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Biomedical subjects

C J Walker

Publications and source records attributed to C J Walker.

At least 19 recordsLinked to original sources
PubMed2001

Adsorption of carbon monoxide on Ni(110) above atmospheric pressure investigated with surface X-ray diffraction.

The crystallographic structure of CO adsorbed on Ni(110) has been investigated with x-ray diffraction at ambient pressures of CO ranging from 2.3 to 10(-10) bars. At room temperature, the high pressure and vacuum structures are identical. Above room temperature ( approximately 130 degrees C), the high pressure of CO induces a restructuring of the Ni substrate that develops strained (111) microfacets.

Journal Article↗
PubMed1999

Magnesium chelatase subunit D from pea: characterization of the cDNA, heterologous expression of an enzymatically active protein and immunoassay of the native protein.

Mg-chelatase catalyzes the insertion of Mg into protoporphyrin and lies at the branchpoint of heme and (bacterio)chlorophyll synthesis. In prokaryotes, three genes--BchI, D and H--encode subunits for Mg-chelatase. In higher plants, homologous cDNAs for the I, D and H subunits have been characterized. Since the N-terminal half of the D subunit is homologous to the I subunit, the C-terminal portion of the pea D was used for antigen production. The antibody recognized the chloroplast D subunit and was used to demonstrate that this subunit associated with the membranes in the presence of MgCl2. The antibody immunoprecipitated the native protein and inhibited Mg-chelatase activity. Expression in Escherichia coli with a construct for the full-length protein (minus the putative transit peptide) resulted in induction of 24.5 kDa (major) and 89 kDa (minor) proteins which could only be solubilized in 6 M urea. However, when host cells were co-transformed with expression vectors for the full-length D subunit and for the 70 kDa HSP chaperonin protein, a substantial portion of the 89 kDa protein was expressed in a soluble form which was active in a Mg-chelatase reconstitution assay.

Amino Acid Sequence↗
PubMed1997

Mechanism and regulation of Mg-chelatase.

Mg-chelatase catalyses the insertion of Mg into protoporphyrin IX (Proto). This seemingly simple reaction also is potentially one of the most interesting and crucial steps in the (bacterio)chlorophyll (Bchl/Chl)-synthesis pathway, owing to its position at the branch-point between haem and Bchl/Chl synthesis. Up until the level of Proto, haem and Bchl/Chl synthesis share a common pathway. However, at the point of metal-ion insertion there are two choices: Mg2+ insertion to make Bchl/Chl (catalysed by Mg-chelatase) or Fe2+ insertion to make haem (catalysed by ferrochelatase). Thus the relative activities of Mg-chelatase and ferrochelatase must be regulated with respect to the organism's requirements for these end products. How is this regulation achieved? For Mg-chelatase, the recent design of an in vitro assay combined with the identification of Bchl-biosynthetic enzyme genes has now made it possible to address this question. In all photosynthetic organisms studied to date, Mg-chelatase is a three-component enzyme, and in several species these proteins have been cloned and expressed in an active form. The reaction takes place in two steps, with an ATP-dependent activation followed by an ATP-dependent chelation step. The activation step may be the key to regulation, although variations in subunit levels during diurnal growth may also play a role in determining the flux through the Bchl/Chl and haem branches of the pathway.

Amino Acid Sequence↗
PubMed1996

Urinary testosterone (T) to epitestosterone (E) ratios by GC/MS. I. Initial comparison of uncorrected T/E in six international laboratories.

Six laboratories in six countries collaborated to investigate the analytical method for estimating the testosterone to epitestosterone ratio (T/E) in urine by gas chromatography/mass spectrometry in the context of detecting the application of T as a doping agent in sport. The protocol specified many but not all details of reagents and instrument conditions. The design included the distribution and analysis of four urines with different T/E values, three replicates per value, and one standard. The ranges of mean T/E values for the four urines estimated by peak area (PA) were 0.32-0.42, 0.72-0.94, 0.91-1.14 and 3.19-5.48. The analyses of variance for these data and for the peak height (PH) data were significant for the laboratory factor (p < 0.0001). In addition there was a significant interaction between the urine factor and the laboratory factor which indicates the complexity of the analysis. T/E calculated using PA was not significantly different from that using PH. For within-laboratory precision all values for PH and PA were < 8.3%, and for between-laboratory precision all values were < 11.7% except for one (20.1%). The data represent a baseline for future experiments designed to elucidate the sources of within-and between-laboratory variance, and to harmonize estimates of T/E.

