Warfarin-fluconazole. II. A metabolically based drug interaction: in vivo studies

Consistent with expectations based on human in vitro microsomal experiments, administration of fluconazole (400 mg/day) for 6 days to six human volunteers significantly reduced the cytochrome P450 (P450)-dependent metabolic clearance of the warfarin enantiomers. In particular, P4502C9 catalyzed 6- and 7-hydroxylation of (S)-warfarin, the pathway primarily responsible for termination of warfarin's anticoagulant effect, was inhibited by approximately 70%. The change in (S)-warfarin pharmacokinetics caused by fluconazole dramatically increased the magnitude and duration of warfarin's hypoprothrombinemic effect. These observations indicate that co-administration of fluconazole and warfarin will result in a clinically significant metabolically based interaction The major P450-dependent, in vivo pathways of (R)-warfarin clearance were also strongly inhibited by fluconazole. 10-Hydroxylation, a metabolic pathway catalyzed exclusively by P4503A4, was inhibited by 45% whereas 6-, 7-, and 8-hydroxylations were inhibited by 61, 73, and 88%, respectively. The potent inhibition of the phenolic metabolites suggests that enzymes other than P4501A2 (weakly inhibited by fluconazole in vitro) are primarily responsible for the formation of these metabolites in vivo as predicted from in vitro kinetic studies. These data suggest that fluconazole can be expected to interact with any drug whose clearance is dominated by P450s 2C9, 3A4, and other as yet undefined isoforms. Overall, the results strongly support the hypothesis that metabolically based in vivo drug interactions may be predicted from human in vitro microsomal data.

Molecular dynamics simulations of adipocyte lipid-binding protein: effect of electrostatics and acyl chain unsaturation

Molecular dynamics (MD) simulations have been performed on adipocyte lipid-binding protein, using the apo and holo forms, bound with stearic and oleic acid. The contribution of electrostatics to protein dynamics and ligand stabilization was assayed by perturbing the electrostatic charge of Arg106 and Arg126 (positive-->neutral) and the fatty acid (132H) headgroup (negative-->neutral). MD simulations for charged holo forms demonstrated significantly greater electrostatic binding energy and a more stabilized hydrogen bond network than simulations performed using neutral forms. Electrostatics, however, appeared to have little effect on fatty acid behavior, e.g., fluctuation of the dihedral head group; number of dihedral transitions within the acyl chain; and change in the end-to-end distance for fatty acid. Instead, fatty acid behavior appeared to be dictated by the presence or absence of an unsaturated bond within the acyl chain. A significantly greater number of transitions were observed during MD simulations in oleic than stearic acid. In addition, significantly greater fluctuation was observed for oleic acid, within the C2 headgroup and C9 and C11 dihedrals (which lie adjacent to the olefin bond of oleic acid). The dynamic behavior of the acyl chain may thereby be more a property of van der Waals contact, and the degree of acyl chain unsaturation, than a function of electrostatics. In the absence of fatty acid, an increase in distance between guanidino carbon centered atoms of Arg126 and Arg106 was observed during MD simulations of the charged apo form. This effect not observed with the neutral apo form or in any of the holo complexes and, presumbably, was a result of repulsion between the negatively charged arginine sidechains. Conserved waters reflected substantially lower mean-square displacement (msd) in all simulations, except the neutral apo form. This suggests that the presence of either charged amino acids or lipid provides increased order for water within the binding pocket. These results provide a dynamic perspective of the interactive nature within the FABP binding pocket regulated in a complex manner by the electrostatics within the binding cavity, acyl chain structure and behavior, and water energetics.

Purification of meningococcal lipo-oligosaccharide by FPLC techniques

A rapid and efficient method for the preparation of highly pure meningococcal lipo-oligosaccharide (LOS) was developed. This used a Superose 6 column on a FPLC system to purify LOS from phenol-water extracts of cell lysates of Neisseria meningitidis. The purest LOS preparations, with no detectable protein contamination and less than 0.5% (w/w) residual RNA, were obtained when cell lysates had been treated with RNase ONE before phenol extraction and chromatographic separation. Preparations that had received no ribonuclease treatment had 2-3% residual RNA contamination and predigestion of samples with RNase A, which only partially degraded the RNA present in the crude extracts, resulted in LOS samples contaminated with 15-20% residual RNA. The LOS purified from RNase ONE-treated extracts was highly endotoxic, and showed no reduction in antibody binding or specific endotoxin activity compared to unpurified material. Approximately 80% of the LOS applied to the chromatography column was recovered as purified material.

Endotoxin exposure-response in a fiberglass manufacturing facility

Peak expiratory flow (PEF) and workplace exposure to endotoxin, phenolic resin, and formaldehyde were measured to investigate asthma symptoms and medication use among employees in a fiberglass wool manufacturing plant. Self-recorded PEF was obtained from 37 workers, for a total of 181 days off work and 187 days at work with concurrent personal exposure monitoring. Pre- and post-shift spirometry were obtained on at least 2 days. The 8 hr time-weighted average personal exposure ranges were endotoxin; 0.4-759 ng/m3; phenolic resin, 5.7-327 micrograms/m3; and formaldehyde, 1.2-265 micrograms/m3. Amplitude percent mean peak flow was associated with years since starting regular work in the highest endotoxin exposure area, although current assignment in that area was associated with reduced amplitude--evidence for a healthy worker effect. Exposure-response was analyzed by regression of lung function change on exposure using generalized estimating equations with robust variance estimates. Endotoxin exposure above 4 ng/m3 (8 hr time-weighted average) was associated with a decline in lung function across the work shift, and with drops in lung function 16-20 hr after exposure. Phenolic resin exposure was not consistently associated with decrements, and formaldehyde was not associated with decrements in lung function.

