Elecsys Cerebrospinal Fluid Assays Accurately Distinguish Alzheimer's Disease from Frontotemporal Lobar Degeneration

BACKGROUND

Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) are heterogeneous in their clinical presentation and underlying pathology, but they often have overlapping features. Diagnostic accuracy is critical for guiding patient management. Cerebrospinal fluid (CSF) diagnostic assays for the differentiation of AD and FTLD may increase diagnostic accuracy.

OBJECTIVES

In this study, we aimed to understand the potential role of CSF biomarkers and biomarker ratios, measured using Elecsys® CSF immunoassays (Roche Diagnostics International Ltd, Rotkreuz, Switzerland), in the differential diagnosis of AD and FTLD.

DESIGN

This study was conducted at a single center in Munich, Germany between July 2019 and July 2020. Patient CSF samples were retrospectively collected from the study center biobank.

PARTICIPANTS

A total of 130 patients with cognitive impairment were included in the study; 86 patients were diagnosed with AD and 44 with FTLD (behavioral variant frontotemporal dementia, semantic variant of primary progressive aphasia, and non-fluent variant of primary progressive aphasia), based on core clinical criteria and a non-CSF biomarker, a typical pattern of regional hypometabolism on [18F] fluorodeoxyglucose positron emission tomography.

MEASUREMENTS

Patient CSF biomarker concentrations were measured using Elecsys CSF immunoassays. Receiver operating characteristic analyses were conducted to determine areas under the curve (AUCs) for CSF biomarker performance. Sensitivity and specificity analyses were conducted to evaluate the performance of established cut-offs (Aβ42 ≤1000 pg/mL, pTau181/Aβ42 ratio >0.024, and tTau/Aβ42 ratio >0.28) and optimized cut-offs based on Youden's index.

RESULTS

AUC-based performance was similarly good for the pTau181/Aβ42 ratio (AUC=0.841; 95% CI: 0.759-0.923), pTau181/Aβ40 ratio (AUC=0.837; 95% CI: 0.754-0.919), Aβ42/Aβ40 ratio (AUC=0.829; 95% CI: 0.746-0.912), tTau/Aβ42 ratio (AUC=0.822; 95% CI: 0.736-0.908), pTau181/Aβ42/Aβ40 ratio (AUC=0.817; 95% CI: 0.734-0.901), and Aβ42 (AUC=0.812; 95% CI: 0.722-0.902). Performance was slightly lower for the tTau/Aβ42/Aβ40 ratio (AUC=0.799; 95% CI: 0.713-0.885), pTau181 alone (AUC=0.793; 95% CI: 0.707-0.880), tTau/Aβ40 ratio (AUC=0.751; 95% CI: 0.657-0.844), and tTau alone (AUC=0.706; 95% CI: 0.613-0.799). The highest qualitative performance was observed with the pTau181/Aβ42 ratio with an established cut-off value of >0.024 and optimized cut-off value of >0.022: sensitivity and specificity values were 0.892 and 0.773, respectively.

CONCLUSIONS

Elecsys CSF immunoassays demonstrate good diagnostic accuracy in differentiating patients with AD from those with FTLD. These immunoassays have the potential to support clinical decision making, i.e. in diagnosing patients with FTLD by excluding patients with amyloid positivity, which is indicative of underlying AD.

Maximizing the production of butyric acid from food waste as a precursor for ABE-fermentation

The current study reports on the maximization of butyric acid production from food waste using a mixed microbial fermentation. In semi-continuous fermentations the effect of three different pH values (5.5, 7.0 and 9.0), three different temperatures (37°C, 55°C and 70°C) and two levels of hydraulic retention time (HRT, 2days and 6days) on the formation of butyric acid as well as total volatile fatty acid production (tVFA) were investigated. Overall, pH5.5 provided the lowest butyric acid concentrations regardless of the temperature and the HRT. At mesophilic temperature (37°C) alkaline conditions (pH9.0) lead to a strong incline of tVFA as well as butyric acid concentration probably due to a decreased solubilization of the substrate. However, most efficient in terms of butyric acid production was the fermentation conducted at 55°C and pH7 where a butyric acid concentrations of 10.55g/L (HRT 2days) and 13.00g/L (HRT 6days) were achieved. Additional experiments at 70°C showed declining butyric acid production. Increase of the HRT from 2days to 6days provided an increment of butyric acid concentration throughout almost all experimental settings. However, regarding volumetric productivity the increase in concentration does not compensate for the bigger reactor volume required to establish a higher HRT. At pH7 and 55°C the resulting volumetric production rates were 5.27g/L∗d at a HRT 2days and only 2.17g/L∗d at a HRT of 6days.