Analysis of Variance↗
PubMed1996

Effect of slurry mineralogy on slurry ICP-AES.

Direct slurry analysis by FI ICP-AES has been tested on seven iron-containing and five zinc-containing minerals. Results indicate that the method can be applied for traces and majors in a range of different materials.

Journal Article↗
PubMed1996

The pentothal test in the management of orthopaedic patients with hysterical illness.

Hysterical illness may be defined as symptoms normally associated with disease without pathological basis. Orthopaedics attracts more of these patients than most specialties. They are intensively and expensively investigated, yet clinicians remain anxious they are missing genuine pathology. We use examination under pentothal, without analgesia, to help distinguish real from imagined symptoms. The results of a review of patients evaluated under pentothal over the past two years are presented.

Adolescent↗
PubMed1995

Proposed confirmatory procedure for detecting 5 alpha-dihydrotestosterone doping in male athletes.

Currently, there is no recommended confirmatory procedure for detecting doping with the anabolic steroid 5 alpha-dihydrotestosterone (DHT) in sportsmen. To develop a method, we determined ratios of hormone concentrations in urine from 120 healthy men and used these to set discrimination limits. These limits were then applied to results for urine specimens from 10 volunteers given DHT percutaneously (125 mg twice daily for 4 days). The ratio of DHT to epitestosterone (EpiT) was chosen as the primary marker of DHT administration, and ratios of 5 alpha-androstane-3 alpha, 17 beta-diol (5 alpha-ADIOL) to EpiT, 5 alpha-ADIOL to luteinizing hormone, and 5 alpha-ADIOL to 5 beta-androstane-3 alpha, 17 beta-diol were proposed as secondary markers. To evaluate method suitability, we analyzed 194 samples from sports competitors; results for 193 samples were negative, but the ratios in 1 sample greatly exceeded all the chosen limits. In conclusion, we propose that a test scheme based on our recommendations be considered for implementation in all Olympic drug-testing laboratories.

Adolescent↗
PubMed1995

Reasons for the loss of sensitivity and specificity of methodologic MeSH terms and textwords in MEDLINE.

OBJECTIVE: To determine the reasons for the loss of sensitivity and specificity of methodologic MeSH terms and textwords in MEDLINE for identifying sound clinical studies of the etiology, prognosis, diagnosis, prevention, or treatment of disorders in adult general medicine. DESIGN: Analytic survey of the information retrieval properties of methodologic MeSH terms and textwords selected to detect studies meeting basic methodologic criteria for direct clinical use in general adult medicine. MEASURES: Frequency of non-use and misuse of relevant methodologic MeSH terms and textwords among studies meeting and not meeting the basic criteria for clinical practice as determined by the manual review (the gold standard) of all articles in 10 internal and general medicine journals for 1986 and 1991. RESULTS: Loss of sensitivity due to the non-use of relevant methodologic terms among articles meeting basic methodologic criteria was more pronounced in the areas of diagnosis, prognosis, and etiology than treatment in 1991 and 1986. The use of relevant methodologic terms has improved from 1986 to 1991 in all areas except prognosis. Loss of specificity due to the use of relevant methodologic terms among articles not meeting basic methodologic criteria occurred most frequently in the areas of treatment and etiology. CONCLUSIONS: Although the appropriate use of methodologic MeSH and textwords has improved from 1986 to 1991 among studies meeting basic methodologic criteria for direct clinical use in general adult medicine much improvement is still needed in the areas of diagnosis, prognosis, and etiology. Improvement is needed in assigning the relevant methodologic index terms to studies that meet the methods criteria and in having the authors use the relevant methodologic textwords in the title or abstract. Some improvement is also needed in not using methodologic terms when the study clearly does not meet the methods criteria.

Abstracting and Indexing↗
PubMed1995

Preliminary assessment of the effect of more informative (structured) abstracts on citation retrieval from MEDLINE.