Prediction of polyelectrolyte polypeptide structures using Monte Carlo conformational search methods with implicit solvation modeling

Many interesting proteins possess defined sequence stretches containing negatively charged amino acids. At present, experimental methods (X-ray crystallography, NMR) have failed to provide structural data for many of these sequence domains. We have applied the dihedral probability grid-Monte Carlo (DPG-MC) conformational search algorithm to a series of N- and C-capped polyelectrolyte peptides, (Glu)20, (Asp)20, (PSer)20, and (PSer-Asp)10, that represent polyanionic regions in a number of important proteins, such as parathymosin, calsequestrin, the sodium channel protein, and the acidic biomineralization proteins. The atomic charges were estimated from charge equilibration and the valence and van der Waals parameters are from DREIDING. Solvation of the carboxylate and phosphate groups was treated using sodium counterions for each charged side chain (one Na+ for COO-; two Na for CO(PO3)-2) plus a distance-dependent (shielded) dielectric constant, epsilon = epsilon 0 R, to simulate solvent water. The structures of these polyelectrolyte polypeptides were obtained by the DPG-MC conformational search with epsilon 0 = 10, followed by calculation of solvation energies for the lowest energy conformers using the protein dipole-Langevin dipole method of Warshel. These calculations predict a correlation between amino acid sequence and global folded conformational minima: 1. Poly-L-Glu20, our structural benchmark, exhibited a preference for right-handed alpha-helix (47% helicity), which approximates experimental observations of 55-60% helicity in solution. 2. For Asp- and PSer-containing sequences, all conformers exhibited a low preference for right-handed alpha-helix formation (< or = 10%), but a significant percentage (approximately 20% or greater) of beta-strand and beta-turn dihedrals were found in all three sequence cases: (1) Aspn forms supercoil conformers, with a 2:1:1 ratio of beta-turn:beta-strand:alpha-helix dihedral angles; (2) PSer20 features a nearly 1:1 ratio of beta-turn:beta-sheet dihedral preferences, with very little preference for alpha-helical structure, and possesses short regions of strand and turn combinations that give rise to a collapsed bend or hairpin structure; (3) (PSer-Asp)10 features a 3:2:1 ratio of beta-sheet:beta-turn:alpha-helix and gives rise to a superturn or C-shaped structure.

Conformational studies of osteocalcin in solution

1H-NMR and circular dichroism studies have been carried out on osteocalcin, a 49-residue, calcium-binding protein, the sequence of which contains a disulphide bridge, a proline-rich segment and three gamma-carboxyglutamic acid (Gla) residues. These latter residues have been proposed to lie on one face of an alpha helix and interact with the mineral phase, leading to incorporation of the protein into the bone matrix. Circular dichroism shows an increase in the alpha-helical structure on Ca2+ binding to bovine osteocalcin. This induced structure is lost on heating the protein, giving a spectrum close to that of the Ca(2+)-free protein. 1H-NMR studies of rabbit osteocalcin gave a set of resonance assignments and NOEs which could be interpreted in terms of distance constraints. These did not allow a single conformation to be defined for the protein in solution but reflect rather a flexible structure which may be essential for the function of the protein. The calculated structures contain a hydrophobic core (comprising Leu2, Leu32, Val36 and Tyr42, seen to be slowly flipping in the Ca(2+)-bound form) and have the gamma-carboxyglutamic acid side chains exposed on one face of the molecule.

Bias in conditional inference: implications for mental models and mental logic

Three experiments are reported in which subjects are given the opportunity to make any of the four inferences associated with conditional statements: modus ponens (MP), denial of the antecedent (DA), affirmation of the consequent (AC), and modus tollens (MT). The primary purpose of the research was to establish the generality and robustness of polarity biases that may be occasioned by systematic rotation of negative components in the conditional rules. In Experiments 1 & 2, three forms of conditionals were used: "if (not) p then (not) q", "(not) p only if (not) q" and "(not) q if (not) p". Experiment 1 used a conclusion evaluation task, whereas Experiment 2 used a conclusion production task. In Experiment 3, thematic conditionals were presented with and without a preceding scenario. The biases investigated were (a) affirmative premise bias--the tendency to draw more inferences with negative conclusions. The suggestive evidence for affirmative premise bias in the literature was not supported: very little evidence was found for it in the current experiments. Robust findings of negative conclusion bias were, however, found across the three experiments, although the bias was mostly restricted to DA and MT inferences. This suggests that the bias is best regarded as a difficulty with double negation. The results are discussed with respect to both the mental logic and mental model accounts of propositional reasoning. Neither theory as currently formulated can explain all of our findings, although a plausible revision of each is considered.

The ability of predicted internal dose measures to reconcile tumor bioassay data for chloroform

PBPK models are developed in the hope that they will improve our ability to extrapolate from one species to another and from one exposure regime to another. Evidence that a dose measure was successful at reconciling the available animal bioassay data would be encouraging. It would give us some confidence that the dose measure (as evaluated by a PBPK model) might yield reasonable predictions for yet other species (e.g., humans) and other dose routes. We have investigated the ability of a modified version of the Corley et al. (R. A. Corley, A. L. Mendrala, F. A. Smith, D. A. Staats, M. L. Gargas, R. B. Conolly, M. E. Andersen, and R. H. Reitz, 1990, Toxicol. Appl. Pharmacol. 103, 512-527) PBPK model for chloroform to reconcile the available bioassay data. Two rate-dependent dose measures, maximal rate of metabolism in the liver, and percentage of hepatocytes killed per day performed well at reconciling the rodent liver bioassay data, while all rate-independent dose measures performed less well. In contrast, none of the PBPK dose measures were capable of reconciling the rat and mouse kidney tumor response data. Here, administered dose scaled to body surface area performed the best.