[Posterior cortical atrophy. Pathology, diagnosis and treatment of a rare form of dementia]

The syndrome of posterior cortical atrophy (PCA) is a rare clinical manifestation of several neurodegenerative diseases which affect the parieto-occipital cortex. The most frequent underlying pathology is Alzheimer's disease but some cases are caused by Lewy body disease, progressive subcortical gliosis, corticobasal degeneration or prion diseases. The most prominent clinical feature of PCA is complex visual disturbances including object agnosia, simultanagnosia, optical ataxia and oculomotor apraxia while basic visual functions remain intact. These deficits lead to multiple impairments in activities of daily living that require visual control. On progression of the disease amnestic, apraxic and dysexecutive symptoms occur so that a global dementia gradually emerges. At the core of the diagnostic work-up are a detailed patient history, accurate analysis of behavior and neuropsychological testing. Structural and functional brain imaging are suitable to demonstrate the localization of the disease process. Measurement of cerebrospinal fluid proteins (e.g. beta amyloid, tau, phospho-tau and 14-3-3) serves to confirm or exclude Alzheimer's disease or prion diseases. The mainstay of treatment are non-pharmacological interventions to support activities of daily living and personal independence. These treatments include cognitive training and compensatory strategies which can be prescribed as neuropsychological treatment or occupational therapy. If Alzheimer's disease or Lewy body disease is the likely cause, a treatment with cholinesterase inhibitor may be tried. Caregiver education and support are another essential part of the treatment regimen as with all forms of dementia.

Counteracting ammonia inhibition in anaerobic digestion by removal with a hollow fiber membrane contactor

The aim of the current study was to investigate the feasibility of membrane contactors for continuous ammonia (NH₃-N) removal in an anaerobic digestion process and to counteract ammonia inhibition. Two laboratory anaerobic digesters were fed slaughterhouse wastes with ammonium (NH₄⁺) concentrations ranging from 6 to 7.4 g/L. One reactor was used as reference reactor without any ammonia removal. In the second reactor, a hollow fiber membrane contactor module was used for continuous ammonia removal. The hollow fiber membranes were directly submerged into the digestate of the anaerobic reactor. Sulfuric acid was circulated in the lumen as an adsorbent solution. Using this set up, the NH₄⁺-N concentration in the membrane reactor was significantly reduced. Moreover the extraction of ammonia lowered the pH by 0.2 units. In combination that led to a lowering of the free NH₃-N concentration by about 70%. Ammonia inhibition in the reference reactor was observed when the concentration exceeded 6 g/L NH₄⁺-N or 1-1.2 g/L NH₃-N. In contrast, in the membrane reactor the volatile fatty acid concentration, an indicator for process stability, was much lower and a higher gas yield and better degradation was observed. The chosen approach offers an appealing technology to remove ammonia directly from media having high concentrations of solids and it can help to improve process efficiency in anaerobic digestion of ammonia rich substrates.

Measuring clinical competence in preregistration trainees by OSCEs

Focal points

Incorporation of 2,4,6-trinitrotoluene (TNT) transforming bacteria into explosive formulations

Pseudomonas putida GG04 and Bacillus SF have been successfully incorporated into an explosive formulation to enhance biotransformation of TNT residues and/or explosives which fail to detonate due to technical faults. The incorporation of the microorganisms into the explosive did not affect the quality of the explosive (5 years storage) in terms of detonation velocity while complete biotransformation of TNT moieties upon transfer in liquid media was observed after 5 days. The incorporated microorganisms reduced TNT sequentially leading to the formation of hydroxylaminodinitrotoluenes (HADNT), 4-amino-2,6-dinitrotoluenes; 2-amino-4,6-dinitrotoluenes, different azoxy compounds; 2,6-diaminonitrotoluenes (2,4-DAMNT) and 2,4-diaminonitrotoluenes (2,6-DAMNT). However, the accumulation of AMDNT and DAMNT (major dead-end metabolites) was effectively prevented by incorporating guaiacol and catechol during the biotransformation process.

