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T Chung

Publications and source records attributed to T Chung.

89 records · Page 5Linked to original sources
PubMed1988

Glyoxylate bypass operon of Escherichia coli: cloning and determination of the functional map.

In Escherichia coli, a single operon encodes the metabolic and regulatory enzymes of the glyoxylate bypass. The metabolic enzymes, isocitrate lyase and malate synthase, are expressed from aceA and aceB, and the regulatory enzyme, isocitrate dehydrogenase kinase/phosphatase, is expressed from aceK. We cloned this operon and determined its functional map by deletion analysis. The order of the genes in this operon is aceB-aceA-aceK, with aceB proximal to the promoter, consistent with the results of previous experiments using genetic techniques. The promoter was identified by S1 nuclease mapping, and its nucleotide sequence was determined. Isocitrate lyase and malate synthase were readily identified by autoradiography after the products of the operon clone were labeled by the maxicell procedure and then resolved by electrophoresis. In contrast, isocitrate dehydrogenase kinase/phosphatase, expressed from the same plasmid, was undetectable. This observation is consistent with a striking downshift in expression between aceA and aceK.

Base Sequence↗
PubMed1988

Nucleotide sequence of aceK, the gene encoding isocitrate dehydrogenase kinase/phosphatase.

In Escherichia coli, the phosphorylation and dephosphorylation of isocitrate dehydrogenase (IDH) are catalyzed by a bifunctional protein kinase/phosphatase. We have determined the nucleotide sequence of aceK, the gene encoding IDH kinase/phosphatase. This gene consists of a single open reading frame of 1,734 base pairs preceded by a Shine-Dalgarno ribosome-binding site. Examination of the deduced amino acid sequence of IDH kinase/phosphatase revealed sequences which are similar to the consensus sequence for ATP-binding sites. This protein did not, however, exhibit the extensive sequence homologies which are typical of other protein kinases. Multiple copies of the REP family of repetitive extragenic elements were found within the intergenic region between aceA (encoding isocitrate lyase) and aceK. These elements have the potential for combining to form an exceptionally stable stem-loop structure (delta G = -54 kcal/mol [ca. -226 kJ/mol]) in the mRNA. This structure, which masks the ribosome-binding site and start codon for aceK, may contribute to the downshift in expression observed between aceA and aceK. Another potential stem-loop structure (delta G = -29 kcal/mol [ca. 121 kJ/mol]), unrelated to the REP sequences, was found within aceK.

Amino Acid Sequence↗
PubMed1987

The effect of valproic acid on the 5-hydroxyindoleacetic, homovanillic and lactic acid levels of cerebrospinal fluid.

In this study the cisternal cerebrospinal fluid (CSF) contents of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) were sequentially measured in free-moving rats which were administered 50-500 mg X kg-1 valproic acid. Animals receiving 100-500 mg X kg-1 valproic acid showed significant increases in CSF 5-HIAA and HVA content, with maximal accumulation rates of 1.80-2.10 and 0.25-0.30 nmol X ml-1 X h-1, respectively, being reached at the 250-mg X kg-1 dose. The combination of valproic acid 500 mg X kg-1 and probenecid 300 mg X kg-1 failed to increase the accumulation rates of 5-HIAA and HVA over those seen with valproic acid 500 mg X kg-1 or probenecid 300 mg X kg-1 alone. This pattern of change indicates that valproic acid and probenecid share a common site of action in blocking the clearance of 5-HIAA and HVA from CSF. The tranquillizer diazepam produced progressive increases in CSF 5-HIAA and HVA content which suggested a similar action to that of valproic acid and probenecid. The anticonvulsants phenytoin and phenobarbital produced selective increases in 5-HIAA, whereas the tranquillizer chlorpromazine produced proportionally larger increases in HVA, changes which seem to indicate a more selective effect of these drugs on the serotonergic or dopaminergic systems, respectively. Valproic acid was associated with increases in CSF lactate which occurred in the absence of similar increases of blood or tissue lactate. This indicated that valproic acid, like probenecid, can inhibit the monocarboxylic acid transport system which removes lactate from the CSF.

Animals↗
PubMed1985

A single gene codes for the kinase and phosphatase which regulate isocitrate dehydrogenase.

The gene which codes for isocitrate dehydrogenase kinase/phosphatase of Escherichia coli, aceK, has been cloned. Physical and functional mapping of this clone indicated that both the isocitrate dehydrogenase kinase and isocitrate dehydrogenase phosphatase activities are encoded by an 1800-base pair sequence. This sequence produced a polypeptide with an apparent molecular weight of 66,000, which is identical to that of the purified protein. Since a protein of this size would require an 1800-base pair coding sequence, we conclude that isocitrate dehydrogenase kinase and isocitrate dehydrogenase phosphatase are expressed from a single gene. This strongly suggests that both activities reside on the same polypeptide chain. The cloning of aceK was made possible by the fortuitous addition of a second origin of replication to the expression vectors which were employed. These expression vectors were found to inhibit the growth of E. coli on the minimal acetate selective medium. The inclusion of a second origin of replication reduced the copy number and so reduced the inhibitory effects of these vectors. Control of the copy number through the addition of replication origins may have a general facility when manipulating plasmids which are potentially toxic to E. coli.