De novo prediction of polypeptide conformations using dihedral probability grid Monte Carlo methodology

We tested the dihedral probability grid Monte Carlo (DPG-MC) methodology to determine optimal conformations of polypeptides by applying it to predict the low energy ensemble for two peptides whose solution NMR structures are known: integrin receptor peptide (YGRGDSP, Type II beta-turn) and S3 alpha-helical peptide (YMSEDEL KAAEAAFKRHGPT). DPG-MC involves importance sampling, local random stepping in the vicinity of a current local minima, and Metropolis sampling criteria for acceptance or rejection of new structures. Internal coordinate values are based on side-chain-specific dihedral angle probability distributions (from analysis of high-resolution protein crystal structures). Important features of DPG-MC are: (1) Each DPG-MC step selects the torsion angles (phi, psi, chi) from a discrete grid that are then applied directly to the structure. The torsion angle increments can be taken as S = 60, 30, 15, 10, or 5 degrees, depending on the application. (2) DPG-MC utilizes a temperature-dependent probability function (P) in conjunction with Metropolis sampling to accept or reject new structures. For each peptide, we found close agreement with the known structure for the low energy conformational ensemble located with DPG-MC. This suggests that DPG-MC will be useful for predicting conformations of other polypeptides.

Uncertainty in fitted estimates of apparent in vivo metabolic constants for chloroform

It has become common practice to rely on fitted estimates of apparent in vivo metabolic constants (e.g., Vmax and KM) in the parameterization of PBPK models. Yet, quantitative estimates of precision in these fitted parameters are not routinely reported. Such information is needed to assess the reliability of model predictions. The purpose of this study was to assess the precision in estimates of Vmax and KM for chloroform, accounting for both the statistical uncertainties in parameter estimates from individual data sets and any additional uncertainty due to differences in the parameter estimates derived from various experiments. Joint confidence regions for Vmax and KM from each experiment, generated using maximum likelihood techniques, were used to evaluate these questions. Three previously published data sets were considered. Estimates of Vmax and KM obtained from these data sets differed more than could be explained as a consequence of a limited number of observations, measurement error, or stochastic error. Issues associated with the use of maximum likelihood techniques to estimate joint confidence regions, the estimation of metabolic constants from individual experiments within a gas uptake study versus the full data set, the degree of overlap in the joint confidence regions for metabolic constants obtained from separate data sets, and the implications for risk assessment are discussed.

Benchmark Dose Workshop: criteria for use of a benchmark dose to estimate a reference dose

The purpose of the Benchmark Dose Workshop was to assess the feasibility and implications of replacing the no observed adverse effect level (NOAEL) with a benchmark dose (BMD) when deriving reference doses and concentrations (RfDs and RfCs). The workshop participants supported the use of the BMD method to remove many of the limitations inherent in using the NOAEL approach. Participants endorsed in general the use of a BMD for all quantal noncancer health effects and endorsed in particular the BMD for assessing developmental toxicity based on data presented at the workshop. The discussions of implementation recognized the need to demonstrate that changing from a NOAEL to a BMD gives the risk manager more certain information on which to base decisions. Most participants agreed that the current NOAEL-derived RfDs and RfCs are sufficiently protective and should only be changed as data become available for estimating a BMD. It was recognized that to achieve general acceptance of the BMD approach, it will have to be applied to a variety of endpoints.

Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults

Time-series, cross-sectional, and prospective cohort studies have observed associations between mortality and particulate air pollution but have been limited by ecologic design or small number of subjects or study areas. The present study evaluates effects of particulate air pollution on mortality using data from a large cohort drawn from many study areas. We linked ambient air pollution data from 151 U.S. metropolitan areas in 1980 with individual risk factor on 552,138 adults who resided in these areas when enrolled in a prospective study in 1982. Deaths were ascertained through December, 1989. Exposure to sulfate and fine particulate air pollution, which is primarily from fossil fuel combustion, was estimated from national data bases. The relationships of air pollution to all-cause, lung cancer, and cardiopulmonary mortality was examined using multivariate analysis which controlled for smoking, education, and other risk factors. Although small compared with cigarette smoking, an association between mortality and particulate air pollution was observed. Adjusted relative risk ratios (and 95% confidence intervals) of all-cause mortality for the most polluted areas compared with the least polluted equaled 1.15 (1.09 to 1.22) and 1.17 (1.09 to 1.26) when using sulfate and fine particulate measures respectively. Particulate air pollution was associated with cardiopulmonary and lung cancer mortality but not with mortality due to other causes. Increased mortality is associated with sulfate and fine particulate air pollution at levels commonly found in U.S. cities. The increase in risk is not attributable to tobacco smoking, although other unmeasured correlates of pollution cannot be excluded with certainty.

The binding of distinct segments of actin to multiple sites in the C-terminus of caldesmon: comparative aspects of actin interaction with troponin-I and caldesmon

Thin-filament-based regulation of the contractile response is considered to involve the interaction of actin with troponin-I in striated muscle and the interaction of actin with caldesmon in smooth muscle. The nature of the interaction with actin of these inhibitory proteins has been studied by proton magnetic resonance spectroscopy using segments of caldesmon and troponin-I which mimic their functional properties. Caldesmon is shown to interact with two distinct sites on the N-terminal residues 1-44 of actin subdomain 1 with corresponding contacts on caldesmon domain 3 and domain 4 at its C-terminus. We demonstrate that, whereas inhibition by the troponin-I fragment (residues 96-117) is effected by its interaction with the N-terminal region of actin, the separate inhibitory ability of different regions of the C-terminus of caldesmon (domains 4a and 4b) is mediated by interaction with noncontiguous segments on subdomain 1 of actin. Our studies of the spatial relationship of these actin contacts on caldesmon further suggest that one molecule of caldesmon may associate with two actin monomers. The demonstrated interactive nature of these caldesmon attachments to distinct regions of actin is relevant to the mechanism of calcium modulation of inhibition of actomyosin ATPase by caldesmon.

General practitioners' tacit and stated policies in the prescription of lipid lowering agents

BACKGROUND

Research into general practitioners' prescribing behaviour with regard to lipid lowering agents has relied on survey methods which presume that doctors have insight into their prescribing behaviour and can describe it accurately.

AIM

This study set out to measure the tacit policies used by general practitioners in prescribing lipid lowering agents and to compare these with their stated policies.

METHOD

Effects of 13 separate cues on decisions to prescribe were examined. The cues included cholesterol levels and a number of associated risk factors for coronary heart disease. Doctors rated 130 imaginary cases presented by a computer. Thirty five general practitioners in the Plymouth area participated in the study. Their ages ranged from 31 to 55 years and all but four were men. The raw data in each case was a rating of the likelihood that the doctor would prescribe for the patient described. These were converted into statistical weightings by use of multiple linear regression. The pattern of (standardized) weights constituted the tacit policy for each doctor. Stated policies were measured in a subsequent interview by asking doctors to rate the influence of each cue.

