance liquid chromatography methods. One patient did not complete the study. The interaction of meloxicam and MTX was examined by equivalence testing. The endpoints AUCMTX, VssMTX, CltotMTX, and CmaxMTX were analyzed parametrically, whereas endpoints MRTMTX, CltotMTX, t1/2MTX, and tmaxMTX were analyzed nonparametrically. RESULTS: The MTX plasma concentrations over time, with and without meloxicam, did not differ significantly. The point estimator for the ratio of log transformed data of the primary endpoint AUCMTX was 108%; the lower 95% confidence limit was 100% and the upper 95% confidence limit was 117%. Clinical laboratory values and adverse events revealed no increased MTX toxicity during concomitant treatment with meloxicam. CONCLUSION: In this short term interaction study there was no statistically significantly effect of meloxicam on the pharmacokinetics of MTX. The combination of MTX and meloxicam did not lead to increased MTX toxicity.

Adolescent↗