Base Sequence↗
PubMed1983

Mechanisms of leukotriene formation: hemoglobin-catalyzed transformation of 15-HPETE into 8,15-DiHETE and 14,15-DiHETE isomers.

Four isomers of 8,15-diHETE as well as 14,15-diHETEs are isolated and characterized after exposure of 15-HPETE to hemoglobin. It is found that 83% of the C-8 oxygen atoms in 8(R), 15(S)-diHETE and 8(S), 15(S)-diHETE, and 41% of the C-8 oxygen atoms in 8(R), 15(S)-11Z-diHETE and 8(S), 15(S)-11Z-diHETE are derived from H2(18)O. These results suggest that hemoglobin catalyzes the transformation of 15-HPETE into these products via a free radical process, possibly involving the intermediacy of 14,15-LTA. Intact human leukocytes contain a distinct enzyme system for catalyzing the conversion of 15-HPETE into 14,15-LTA. This enzyme activity is inhibited by ETYA and is rapidly denatured upon homogenization of the intact leukocytes.

Animals↗
PubMed1982

The effects of imidazole-4-acetic acid on cerebral carbohydrate metabolism.

The effects of intravenous administration of 50-400 mg/kg imidazole-4-acetic acid (IMA) on the carbohydrate metabolism of the rat brain were assessed by measurement of the cerebral hemisphere contents of energy phosphates and glycolytic--citric acid cycle metabolites. IMA (100-400 mg/kg) produced a spectrum of electroencephalographic (EEG) change ranging from desynchronization to electrical suppression which was associated with unchanged tissue contents of ATP, ADP, and AMP, increasing levels of phosphocreatine, glucose, and aspartate, and decreasing levels of pyruvate, lactate, alpha-ketoglutarate, and malate. The changes in glycolytic intermediates were present within 5 min of injecting IMA (200 mg/kg) and the pattern suggested a suppression of glycolysis. The EEG stage of electrical suppression with episodic spiking (400 mg/kg) was associated with a 30% reduction of cortical high-energy phosphate use. The lowest dose of IMA (50 mg/kg) resulted in episodic EEG desynchronization which was associated with no significant changes of the measured metabolites. The results indicate that IMA is associated with metabolite changes that are compatible with a state of cerebral depression and that the desynchronous EEG pattern is without a biochemical correlate of increased neuronal activity.

Adenosine Triphosphate↗
PubMedDate not supplied

Heart size on chest x-ray as a predictor of cardiac enlargement by echocardiography in children.

To determine the usefulness of heart size on chest radiograph (CXR) in predicting cardiac enlargement (CE) in children, we prospectively evaluated 95 consecutive outpatients, who had both a CXR and echocardiography performed. Their median age was 5.0 years (2 days to 19.9 years). All patients underwent CXR assessment by a pediatric radiologist, with classification of cardiac silhouette as normal, borderline, or enlarged. Echocardiographic assessment of CE was performed by a pediatric echocardiographer. Sensitivity, specificity, and predictive values of the pediatric radiologist's interpretation of heart size on CXR were estimated. The presence of CE by echocardiography was used as the gold standard. Seventy-nine patients (83.2%) had no CE on CXR, and 16 patients (16.8%) had CE. Sensitivity of the CXR to identify CE was 58.8%, 95% confidence interval (CI) [32.9, 81.6], with a positive predictive value of 62.5% [35.4, 84.8]. Specificity was 92.3% [84.0, 97.1], with a negative predictive value of 91.1% [82.6, 96.4]. These data suggest that the assessment of CE on CXR to predict CE by echocardiography has a relatively high specificity and negative predictive value, but a low sensitivity and positive predictive value. The limitations of CXR as a diagnostic test should be understood by clinicians using the test when screening children for cardiac disease.

Adolescent↗
PubMedDate not supplied

From signal to image: magnetic resonance imaging physics for cardiac magnetic resonance.

Magnetic resonance imaging (MRI) is a powerful tool which enables the visualization of anatomy and the assessment of many physiological aspects of organ function. MRI and magnetic resonance angiography and magnetic resonance spectroscopy will play critical roles in cardiac applications during the next millennium. Thus, it is important to have a basic understanding of the most important physical processes in MR--the generation of nuclear magnetic resonance signals and their transformation into images. A conceptual description of these processes is the primary focus of this article. Also discussed are some additional signal manipulations specific to the needs of cardiac MRI, a field readily identified as the most significant in modern MRI technology development.

Heart Diseases↗
PubMedDate not supplied

Assessment of cardiovascular anatomy in patients with congenital heart disease by magnetic resonance imaging.

The following discussion addresses the assessment of cardiovascular anatomy in patients with congenital heart disease by magnetic resonance (MR). The focus of this review is on the techniques of performing the MR examination. In particular, individual pulse sequences are described and illustrated with their strengths and weaknesses. Imaging strategies using the described pulse sequences are proposed. The pulse sequences described are widely available on most MR scanners. Therefore, the proposed imaging strategies are clinically proven to be simple and effective ways to perform cardiac MR examination for the assessment of cardiovascular anatomy in patients with congenital heart disease. Functional imaging, such as flow analysis and ventricular function assessment, are discussed elsewhere in this issue.