RESULTS

Both tacit and stated policies diverged widely between different doctors. Most doctors overestimated the number of cues that had actually influenced their decisions, and many believed that they had taken into account associated factors for coronary heart disease when they had not. On lifestyle related risks doctors were generally less likely to treat overweight people and most stated this as their policy. Most were also less likely to treat smokers but some had the opposite policy. Those less likely to treat smokers were also less likely to treat obese patients. There was also considerable variation in the extent to which the doctors took account of the attitude of the patient to receiving treatment.

CONCLUSION

Doctors' policies are highly variable and particularly inconsistent in the treatment of smokers. Relevant risk factors may be ignored--even though they are understood--because the risk assessment involved is too psychologically complex a task to be performed intuitively. Decision aids and clear protocols are needed in this area.

Structural determinants of substrate selection by the human insulin-receptor protein-tyrosine kinase

Using NMR spectroscopy to visualise tyrosine phosphorylation kinetics in real time, we have investigated the sequence-dependent determinants of the selectivity of the human insulin receptor protein-tyrosine kinase for different tyrosine residues. The peptides used encompass the multiple-tyrosine-containing autophosphorylation site sequences from the insulin receptor kinase core domain (Tyr1158, Tyr1162 and Tyr1163) and from its specific C-terminal tail domain (Tyr1328 and Tyr1334). Comparison of the phosphorylation kinetics with those found for the tyrosine residues on a peptide comprising the regulatory tyrosine phosphorylation site of cdc2 points to the role of the primary sequence context of the phosphate acceptor. The particularly deleterious influence of a basic residue immediately C-terminal to the tyrosine is discussed in relation to the autophosphorylation properties of the regulatory loop regions of the insulin and epidermal growth factor receptor kinases. The data further suggest that receptor tyrosine kinase active sites and their substrate targets act in concert to ensure that specific downstream effects are activated.

Phosphophoryn, an "acidic" biomineralization regulatory protein: conformational folding in the presence of Cd(II)

The divalent cation-induced protein folding properties of the template macromolecule, bovine dentine phosphophoryn (BDPP), have been examined by 1H/31P/13C/113Cd-nmr spectroscopy. Cd(II) was employed, exploiting the sensitivity of 113Cd-nmr to ligand-binding interactions and kinetics. Cation binding was studied over the stoichiometric range of 0-50: 1 Cd(II): protein (mole ratio), well below the range of Cd(II) concentration required to induce protein precipitation. The stepwise titration of divalent cation-depleted phosphophoryn at pH 7.2 in H2O/D2O with 113CdCl2 revealed that (PSer)n, (PSerAsp)n, and (Asp)n polyelectrolyte cation-binding domains undergo two major transitions in their secondary and tertiary structures: the first transition, occurring between 1:10 and 1:1 Cd(II): protein stoichiometry, and the second, between 10:1 and 50:1. By monitoring the amide NH intensities, 31P-nmr chemical shift, and 13C Asp-C, resonances, it was concluded that Cd(II) ions exhibit a binding-site preference for polyelectrolyte cation-binding domains, in the order (PSer)n > (PSerAsp)n > (Asp)n This preference correlates with the degree of negative charge density for each sequence motif. Accompanying the backbone conformational transitions at the polyelectrolyte regions were conformational transitions in the flanking hinge domains, indicating that the hinge domains participate in the folding of the phosphophoryn molecule as divalent cation binding occurs at the polyelectrolyte domains. We were unsuccessful in detecting phosphophoryn-bound Cd(II) species by 113Cd-nmr because of chemical exchange modulation. However, using a smaller 21-residue peptide mimetic of phosphophoryn, we have observed three stoichiometric-dependent 113Cd resonances that differ in terms of the oxoanion coordination number. Our observation of multiple Cd(II) species in the presence of the peptide supports our contention that Cd(II) has many chemically distinct coordination sites on phosphophoryn, each in multiple equilibria with H2O, Cl-, and side-chain oxoatoms.

Use of probabilistic expert judgment in uncertainty analysis of carcinogenic potency

A new approach to characterizing the state of knowledge about carcinogenic potency is described. In this approach, the carcinogenic risk posed by a specific dose is characterized by a probability distribution, indicating the relative likelihood of different risk estimates. The approach utilizes expert judgment and a probability tree and is illustrated in a case study of chloroform exposure. Experts in cancer biology/toxicology, pharmacokinetics, and dose-response modeling were identified by a panel of science-policy specialists. In a workshop, experts reviewed the chloroform data, received training in probability elicitation, and constructed a consensual probability tree based on biological theories of cancer causation. Distributions of carcinogenic risk were developed based on the probability tree, chloroform data, judgmental probabilities provided by the experts, and classical statistical techniques. Risk distributions varied considerably between experts, with some predicting essentially no risk from 100 ppb chloroform in drinking water while other have at least some probability on risks generally considered of regulatory significance. Estimated human risk was much lower when extrapolating from liver tumors in animals than from kidney tumors. Issues of scientific disagreement leading to different risk distributions between experts are discussed. The resulting risk distributions are compared to standard EPA risk calculations for the same exposure scenario as well as to the expert judgement of epidemiologists about cancer risks of chlorinated drinking water. Issues in combining expert judgments are discussed, and several alternative methods are presented. Strengths and weaknesses of the distributional approach are discussed.

A novel equipment bridge for helicopter transport of critically ill patients

A three-piece equipment bridge designed to simplify transfer of critically ill patients by helicopter or land ambulance is described. A lightweight, support frame is first secured onto a standard stretcher while still allowing access to the patient. An equipment module can then be attached to the frame. Emergency access to the patient is quickly achieved by detaching the equipment module, while ventilation and monitoring continue uninterrupted. The unit is fully self-contained and has a low centre of gravity to assist stability.