OBJECTIVE: To compare the recall and precision of citation retrieval from MEDLINE when searching in journals containing traditional abstracts with journals containing abstracts prepared according to the guidelines of more informative abstracts (MIAs) for clinical journal articles. DESIGN: Analytic survey of operating characteristics of MeSH terms and textwords selected to detect studies meeting basic methodological criteria for direct clinical use in general adult medicine in priority journals with and without MIAs. MEASURES: The sensitivity, specificity, and precision of search terms were determined by comparison with a manual review of all articles (the gold standard) in 10 internal and general medicine journals for 1986 and 1991. The performance of each search term was compared before and after the implementation of MIAs and in two journal sets with and without MIAs in 1991. RESULTS: The sensitivity of some terms increased with the use of MIAs and also increased by the same amount in journals with usual abstracts in 1991 compared to 1986. The use of MIAs had the greatest impact in the areas of etiology and prognosis. Many MeSH and textwords had comparable performance before and after the implementation of MIAs and between journals with and without MIAs in 1991. CONCLUSIONS: More informative abstracts improved the retrieval properties of some, but not all, textwords and medical subject headings. Further work may be needed to enhance the effects of MIAs on bibliographic retrieval.

Abstracting and Indexing↗
PubMed1994

The magnesium-insertion step of chlorophyll biosynthesis is a two-stage reaction.

Mg(2+)-chelatase catalyses the first step unique to chlorophyll synthesis, namely the insertion of Mg2+ into protoporphyrin IX. When pea (Pisum sativum L., cv. Spring) chloroplasts are lysed in a buffer lacking Mg2+ and the thylakoids removed by centrifugation, the remaining mixture of light membranes and soluble proteins (LM/S) has high Mg(2+)-chelatase activity. Several lines of evidence are presented to show that the Mg2+ insertion catalysed by this preparation is a two-step reaction consisting of activation followed by Mg2+ chelation. An activated state of Mg(2+)-chelatase is achieved by preincubating LM/S with ATP. The activated state is observed as the elimination of the approx. 6 min lag in the rate of Mg2+ chelation on addition of the porphyrin substrate. The activity of LM/S assayed at low protein concentrations can be greatly enhanced by preincubating at high protein concentrations (12 mg/ml is optimal). This activation effect requires the presence of both LM and S fractions, as well as ATP. Both steps require ATP, but at different concentrations; the first step is optimal at > 0.5 mM (EC50 = 0.3 mM) and the second step is optimal at 0.3 mM (EC50 < 0.2 mM). ATP in the first step could be replaced by ATP[S]; this analogue could not sustain activity in the second step. This activated state was stable for at least 30 min at room temperature, but chilling of preincubated LM/S on ice for 30 min caused an almost complete loss of the activated state.

Adenosine Triphosphate↗
PubMed1994

Biosynthetic studies on chlorophylls: from protoporphyrin IX to protochlorophyllide.

The series of reactions leading from protoporphyrin IX to protochlorophyllide have been studied over the last 15 years in the authors' laboratories at Davis and Clemson. Here, two crucial steps are emphasized, the discovery of the ATP requirement for Mg2+ chelation, and the oxidative cyclization of Mg-protoporphyrin IX monomethyl ester to protochlorophyllide. The in vitro systems for the chelation of Mg2+ and for the oxidative cyclization of Mg-protoporphyrin IX monomethyl ester both require membrane-associated and soluble heat-labile components. We speculate about the enzymological mechanisms of these important reactions, their sub-plastidic localization and the relationship of these individual steps to the broader questions of chloroplast and cell development.

Adenosine Triphosphate↗
PubMed1994

Quantitative comparison of pre-explosions and subheadings with methodologic search terms in MEDLINE.

OBJECTIVE: To compare the retrieval characteristics of subheadings with methodologic textwords and MeSH terms in MEDLINE for identifying sound clinical studies on the etiology, prognosis, diagnosis, prevention and treatment of disorders in general adult medicine. DESIGN: Analytic survey of the information retrieval properties of methodologic textwords, single methodologic MeSH terms, pre-explosions and subheadings selected to detect studies meeting basic methodologic criteria for direct clinical use in general adult medicine. MEASURES: The sensitivity, specificity, and precision of search terms were determined by comparing the citations retrieved by the search strategies in MEDLINE with that of a manual review (the gold standard) of all articles in 10 internal and general medicine journals for 1986 and 1991. RESULTS: For treatment and diagnosis in 1991, and treatment, diagnosis, and etiology in 1986, pre-explosions yielded the highest sensitivity, with typical absolute increases exceeding 15%. For etiology and prognosis in 1991, and prognosis in 1986, textwords or MeSH terms yielded the highest sensitivity. In all cases the increase in sensitivity was coupled with a loss in specificity and precision. CONCLUSIONS: Compared with searching with single methodologic textwords and subject headings, the detection of sound clinical studies on the diagnosis and treatment of disorders in general adult medicine was consistently enhanced by searching with pre-explosions, but at a price of decreased specificity and precision.