Subpixel tracking for the analysis of outer hair cell movements

CONCLUSION

Videomicroscopy with subpixel analysis is an excellent system for quantification of outer hair cell (OHC) movements. The resolution of a few nanometers is accurate enough to show induced differences of electromotility.

OBJECTIVE

Electromotility of OHCs is a voltage-dependent process resulting from a membrane protein named prestin. Voltage sensitivity is conferred to prestin by intracellular anions. Reduction of these anions reduces electromotility. Videomicroscopy and subpixel tracking combine video-based analysis with a resolution of few nanometers. The aim of this study was to show the feasibility of a system for quantification of OHC movements.

MATERIALS AND METHODS

Electromotility was investigated under normal and reduced intracellular chloride conditions. Cells were stimulated by the patch-clamp technique. Voltage steps were 500 ms long, ranging from -170 to +30 mV in 10 mV steps.

RESULTS

As in previous studies our results show the following. The direction of OHC movement depends on the polarity of voltage steps, length changes are not equal for symmetrical voltage steps of opposite polarity, average shortening for a depolarizing step (-70 mV to +30 mV) is about 13 nm/mV. Hyperpolarization (-70 mV to -170 mV) on average evokes elongations of about 3 nm/mV. Half maximal chloride concentration reduces motility by 14%; half maximal electromotility is reached by a 94% reduction of chloride.

Cold atoms and molecules in self-assembled dipolar lattices

We study the realization of lattice models, where cold atoms and molecules move as extra particles in a dipolar crystal of trapped polar molecules. The crystal is a self-assembled floating mesoscopic lattice structure with quantum dynamics given by phonons. We show that within an experimentally accessible parameter regime extended Hubbard models with tunable long-range phonon-mediated interactions describe the effective dynamics of dressed particles.

Behandlung vital gefährdeter Anorexia-nervosa-Patienten unter Berücksichtigung der Möglichkeiten des Betreuungsrechts

Anorexia nervosa is a psychiatric disorder occurring primarily in young women. Especially when chronic or the body mass index is less than 12 kg/m(2), it has a mortality of up to 20%. Often these patients are admitted to emergency units and treated internistically. If they accept psychiatric treatment, they are normally transferred to specialized psychosomatic units for further therapy. If patients are not able to accept necessary further therapy to overcome the danger to health and life, the question arises in terms of guardianship law of whether they are able to handle their personal concerns alone. After several vitally endangered anorexia nervosa patients had been admitted to our closed psychiatric ward, we developed a therapeutic concept for this subgroup of patients, taking the possibilities of guardianship law into account. This concept aims at restoring the body weight continuously and finally enable the patients to be transferred to less restrictive psychosomatic units. The chosen treatment is shown with the therapy courses of 25 patients treated according to this concept.

Diagnostic yield of wireless capsule enteroscopy in comparison with computed tomography enteroclysis

BACKGROUND AND STUDY AIMS

It is still difficult to visualize changes in the small intestine. Wireless capsule enteroscopy is a new method that promises to provide new insights into the small intestine. In a prospective study, the diagnostic yield of wireless enteroscopy was therefore compared with computed tomography (CT) enteroclysis.

PATIENTS AND METHODS

Twenty-two patients with suspected small-bowel pathology underwent CT enteroclysis and wireless capsule enteroscopy examinations, conducted by two independent blinded investigators. The results of the two investigations (diagnoses and the number, extent, and location of lesions detected) were compared by a third investigator.