Phosphophoryn, a biomineralization template protein: pH-dependent protein folding experiments

The protein folding behavior of a polyelectrolyte protein, bovine dentine phosphophoryn (BDPP), in the pH range of 1.82-11.0 has been investigated. One- and two-dimensional nmr spectroscopy has been utilized to obtain proton spin assignments for amino acid residues in D2O and in H2O. One-dimensional 31P-nmr experiments verify the existence of three separate classes of O-phosphoserine (PSer) resonances in BDPP (alpha, beta, chi), representing three distinct PSer residue populations at pH 6.94. By means of pH titration and 1H-nmr, five populations of Asp residues can be identified. Three of these populations exhibit secondary inflection points on their pH titration curves that correspond to an observed pKa of 6.17-6.95. The presence or absence of secondary inflection points for Asp populations and the 31P-nmr chemical shift dispersion for the three PSer residue populations indicate that BDPP may be comprised of homologous (Asp-Asp)n. (PSer-PSer)n, and heterologous (PSer-Asp)n sequences arranged into polyelectrolyte cluster regions. The pH titration also revealed that certain populations of Ser, Gly, and Pro residues in BDPP exhibit pH-dependent resonance frequency shifts. The "apparent" pKa for the transition points of these frequency shifts corresponds to either the pK1a of Pser monophosphate ester and/or the pKa of Asp COOH group of BDPP polyelectrolyte regions. On the basis of these transition points, we can assign four types of Ser, Gly, or Pro-containing "intervening" regions in BDPP, based on their sensitivity to protonation and deprotonation events occurring at (Asp)n, (PSer)n, or (PSer-Asp)n anionic cluster regions that flank the intervening regions. Our 1H-nmr experiments also reveal that BDPP assumes a folded conformation at low pH. As the pH increases, this conformation undergoes several unfolding transitions as the BDPP molecule assumes more open conformations in response to increased electrostatic repulsion between polyelectrolyte anionic regions in the protein. These folding-unfolding transitions are mediated by the intervening regions, which act as "hinges" to allow the polyelectrolyte regions to fold relative to one another.

Pyrimidine ring fragmentation products. Effects of lesion structure and sequence context on mutagenesis

Free radicals produce a broad spectrum of damages to DNA, a major proportion of which includes ring fragmentation and contraction products of DNA bases as well as abasic sites. In this study, the mutagenic potential fo two pyrimidine ring fragmentation products, urea and beta-ureidoisobutyric acid (UBA), was analyzed using the i-d region of the Escherichia coli lacI gene contained in a single-stranded f1-K12 phage hybrid vector. Single-stranded DNA was used so that the in vivo interactions between the damage and the DNA polymerase could be assessed in the absence of excision repair. The i-d region contains 20 mutable thymine sites so that 20 separate sequence contexts containing a unique lesion at a position of thymine can be analyzed simultaneously. Urea and UBA residues were uniquely introduced into f1-K12 DNA by chemical and enzymatic methods and primer extension and piperidine analysis of the damage-containing template molecules demonstrated that the potential mutable thymine sites contained randomly distributed lesions. Both fragmentation products were poorly bypassed by DNA polymerases in vitro and in the cell; although in the presence of SOS-induction, UBA was bypassed more efficiently than urea. UBA was a potent premutagenic lesion with a rate of mutation induction more than sixfold above that observed with abasic sites derived from purines. Urea residues were about as mutagenic as abasic sites derived from purines, which in turn were more mutagenic than abasic sites derived from thymine. Mutations derived from urea, UBA and abasic sites were all dependent on SOS-induction of the host cells. Since both urea and UBA were derived from DNA thymine, these data demonstrate that adenine is not routinely inserted opposite products that no longer retain the structural integrity of the pyrimidine ring. Sequence analysis showed that the mutations were targeted at thymine with 62% of the urea-derived mutations being T to C transitions and 62% of the UBA derived mutations being T to A transversions. Thus, the two fragmentation products appeared to direct specific misinsertions. The mutations were not randomly distributed over the i-d region for either fragmentation product and hotspots were observed for both damages. The presence of hotspots suggests that in addition to lesion structure, sequence context plays an important role in base selectivity by DNA polymerases opposite DNA lesions. Energy minimization calculations were used to model the urea and UBA lesions at two contrasting hotspot sites. In both cases, there was significant agreement between the computational and biological data sets.

A distributional approach to characterizing low-dose cancer risk

Since cancer risk at very low doses cannot be directly measured in humans or animals, mathematical extrapolation models and scientific judgment are required. This article demonstrates a probabilistic approach to carcinogen risk assessment that employs probability trees, subjective probabilities, and standard bootstrapping procedures. The probabilistic approach is applied to the carcinogenic risk of formaldehyde in environmental and occupational settings. Sensitivity analyses illustrate conditional estimates of risk for each path in the probability tree. Fundamental mechanistic uncertainties are characterized. A strength of the analysis is the explicit treatment of alternative beliefs about pharmacokinetics and pharmacodynamics. The resulting probability distributions on cancer risk are compared with the point estimates reported by federal agencies. Limitations of the approach are discussed as well as future research directions.

Reasoning, decision making and rationality

It is argued that reasoning in the real world supports decision making and is aimed at the achievement of goals. A distinction is developed between two notions of rationality: rationality which is reasoning in such a way as to achieve one's goals--within cognitive constraints--and rationality which is reasoning by a process of logic. This dichotomy is related to the philosophical distinction between practical and theoretical reasoning. It is argued that logicality (rationality) does not provide a good basis for rationality and some psychological research on deductive reasoning is re-examined in this light. First, we review belief bias effects in syllogistic reasoning, and argue that the phenomena do not support the interpretations of irrationality that are often placed upon them. Second, we review and discuss recent studies of deontic reasoning in the Wason selection task, which demonstrate the decision making, and rational nature of reasoning in realistic contexts. The final section of the paper examines contemporary decision theory and shows how it fails, in comparable manner to formal logic, to provide an adequate model for assessing the rationality of human reasoning and decision making.