Abstracting and Indexing↗
PubMed1993

A program to enhance clinical use of MEDLINE. A randomized controlled trial.

OBJECTIVE: To determine if a preceptor and individualized feedback improves the performance of physicians in searching MEDLINE in clinical settings. DESIGN: Randomized controlled trial with 2 to 10 months follow-up. SETTING: A 300-bed teaching hospital. PARTICIPANTS: All 392 physicians and physicians-in-training from 6 major clinical departments were invited to participate if they made patient-care decisions during the study period; 79.4% agreed. INTERVENTIONS: All participants were given 2 hours of basic training, then randomized to a control group (no further intervention) or an intervention group in which each person chose a clinical preceptor experienced in MEDLINE searching and received individualized feedback from a study librarian on each of their 1st 10 searches. MAIN OUTCOME MEASURES: The number and proportion of relevant and irrelevant references retrieved for 1st, 4th, and 8th searches of participants were compared with independent librarian searches on the same topics. RESULTS: Intervention group members did not search more often than controls (5.9 searches per month versus 4.7, respectively; P = 0.26) and there were no significant differences in the quality of searches. Rather, search performance for both groups improved, with the average number of relevant references retrieved per search increasing from 4.5 to 7.4 (P < 0.01). The librarian retrieved more relevant citations than participants for the 1st search (P = 0.001) but not for the 4th (P = 0.60) or 8th (P = 0.76) searches. CONCLUSIONS: A program of assigning preceptors and providing feedback on individual searches did not enhance the quantity or quality of searches. Soon after a basic introduction to searching, however, clinicians in both groups improved their search performance.

Clinical Medicine↗
PubMed1993

Assessment of methodologic search filters in MEDLINE.

OBJECTIVE: To determine the retrieval characteristics of methodologic textwords and MeSH terms in MEDLINE for identifying methodologically sound studies on the etiology, prognosis, diagnosis, and prevention and treatment of disorders in general adult medicine. DESIGN: Comparison of methodologic search terms and phrases for the retrieval of citations in MEDLINE with a manual hand search of the literature (the gold standard) for 10 internal and general medicine journals for 1986 and 1991. MEASURES: Sensitivity (proportion of methodologically sound and correct topic studies retrieved) and specificity (proportion of unsound or wrong topic articles not retrieved) of the search strategies. RESULTS: The individual terms yielding the best sensitivity for 1991 by purpose category were: risk (tw) for etiology; exp cohort studies for prognosis; sensitivity (tw) for diagnosis; and clinical trial (pt) for treatment. The corresponding terms for 1986 were: risk (tw) for etiology; prognosis: (tw) for prognosis; sensitivity (tw) for diagnosis; and random: (tw) for treatment. CONCLUSIONS: The performance of methodologic MeSH terms and textwords varied greatly in MEDLINE and changed from 1986 to 1991. More complex search strategies may be required to optimize retrieval.

Information Storage and Retrieval↗
PubMed1992

Possible indices for the detection of the administration of dihydrotestosterone to athletes.

Dihydrotestosterone (DHT) can be used by an athlete as an anabolic steroid to evade the current International Olympic Committee approved drug tests. To investigate the possibility of a method for its detection, the heptanoate ester of DHT was administered to two male subjects (150 mg i.m.). Urine samples, collected before and after the injection, were subjected to enzymatic hydrolysis and the excretion rates of DHT, 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-diol) and testosterone (T) were determined by radioimmunoassay. Relative changes in the excretion of DHT, 3 alpha-diol, 5 alpha-androstane-3 beta,17 beta-diol (3 beta-diol), 5 beta-androstane-3 alpha, 17 beta-diol (5 beta-diol), T and epitestosterone (17 alpha hydroxyandrost-4-en-3-one; Epi-T) were determined by gas chromatography-mass spectrometry (GC-MS). Following administration of DHT, the urinary excretion rates of DHT, 3 alpha-diol and 3 beta-diol increased when compared to those of T, Epi-T, 5 beta-diol and luteinizing hormone (LH). Concentrations of DHT in the plasma increased whereas those of T, LH and follicle stimulating hormone decreased. The changes following such modest doses of DHT suggest that these ratios of urinary hormones may be used for the detection of doping with DHT.

Adult↗