RESULTS

The patients included in the study had obscure gastrointestinal bleeding (n = 8), Crohn's disease (n = 8), unexplained diarrhea (n = 5), or suspected carcinoid tumor (n = 1). Pathological lesions were detected using capsule enteroscopy in 13 patients (59 %) and using CT enteroclysis in eight (36 %; P = 0.12). In seven patients (one case each of colonic Crohn's disease, diverticulitis, Meckel's diverticulum, carcinoid tumor, mesothelioma, colonic polyps, and irritable bowel syndrome), no pathological changes were found in the small intestine using either method. The diagnosis was established by wireless capsule enteroscopy in four patients with obscure bleeding, whereas CT enteroclysis was positive in only one patient ( P = 0.1). Crohn's disease was found in two patients with unexplained diarrhea. Small-bowel lesions were identified in six patients with known Crohn's disease using capsule enteroscopy or CT enteroclysis. The only side effect of wireless capsule enteroscopy observed was abdominal pain in one patient with Crohn's disease. There were no serious side effects with CT enteroclysis.

CONCLUSIONS

Wireless capsule enteroscopy detects more small-bowel lesions than CT enteroclysis in patients with obscure gastrointestinal bleeding and Crohn's disease.

Methylation status of p14ARF and p16INK4a as detected in pancreatic secretions

The clinical management of pancreatic disease is often hampered by a lack of tissue diagnosis. Endoscopic pancreatography offers the opportunity to investigate exfoliated cells. However, the significance of mere cytological investigation is compromised by an insufficient sensitivity. The evaluation of the molecular background of carcinogenesis hopefully is capable of providing more sensitive diagnostic markers. The p16INK4a-/retinoblastoma tumour-suppressive pathway has been shown to be involved in the development of near to all pancreatic neoplasms. p14ARF is another tumour suppressor located in the immediate neighbourhood of p16INK4a. Promoter methylation has been demonstrated to be a major inactivating mechanism of both genes. We sought to further evaluate the role of the gene locus INK4a methylation status in the endoscopic differentiation of chronic inflammatory and neoplastic pancreatic disease. Pancreatic fluid specimens of 61 patients with either pancreatic carcinoma (PCA: 39), chronic pancreatitis (CP: 16) or a normal pancreatogram (NAD: 6) were retrieved. In order to detect methylation of either the p14ARF or the p16INK4a promoter a methylation-specific PCR protocol was applied. While 19 out of 39 patients with PCA showed p16 promoter methylation (49%), none of the 16 patients with CP revealed p16 promoter methylation. p14ARF methylation was found in a lower percentage of PCA specimens and in none of the samples of patients with CP. These results suggest a specific significance of INK4a for the development of malignant pancreatic disease. Our data further indicate a potential role for INK4a methylation as a diagnostic marker in the endoscopic differentiation of benign and malignant pancreatic disease.

Photodynamic therapy for cholangiocarcinoma

The prognosis of nonresectable cholangiocarcinoma is dismal, and in Bismuth type III and IV tumors relief of jaundice is seldom achieved, despite successful endoprosthesis insertion. Therefore, we evaluated additional photodynamic therapy in patients who failed to respond to endoprostheses insertion. All patients showed good clinical results in regard to jaundice, quality of life; and survival time (median, 439 days). Before initiating a randomized multicenter trial we wanted to evaluate these preliminary results in a greater number of patients. Twenty-one patients underwent photodynamic therapy in addition to endoscopic drainage. The hematoporphyrin derivative Photofrin was infused intravenously (2 mg/kg body weight), and intraluminal photoactivation was performed 2 days later. Bilirubin decreased from a mean level of 201.26 +/- 189.25 micromol/l after stenting alone to 68.87 +/- 78.27 micromol/l (P = 0.0051), and the Karnofsky index improved from 49.04% +/- 28.79% to 72.85 +/- 19.01 (P = 0.003). Thirteen patients have died and 8 patients are still alive, with a follow-up period of 82-739 days. The 6-month survival time is 95%. Similar results were obtained by another group, with 75% overall (stage M1 and M0) survival after 6 months and beyond 80% for stage M0. Other authors treated 7 patients and saw a remarkable reduction of bile duct stenosis and bilirubin decrease in all patients.