Adolescent↗
PubMedDate not supplied

Phase-velocity cine magnetic resonance imaging measurement of pulsatile blood flow in children and young adults: in vitro and in vivo validation.

Quantification of blood flow in vessels provides valuable information that aids management decisions in a variety of cardiac conditions. Current flow measurement techniques are often limited by accuracy, time resolution, convenience, or anatomic localization. This study examined the accuracy of a commercially available phase-velocity cine magnetic resonance imaging (PVC MRI) technique to quantify flow rate in a pulsatile flow phantom. In addition, the equivalence of PVC MRI measurements of pulmonary and systemic flow was evaluated in children and adults without any pathologic shunt. Using a pulsatile flow phantom, volume flow rates measured by PVC MRI were compared to those by a transit-time ultrasound flowmeter over a range of flow rates (1.25-3.5 L/min, 13 trials). Close agreement was found between these techniques (y = 1.02x - 0.02, r = 0.99, Bland-Altman bias = -0.045 L/min, 95% limits of agreement = -0. 19-0.10 L/min). Twenty subjects (median age 12.8 years, range 0.7-49 years) with no pathologic shunt underwent PVC MRI measurement of blood flow in the main pulmonary artery (Q(p)) and the ascending aorta (Q(s)). Data processing time for each location was 20 minutes. The Q(p)/Q(s) ratio closely approximated unity (mean = 0.99, SD = 0. 10, range 0.85-1.19). Interobserver agreement was excellent (Bland-Altman bias = 0.09 L/min, 95% limits of agreement = 0.15-0.33 L/min). PVC MRI is an accurate technique to quantify pulsatile blood flow at a specific location. It can be used to noninvasively calculate Q(p) and Q(s) under normal flow conditions.

Adolescent↗
PubMedDate not supplied

Illness severity and self-efficacy as course predictors of DSM-IV alcohol dependence in a multisite clinical sample.

Illness severity and self-efficacy are two constructs of growing interest as predictors of clinical response in alcoholism. Using alternative measures of illness severity (DSM-IV symptom count, Alcohol Dependence Scale, and Addiction Severity Index) and self-efficacy (brief version of the Situational Confidence Questionnaire) rigorously controlled for theoretically important background variables, we studied their unique contribution to multiple indices of relapse, relapse latency, and use of alternative coping behaviors in a large, heterogeneous clinical sample. The Alcohol Dependence Scale contributed to the prediction of 4 of 5 relapse indicators. The SCQ failed to predict relapse behavior or its precursor, coping response. The findings emphasize the predictive validity of severity of dependence as a course specifier and underline the need for more sensitive and externally valid measures of cognitive processes such as self-efficacy for application in future studies of posttreatment behavior.

Adaptation, Psychological↗
PubMedDate not supplied

In vivo evaluation of Fontan pathway flow dynamics by multidimensional phase-velocity magnetic resonance imaging.

BACKGROUND: Hemodynamic efficiency of Fontan circulation is believed to be a major determinant of outcome. Prior research on flow dynamics in different modifications of Fontan circulation used in vitro models and computer-based simulation. This study was designed to compare in vivo flow dynamics in the systemic venous pathway between patients with atriopulmonary anastomosis (APA) and those with total cavopulmonary connection (TCPC). METHODS AND RESULTS: Multidimensional phase-velocity magnetic resonance imaging (PV-MRI) studies were performed on 10 patients who had undergone a modified Fontan operation (5 with TCPC and 5 with APA) and were free of symptoms. The groups were comparable in terms of age and body surface area. The interval since surgery was longer for APA than for TCPC subjects. In each subject, the phase-velocity data sets were used to generate dynamic velocity-vector maps and to calculate quantitative flow indices describing the 3-dimensional blood-flow patterns throughout the cardiac cycle at the widest diameter of the Fontan pathway. Mean flow rate was comparable between groups. Velocity-vector maps showed areas of flow reversal, flow stagnation, and circular flow within APA but not TCPC pathways. Analysis of quantitative flow indices showed that compared with the APA group, flow velocities in the TCPC patients were significantly higher (mean velocity, 14+/-6 cm/s versus 5+/-3 cm/s; P=0.02), less variable (coefficient of variation, 19+/-2% versus 37+/-3.5%; P<0.0001), and more unidirectional (degree of unidirectionality, 89+/-7% versus 71+/-12%; P=0.03). APA pathways were significantly more dilated than were TCPC pathways (P<0.01) and showed a trend toward larger diameter with increased interval since surgery (R2=0.6, P=0.09). Fontan pathway dilatation correlated with flow velocity variability (R2=0.57, P=0.01) and inversely with flow unidirectionality (R2=0.75, P=0.001). CONCLUSIONS: Blood flow patterns are more organized and uniform in TCPC than in APA pathways and are significantly influenced by pathway diameter. We speculate that TCPC may result in a more hemodynamically efficient circulation than APA because of differences in pathway dimension and uniformity.

Adolescent↗