Workers' breath as a source of perchloroethylene (Perc) in the home

Wallace et al. (1991a) measured elevated levels of perchloroethylene (Perc) in the homes of six dry cleaning workers who used Perc in the workplace. This paper considers the hypothesis that dry cleaning workers introduce Perc into their homes by exhaling Perc accumulated in their bodies during the work day. We simulated worker exposure considering current occupational guidelines, uptake and body burden using a physiologically-based pharmacokinetic (PBPK) model for Perc, and home concentrations using a simple model for a "typical" home. These results suggest that workers' body burdens of Perc may be sufficient to explain elevated home concentrations. The implications of these results, for design of exposure monitoring studies, are discussed, along with some other policy implications.

The value of animal test information in environmental control decisions

Value of information (VOI) analytic techniques are used to evaluate the benefit of performing animal bioassays to provide information about the cancer potency of specific chemical compounds. These tools allow the identification of the conditions in which the cost of reducing uncertainty about potency, by performing a subchronic or chronic bioassay, is justified by the benefit of having improved information for making control decisions. The decision analytic results are readily scaled to apply to a range of human contact rates (exposures) and a variety of control strategies. The sensitivity of results to uncertainty about animal to human extrapolation and the design of the bioassay is explored. An evaluation of the possible gains in general understanding about the mechanisms of carcinogenicity resulting from chronic bioassays is beyond the scope of this approach.

The mental model theory of conditional reasoning: critical appraisal and revision

Johnson-Laird and Byrne (1991) present a theory of conditional inference based upon the manipulation of mental models. In the present paper, the theory is critically examined with regard to its ability to account for psychological data, principally with respect to the rate at which people draw the four basic inferences of modus ponens (MP), denial of the antecedent (DA), affirmation of the consequent (AC) and modus tollens (MT). It is argued first that the theory is unclear in its definition and in particular with regard to predictions of problem difficulty. Clarification and specification of principles are consequently provided here. Next, it is argued that there are a number of phenomena in the conditional reasoning literature for which the theory cannot account in its present form. Specifically, (a) the relatively frequency of DA and AC inferences on affirmative conditional is not as predicted by the theory, (b) differences occur between inferences on if then and only if rules beyond the capacity of the theory to explain and (c) there is no account of the "negative conclusion bias" observed when negated components are introduced into the rules. A number of revisions to the mental model theory of conditional reasoning are proposed in order to account for these findings.

An empirical model for estimating the collection efficiency of dust-mist respirators

This paper compares experimental measurements of filter performance with predictions of aerosol collection by dust-mist respirators using the single fibre efficiency model and an empirical model developed by the authors. The former, a theoretically-based semi-empirical model, indicates that mechanical phenomena such as impaction, interception and diffusion are responsible for only a fraction of the collection efficiency of dust-mist respirators, i.e. 30-70% depending on particle size. The difference between these relatively low predicted efficiencies and experimentally observed values, which were greater than 95%, can be attributed to electrostatic forces. Because it was not possible to determine filter fibre charge the single fibre efficiency model could not be used to predict electrostatic collection. An empirical model developed from experimental data with monodisperse latex aerosols was used to predict silica and asbestos collection. The empirical model adequately described silica collection, but predicted much higher asbestos penetration than was observed experimentally.

The source of belief bias effects in syllogistic reasoning

In studies of the belief bias effect in syllogistic reasoning, an interaction between logical validity and the believability of the conclusion has been found; in essence, logic has a larger effect on unbelievable than on believable conclusions. Two main explanations have been proposed for this finding. The selective scrutiny account claims that people focus on the conclusion and only engage in logical processing if this is found to be unbelievable; while the misinterpreted necessity account claims that subjects misunderstand what is meant by logical necessity and respond on the basis of believability when indeterminate syllogisms are presented. Experiments 1 and 2 compared the predictions of these two theories by examining whether the interaction would disappear if only determinate syllogisms were used. It did, thus providing strong support for the misinterpreted necessity explanation. However, the results are also consistent with a version of the mental models theory, and so Experiment 3 was carried out to compare these two explanations. The mental models theory received strong support, as it did also in the follow-up Experiments 4 and 5. It is concluded that people try to construct a mental model of the premises but, if there is a believable conclusion consistent with the first model they produce, then they fail to construct alternative models.

Alternative management of choledocholithiasis

This paper is presented to clarify the role of ERCP in conjunction with laparoscopic cholecystectomy. As the field of surgery moves more toward less invasive procedures, our management schemata may also need revision. With the popularity of laparoscopic cholecystectomy came a large increase in the number of patients treated with cholelithiasis, and therefore an increase in patients with choledocholithiasis. The authors believe that treatment options exist to avoid open common bile duct procedure in most cases.

An early review of 800 laparoscopic cholecystectomies at a university-affiliated community teaching hospital

Laparoscopic cholecystectomy (LC) was first performed at Georgia Baptist Medical Center (GBMC) in December 1989, subsequently becoming the treatment of choice for most patients with symptomatic gallbladder disease. Early in the authors' series, all patients evaluated for cholecystitis were treated laparoscopically, unless the third party refused reimbursement or the attending surgeon was not trained in LC. Indications for LC were no different than for standard open cholecystectomy (OC). Eight hundred patients from December 1989 to March 1991 had an attempted LC at GBMC. The procedure was completed in 782 patients (97.7%) and required conversion to OC in 18 patients, (2.3%) primarily because of technical difficulties such as dense adhesions or gangrenous changes. No patient sustained a trocar injury to the intra-abdominal viscera, bile ducts injury, or major vascular injury. Overall morbidity was 3.1 per cent and mortality 0.13 per cent. Selective cholangiography (SIOC) was used in 14 per cent. Endoscopic retrograde cholangiopancreatography (ERCP), choledochoscopy, and Fogarty catheter techniques were used for common bile duct stone management. Average hospitalization was 0.89 days, with 85 per cent discharged in less than 24 hours. Average operative time was 86 minutes (range: 25 to 353). Patients returned to full activities at home in 8.4 days. Savings on hospital charges to patients averaged $1,100 for inpatient LC and $2,500 for outpatient LC when compared to 1989 costs for OC. Laparoscopic cholecystectomy is the current surgical procedure of choice for most patients with cholecystitis and can be done at least as safely as standard open cholecystectomy. The morbidity appears to be significantly less with LC, but longer follow-up is needed to confirm these preliminary findings.(ABSTRACT TRUNCATED AT 250 WORDS)