Photodynamic therapy in the biliary tract

Biliary tract cancers are rare malignancies usually characterized by slow growth and a low propensity for metastasis. Despite the relatively localized nature of these cancers, the only therapeutic measure with curative potential to date is surgical intervention. Because symptoms occur late, the diagnosis is rarely made at an early stage, and therefore only about half of the patients can have curative surgery. Patients with advanced disease face a dismal prognosis because palliative treatment options are limited. This review outlines recent advances in treatment of biliary cancer. Encouraging results from prospective, single-arm phase II trials of photodynamic therapy in nonresectable cholangiocarcinoma suggest considerable promise for this new palliative treatment modality. However, the apparent benefit of photodynamic therapy on survival, jaundice, and quality of life must be confirmed in a randomized multicenter trial.

[Bile peritonitis in an alcoholic man after traumatic gallbladder perforation--differential ascitic decompensated liver cirrhosis diagnosis]

Traumatic rupture of the gallbladder is a very rare event associated with high mortality. Since clinical symptoms are nonspecific, diagnosis is difficult. We present an alcoholic with biliary ascites after traumatic perforation of the gallbladder. Initially, he was misdiagnosed to have ascites caused by liver cirrhosis. This case demonstrates, that the combination of patients history, clinical investigation and ultrasound allows the diagnosis of traumatic rupture of the gallbladder.

Improved accuracy in the diagnosis of intrahepatic bile duct ectasia in Caroli's disease by combination of ultrasound and endoscopic retrograde cholangiography

Caroli's disease is characterized by dilatation of the intrahepatic bile ducts. Cholangitis, liver cirrhosis and development of a cholangiocarcinoma are possible complications. For optimal therapy, a correct diagnosis of the extent of the disease is mandatory. The present report demonstrates that the combination of endoscopic retrograde cholangiography and ultrasound may lead to a more reliable diagnosis of the extent of Caroli's disease. It is therefore essential to perform ultrasound in all these patients.

Production of IL-12 in gastritis relates to infection with Helicobacter pylori

Increased production of proinflammatory cytokines, including tumour necrosis factor-alpha (TNF-alpha), IL-1beta, IL-6 and IL-8, has been demonstrated in Helicobacter pylori-associated gastric mucosal inflammation. IL-12, a newly characterized cytokine, is thought to be a key mediator in host responses to bacterial infections. The aim of this study was to investigate differences in cytokine patterns between H. pylori-positive and -negative gastritis and normal mucosa. Secretion of IL-12, TNF-alpha, IL-1beta, IL-6, IL-8 and IL-10 was measured in 176 patients with chronic gastritis in whole biopsy cultures. Gastritis was graded for chronic inflammation or acute inflammatory activity, respectively, according to the Sydney system. Biopsies with similar scores were matched for analysis from H. pylori-infected and non-infected patients. Secretion of IL-12 was significantly increased in H. pylori-associated gastritis in comparison with H. pylori-negative gastritis (P < 0.0001). In contrast, secretion of TNF-alpha, IL-1beta, IL-6, and IL-8 correlated with the degree of inflammation but was not different between H. pylori-positive and -negative patients. Moreover, IL-10 secretion was found to be higher in H. pylori-positive than in H. pylori-negative patients. IL-12 may play a specific role in H. pylori-associated gastric disease, whereas production of the proinflammatory cytokines TNF-alpha, IL-1beta, IL-6 and IL-8 does not seem to be restricted to H. pylori-induced inflammation. The contra-inflammatory cytokine IL-10 may be a contributor to the chronicity of H. pylori-associated gastritis by impairing clearance of the pathogen.

Association between intraepithelial Escherichia coli and colorectal cancer

BACKGROUND & AIMS

Although multiple studies have focused on Helicobacter pylori, little is known about the mucosa-associated flora of the colon. The aim of this study was to detect bacteria directly in colonic mucosa from patients screened for colorectal cancer.

METHODS

Bacteria were quantified with the polymerase chain reaction and identified by comparative sequence analysis in colonoscopic biopsy specimens from 31 asymptomatic and 34 symptomatic controls with normal colonoscopic findings, 29 patients with colonic adenoma, and 31 patients with colorectal carcinoma. In 41 patients, intra- and extracellular location of bacteria was confirmed with the gentamicin protection assay.

RESULTS

No bacteria were detected in biopsy specimens from 97% of asymptomatic and 69% of symptomatic controls. In contrast, bacterial concentrations of 10(3)-10(5) colony-forming units per microliter were detected in biopsy specimens from both malignant and macroscopically normal tissue in 90% and 93% of patients with adenoma and carcinoma, respectively. E. coli and coli-like bacteria were shown to colonize the colonic mucosa in 82% of these patients. The gentamicin protection assay indicated that E. coli was partially intracellular in 87% of patients with adenoma and carcinoma and in none of the controls.

CONCLUSIONS

The colonic mucosa of patients with colorectal carcinoma but not normal colonic mucosa is colonized by intracellular E. coli.

Treatment with tumour necrosis factor inhibitor oxpentifylline does not improve corticosteroid dependent chronic active Crohn's disease

BACKGROUND

In Crohn's disease, inflammation is presumably sustained by an increased production of proinflammatory cytokines, in particular tumour necrosis factor alpha (TNF alpha) and interleukin 1 beta (IL 1 beta). TNF alpha can induce a host of cellular effector events resulting in perpetuation of the inflammatory process. In vivo studies with anti-TNF alpha antibody treatment have led to impressive clinical results.

AIMS

To investigate whether treatment with the TNF alpha inhibitor oxpentifylline results in clinical improvement in corticosteroid dependent chronic active Crohn's disease.

METHODS

Sixteen Crohn's disease patients received oxpentifylline 400 mg four times a day in a four week open label study.

RESULTS

Blockade of TNF alpha production in 16 patients with corticosteroid dependent Crohn's disease did not improve the clinical disease activity (CDAI mean (SEM) 188.75 (5.65) versus 185.13 (10.87) or the endoscopic degree of inflammation (CDEIS 14.9 (2.87) versus 14.8 (2.27) or laboratory parameters.

CONCLUSIONS

In this study, use of the TNF alpha inhibitor oxpentifylline does not improve inflammation in Crohn's disease. This finding suggests that there may be more key mediators than only TNF alpha in the inflammatory process in Crohn's disease.

Molecular and structural analysis of a continuous birch profilin epitope defined by a monoclonal antibody

The interaction of a mouse monoclonal antibody (4A6) and birch profilin, a structurally well conserved actin- and phosphoinositide-binding protein and cross-reactive allergen, was characterized. In contrast to serum IgE from allergic patients, which shows cross-reactivity with most plants, monoclonal antibody 4A6 selectively reacted with tree pollen profilins. Using synthetic overlapping peptides, a continuous hexapeptide epitope was identified. The exchange of a single amino acid (Gln-47 --> Glu) within the epitope was found to abolish the binding of monoclonal antibody 4A6 to other plant profilins. The NMR analyses of the birch and the nonreactive timothy grass profilin peptides showed that the loss of binding was not due to major structural differences. Both peptides adopted extended conformations similar to that observed for the epitope in the x-ray crystal structure of the native birch profilin. Binding studies with peptides and birch profilin mutants generated by in vitro mutagenesis demonstrated that the change of Gln-47 to acidic amino acids (e.g. Glu or Asp) led to electrostatic repulsion of monoclonal antibody 4A6. In conclusion the molecular and structural analyses of the interaction of a monoclonal antibody with a continuous peptide epitope, recognized in a conformation similar to that displayed on the native protein, are presented.

Multiple fistulas and tracheobronchial stenoses require extensive stenting of the central airways and esophagus in squamous-cell carcinoma

We report here on two cases of double airway and esophageal stenting in patients with multiple esophagotracheal fistulas and stenoses of the airways and esophagus due to squamous-cell carcinoma. Dumon stents and a Strecker device were used for tracheobronchial stenting. Covered Gianturco Z-stents were implanted into the esophagus. In one case, extrinsic compression of the trachea and tumor progression required recanalization by Nd:YAG laser. Clinical improvement led to discharge of the patients within two weeks after the procedure. The results show that simultaneous implantation of stents in the central airways and covered Gianturco Z-stents in the esophagus is an effective therapeutic strategy in patients with tracheal and esophageal obstructions and esophagorespiratory fistulas. Further systematic studies evaluating double stenting are warranted.