Multipractice analysis of laparoscopic cholecystectomy in 1,983 patients

We analyzed the results of laparoscopic cholecystectomy in 1,983 patients from a variety of practice settings in order to evaluate a large, cross-sectional experience for this new procedure. Twenty general surgeons from 9 clinics in 4 states examined the records and outcome of their laparoscopic cholecystectomy patients through March 1991. In 88 patients (4.5%), the operation was converted to an open procedure, usually because of marked inflammation and unclear anatomy. A total of 644 cases were performed with laser dissection and 1,339 with cautery, and the results of these 2 methods were similar. There were 41 complications. Reoperation for repair was necessary in 18 patients, including 5 with common duct injuries, and, to date, the outcome has been good in each patient. Seventy-six patients (3.8%) have had recognized common duct stones; these were removed preoperatively by endoscopic sphincterotomy (ERS) in 20 patients, during cholecystectomy in 46 patients, and postoperatively by ERS in 4 patients. In six patients, common duct stones became apparent 1 to 4 months after cholecystectomy. We conclude that trained general surgeons can perform laparoscopic cholecystectomy safely with risks comparable to those for conventional open cholecystectomy.

Effects of age and biotin status on postnatal development of plasma biotinidase activity in rats

Biotinidase activity was measured in plasmas of 1-, 7-, 14-, and 21-day-old rats from control dams and dams that had been fed a biotin-depleting diet from Day 15 of gestation. Biotinidase activity increased significantly in the plasma of rats from control and depleted mothers until Postnatal Day 14, after which there was a small but significant decline at Day 21. Differences between the mean activities of the two groups of pups on each sampling day were not significant and there were no significant differences in activity levels attributable to sex. Plasma albumin concentrations increased from birth until Day 21, and plasma biotinidase activity and albumin concentration were significantly correlated (r = +/- 0.43). We suggested that these two proteins may be controlled by a common mechanism in the early postnatal period, and that biotin deficiency does not affect the development of biotinidase activity. Because biotin-depleted neonatal pups show developmental changes in biotinidase activity similar to those of human newborns, and they can be produced reliably by depleting dams from Day 15 of gestation, they may be useful models for studying the developmental abnormalities associated with human biotinidase deficiency.

Biliary infusion therapy in the inspissated bile syndrome of cystic fibrosis

A 3.5-month-old white boy was born with meconium ileus, peritonitis, and jejunal atresia from cystic fibrosis. He subsequently developed unrelenting and severe extrahepatic biliary obstruction as demonstrated by liver biopsy showing periportal inflammation, cholestasis, and fibrosis. Surgical exploration confirmed the diagnosis of extrahepatic biliary obstruction by severely inspissated bile. A cholecystostomy tube was left in place. The cholestasis remained unresponsive to conservative medical therapy. The obstruction was relieved by hydrostatic infusion of 2% N-acetylcysteine into the biliary tree over a 6-day period. The child also received concurrently four i.v. injections of synthetic cholecystokinin. This therapeutic modality was thought to be both safe and effective.

Structure and function of X-Pro dipeptide repeats in the TonB proteins of Salmonella typhimurium and Escherichia coli

The TonB protein is required for several outer membrane transport processes in bacteria. A short 33-residue peptide segment of TonB has been studied by 1H and 13C nuclear magnetic resonance spectroscopy. The sequence of this peptide segment contains multiple Glu-Pro and Lys-Pro dipeptide repeats that maintain rigid, elongated structures and flank a short connecting segment that adopts a beta-strand configuration. This TonB peptide is shown to interact specifically with the FhuA protein, the outer membrane receptor for ferrichrome-iron, providing the first direct evidence that the TonB protein interacts with outer membrane receptors. Interaction with the FhuA protein involves the extended structural element containing positively charged Lys-Pro repeats, and suggests a functional role for this segment of the TonB protein. As TonB is anchored in the cytoplasmic membrane the protein must, uniquely, span the periplasm. These data, together with studies described in the accompanying paper, suggest a model by which TonB serves to transduce conformational information over extended distances, from the cytoplasmic membrane to the outer membrane.

TonB protein of Salmonella typhimurium. A model for signal transduction between membranes

The tonB gene product is required for several outer membrane transport processes in bacteria. The tonB gene from Salmonella typhimurium was sequenced and found to be similar to that of Escherichia coli. The TonB protein is highly proline-rich and includes an unusual segment consisting of multiple X-Pro dipeptide repeats. A synthetic peptide corresponding to this segment has been used to raise anti-TonB antibodies. TonB was shown to be associated with the cytoplasmic membrane, apparently anchored via a single hydrophobic N-terminal segment. Protease accessibility studies, and the use of a series of TonB-beta-lactamase fusions, showed that the rest of the TonB protein is periplasmic. Unusually, export of TonB is not accompanied by cleavage of the N-terminal signal peptide. In the accompanying paper, we show that TonB interacts directly with the outer membrane FhuA (TonA) receptor. Thus, TonB must span the periplasm, providing a link between the cytoplasmic membrane and receptors in the outer membrane. On the basis of these data, and those published by other laboratories, we propose a model whereby TonB serves as a "mechanical" linkage that, by transmitting protein conformational changes from the cytoplasmic membrane across the periplasm, acts as a means of coupling energy to outer membrane transport processes. Such a mechanism has general implications for signal transduction within and between proteins.