Helmholtz free energies of atom pair interactions in proteins

BACKGROUND

Proteins fold to unique three-dimensional structures, but how they achieve this transition and how they maintain their native folds is controversial. Information on the functional form of molecular interactions is required to address these issues. The basic building blocks are the free energies of atom pair interactions in dense protein solvent systems. In a dense medium, entropic effects often dominate over internal energies but free energy estimates are notoriously difficult to obtain. A prominent example is the peptide hydrogen bond (H-bond). It is still unclear to what extent H-bonds contribute to protein folding and stability of native structures.

RESULTS

Radial distribution functions of atom pair interactions are compiled from a database of known protein folds. The functions are transformed to Helmholtz free energies using a recipe from the statistical mechanics of dense interacting systems. In particular we concentrate on the features of the free energy functions of peptide H-bonds. Differences in Helmholtz free energies correspond to the reversible work required or gained when the distance between two particles is changed. Consequently, the functions directly display the energetic features of the respective thermodynamic process, such as H-bond formation or disruption.

CONCLUSIONS

In the H-bond potential, a high barrier isolates a deep narrow minimum at H-bond contact from large distances, but the free energy difference between H-bond contact and large distances is close to zero. The energy barrier plays an intriguing role in H-bond formation and disruption: both processes require activation energy in the order of 2kT. H-bond formation opposes folding to compact states, but once formed, H-bonds act as molecular locks and a network of such bonds keeps polypeptide chains in a precise spatial configuration. On the other hand, peptide H-bonds do not contribute to the thermodynamic stability of native folds, because the energy balance of H-bond formation is close to zero.

Progress in fold recognition

The prediction experiment reveals that fold recognition has become a powerful tool in structural biology. We applied our fold recognition technique to 13 target sequences. In two cases, replication terminating protein and prosequence of subtilisin, the predicted structures are very similar to the experimentally determined folds. For the first time, in a public blind test, the unknown structures of proteins have been predicted ahead of experiment to an accuracy approaching molecular detail. In two other cases the approximate folds have been predicted correctly. According to the assessors there were 12 recognizable folds among the target proteins. In our postprediction analysis we find that in 7 cases our fold recognition technique is successful. In several of the remaining cases the predicted folds have interesting features in common with the experimental results. We present our procedure, discuss the results, and comment on several fundamental and technical problems encountered in fold recognition.

A respirometer for investigating oxidative cell metabolism: toward optimization of respiratory studies

Oxygen flux measurements are critical at low rates of. respiration and at oxygen tensions below air saturation. Difficulties are primarily due to increased oxygen diffusion and consumption processes which may occur in conventional closed respirometric chambers. To avoid measurement errors encountered with standard equipment, a new type of oxygraph was developed. This instrument provides reliable quantification of oxygen flux in small organisms, tissue samples, cells, isolated mitochondria, and chloroplasts. The instrument is available commercially or can be rebuilt in a well-equipped laboratory workshop.

Reversibility of oxygen switch-off effect on Bacillus polymyxa nitrogenase

The objective of this study was to analyse in vivo the effect of oxygen on the nitrogenase of Bacillus polymyxa. The culture technique employed in this study prevented spore formation by B. polymyxa during the entire period of exposure to acetylene. Under these conditions the acetylene-reduction assay allowed quantification of nitrogenase activity over long incubation periods (44 h). Nitrogenase activity was highest in cells harvested in the late logarithmic phase. At PO2 of 0.19 and 0.37 kPa, acetylene reduction was inhibited by 80 and 100%, respectively. This switch-off effect could be reversed through oxygen exhaustion, either by flushing the culture with N2 or by cellular respiration, suggesting a respiratory protection mechanism for the nitrogenase complex in B. polymyxa. Oxygen consumption measured by a closed-chamber respirometer showed a linear increase up to a PO2 of 0.2 kPa. Above 0.3 kPa a saturation in oxygen consumption was observed. Exposure to high oxygen pressures resulted in an irreversible loss of nitrogenase activity. The oxygen inhibition pattern was shown to be similar to that in other microaerophilic and anaerobic nitrogen-fixing microorganisms.