Collection of silica and asbestos aerosols by respirators at steady and cyclic flow

Air purifying dust/mist respirators are presently tested using a silica aerosol under conditions of steady flow. In experiments, the predictive validity of such tests was evaluated by testing respirators using silica and asbestos aerosols under conditions of both steady and cyclic flow. Silica penetration at steady flow was reasonably predictive of silica penetration under cyclic flow. However, asbestos penetration under cyclic flow was not predicted well by penetration of either silica or asbestos at steady flow. Furthermore, the potential for exhalation valve failure under cyclic flow was identified. Current NIOSH protocols for evaluating respirator performance should be reconsidered in light of these findings.

Degradation of thiophene-2-carboxylate, furan-2-carboxylate, pyrrole-2-carboxylate and other thiophene derivatives by the bacterium Vibrio YC1

A bacterium of the genus Vibrio, capable of degrading thiophene derivatives, was isolated from enrichment cultures inoculated with oil-contaminated estuarine mud. Of the thiophene derivatives tested, only thiophene-2-carboxylate (T2C), thiophene-2-acetate and thiophene-2-carbonyl chloride supported growth, but a total of seven were significantly oxidised by resting cells. Furan-2-carboxylate (F2C) and pyrrole-2-carboxylate (P2C) also supported growth and were oxidised. The heteroatoms of T2C and P2C were released into the media as sulphate and ammonia and served as sole sources of sulphur and nitrogen for growth. Mutants defective in T2C utilisation (Thc-) were isolated, many being also defective in F2C and P2C utilisation.

Collection efficiency of respirator filters challenged with monodisperse latex aerosols

Collection efficiency was evaluated for 10 respirator manufacturers' electrostatically-charged dust/mist filters challenged with eight sizes of latex spheres in a range representative of the silica aerosol used in National Institute for Occupational Safety and Health (NIOSH) certification tests. Minimum efficiency occurred at or below the smallest size of 0.102 microns. Appreciable differences were found in the performances of filters distributed by the 10 manufacturers. Filters produced by 1 manufacturer exhibited significantly lower collection efficiency than other filters tested; those produced by 2 manufacturers performed somewhat better than the others tested. Statistical evaluation of Weibull-transformed data using analyses of covariance and Tukey's significant difference test allowed classification of the 10 manufacturers into three performance categories.

The distribution of Student's t-statistic for small samples from lognormal exposure distributions

To assess compliance with industrial hygiene exposure criteria (e.g., TLVs), it may be necessary to perform statistical tests of hypotheses based on relatively small samples. For pollutants with long biological half-lives, the parameter most relevant for determining the risk faced by workers is the long-term arithmetic average concentration of the pollutant. In industrial environments it is common for pollutant concentrations to be approximately lognormal. Unfortunately, when based on small samples from lognormal distributions, the ordinary Student's t-statistic has some undesirable characteristics which are not recognized widely by practicing industrial hygienists. The difficulties in using the ordinary Student's t-statistic to evaluate the average exposure have been demonstrated. The properties of alternative test statistics have been explored. Some general observations on the implications of these findings have been made.

Occult blood in stools and necrotizing enterocolitis. Is there a relationship?

A prospective study was undertaken to determine the incidence of occult hematochezia in enterally fed low-birth-weight neonates (less than 1800 g at birth). The relationship of this occult hematochezia to the development of necrotizing enterocolitis (NEC) was then analyzed. Daily stool specimens from 95 neonates were tested for occult blood during the first six weeks of life. Fifty-four (58%) of the 95 neonates had one or more blood-positive stools. Six neonates (6.3%) developed NEC. Necrotizing enterocolitis occurred in only two of the 54 neonates with one or more blood-positive stools vs four of the 41 neonates with blood-negative stools only. Presence of occult blood in the stools did not correlate with development of NEC.

Biosynthesis of type I procollagen. Characterization of the distribution of chain sizes and extent of hydroxylation of polysome-associated pro-alpha-chains

[3H]Proline-labeled nascent procollagen chains were isolated from chick tendon polysome preparations as peptidyl-tRNA complexes by ion exchange chromatography. Proline hydroxylation of the nascent chains was at least 40% complete, based on radioactive hydroxyproline/proline ratios. These data provide the first direct evidence that hydroxylation of procollagen proline residues does occur on nascent chains. The electrophoretic profiles of [3H]proline-labeled nascent chains and of unlabeled nascent chains visualized by Western blotting with 35S-labeled monoclonal antibodies to the alpha 1(I) N-propeptide or the C-propeptides indicate that there are pauses in the translation of procollagen alpha-chains in the intact cells. Approximately 25% of the radioactivity associated with [3H]proline-labeled polysomes was in fully elongated but underhydroxylated (relative to secreted procollagen) pro-alpha-chains. The association of these completely elongated but only partially modified procollagen chains with the polysome complex may facilitate the carboxyl-terminal interactions which lead to triple helix formation.

Sequence-imposed structural constraints in the TonB protein of E. coli

The solution conformation of a 33-residue peptide segment, derived from the TonB protein which is implicated in bacterial membrane transport processes, has been investigated using high-resolution proton magnetic resonance techniques. This proline-rich peptide possesses sequence-imposed sections of elongated secondary structure that must be retained in the native protein configuration. These structural constraints provide elements of stiffness that imply a purely structural role for TonB and are relevant to the subcellular location and biological role of the protein. On the basis of these data we suggest that this protein spans the periplasmic space, linking the inner and outer membrane components of TonB-dependent transport systems.

The influence of diagnostic radiography on the incidence of breast cancer and leukemia

Little accurate information is available on the influence of diagnostic radiography on the incidence of cancer in the general population, largely because of a lack of knowledge of the numbers and the age distributions of patients undergoing radiographic examinations. We used such information from a closed population in Maine that was followed for a year, along with data from the literature on the absorbed dose from typical examinations and a mathematical model linking dose to the incidence rate of cancer, to estimate the numbers of cases of radiologically induced cancer of the bone marrow and breast. Our data indicate that about 1 percent of all cases of leukemia and less than 1 percent of all cases of breast cancer result from diagnostic radiography. In addition, such tumors generally occur late in life; the incidence of radiation-induced leukemia was highest at 69 years, and the incidence of radiation-induced breast cancer was highest at 76 years. Thus, diagnostic radiography has only a small influence on the occurrence of leukemia and breast cancer.