Radiomics Can Distinguish Pediatric Supratentorial Embryonal Tumors, High-Grade Gliomas, and Ependymomas

BACKGROUND AND PURPOSE

Pediatric supratentorial tumors such as embryonal tumors, high-grade gliomas, and ependymomas are difficult to distinguish by histopathology and imaging because of overlapping features. We applied machine learning to uncover MR imaging-based radiomics phenotypes that can differentiate these tumor types.

MATERIALS AND METHODS

Our retrospective cohort of 231 patients from 7 participating institutions had 50 embryonal tumors, 127 high-grade gliomas, and 54 ependymomas. For each tumor volume, we extracted 900 Image Biomarker Standardization Initiative-based PyRadiomics features from T2-weighted and gadolinium-enhanced T1-weighted images. A reduced feature set was obtained by sparse regression analysis and was used as input for 6 candidate classifier models. Training and test sets were randomly allocated from the total cohort in a 75:25 ratio.

RESULTS

The final classifier model for embryonal tumor-versus-high-grade gliomas identified 23 features with an area under the curve of 0.98; the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were 0.85, 0.91, 0.79, 0.94, and 0.89, respectively. The classifier for embryonal tumor-versus-ependymomas identified 4 features with an area under the curve of 0.82; the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were 0.93, 0.69, 0.76, 0.90, and 0.81, respectively. The classifier for high-grade gliomas-versus-ependymomas identified 35 features with an area under the curve of 0.96; the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were 0.82, 0.94, 0.82, 0.94, and 0.91, respectively.

CONCLUSIONS

In this multi-institutional study, we identified distinct radiomic phenotypes that distinguish pediatric supratentorial tumors, high-grade gliomas, and ependymomas with high accuracy. Incorporation of this technique in diagnostic algorithms can improve diagnosis, risk stratification, and treatment planning.

Radiomic Phenotypes Distinguish Atypical Teratoid/Rhabdoid Tumors from Medulloblastoma

BACKGROUND AND PURPOSE

Atypical teratoid/rhabdoid tumors and medulloblastomas have similar imaging and histologic features but distinctly different outcomes. We hypothesized that they could be distinguished by MR imaging-based radiomic phenotypes.

MATERIALS AND METHODS

We retrospectively assembled T2-weighted and gadolinium-enhanced T1-weighted images of 48 posterior fossa atypical teratoid/rhabdoid tumors and 96 match-paired medulloblastomas from 7 institutions. Using a holdout test set, we measured the performance of 6 candidate classifier models using 6 imaging features derived by sparse regression of 900 T2WI and 900 T1WI Imaging Biomarker Standardization Initiative-based radiomics features.

RESULTS

From the originally extracted 1800 total Imaging Biomarker Standardization Initiative-based features, sparse regression consistently reduced the feature set to 1 from T1WI and 5 from T2WI. Among classifier models, logistic regression performed with the highest AUC of 0.86, with sensitivity, specificity, accuracy, and F1 scores of 0.80, 0.82, 0.81, and 0.85, respectively. The top 3 important Imaging Biomarker Standardization Initiative features, by decreasing order of relative contribution, included voxel intensity at the 90th percentile, inverse difference moment normalized, and kurtosis-all from T2WI.

CONCLUSIONS

Six quantitative signatures of image intensity, texture, and morphology distinguish atypical teratoid/rhabdoid tumors from medulloblastomas with high prediction performance across different machine learning strategies. Use of this technique for preoperative diagnosis of atypical teratoid/rhabdoid tumors could significantly inform therapeutic strategies and patient care discussions.

Deep Learning for Pediatric Posterior Fossa Tumor Detection and Classification: A Multi-Institutional Study

BACKGROUND AND PURPOSE

Posterior fossa tumors are the most common pediatric brain tumors. MR imaging is key to tumor detection, diagnosis, and therapy guidance. We sought to develop an MR imaging-based deep learning model for posterior fossa tumor detection and tumor pathology classification.

MATERIALS AND METHODS

The study cohort comprised 617 children (median age, 92 months; 56% males) from 5 pediatric institutions with posterior fossa tumors: diffuse midline glioma of the pons (n = 122), medulloblastoma (n = 272), pilocytic astrocytoma (n = 135), and ependymoma (n = 88). There were 199 controls. Tumor histology served as ground truth except for diffuse midline glioma of the pons, which was primarily diagnosed by MR imaging. A modified ResNeXt-50-32x4d architecture served as the backbone for a multitask classifier model, using T2-weighted MRIs as input to detect the presence of tumor and predict tumor class. Deep learning model performance was compared against that of 4 radiologists.

RESULTS

Model tumor detection accuracy exceeded an AUROC of 0.99 and was similar to that of 4 radiologists. Model tumor classification accuracy was 92% with an F₁ score of 0.80. The model was most accurate at predicting diffuse midline glioma of the pons, followed by pilocytic astrocytoma and medulloblastoma. Ependymoma prediction was the least accurate. Tumor type classification accuracy and F₁ score were higher than those of 2 of the 4 radiologists.

CONCLUSIONS

We present a multi-institutional deep learning model for pediatric posterior fossa tumor detection and classification with the potential to augment and improve the accuracy of radiologic diagnosis.

Variable Refocusing Flip Angle Single-Shot Imaging for Sedation-Free Fast Brain MRI

BACKGROUND AND PURPOSE

Conventional single-shot FSE commonly used for fast MRI may be suboptimal for brain evaluation due to poor image contrast, SNR, or image blurring. We investigated the clinical performance of variable refocusing flip angle single-shot FSE, a variation of single-shot FSE with lower radiofrequency energy deposition and potentially faster acquisition time, as an alternative approach to fast brain MR imaging.

MATERIALS AND METHODS

We retrospectively compared half-Fourier single-shot FSE with half- and full-Fourier variable refocusing flip angle single-shot FSE in 30 children. Three readers reviewed images for motion artifacts, image sharpness at the brain-fluid interface, and image sharpness/tissue contrast at gray-white differentiation on a modified 5-point Likert scale. Two readers also evaluated full-Fourier variable refocusing flip angle single-shot FSE against T2-FSE for brain lesion detectability in 38 children.

RESULTS

Variable refocusing flip angle single-shot FSE sequences showed more motion artifacts (P < .001). Variable refocusing flip angle single-shot FSE sequences scored higher regarding image sharpness at brain-fluid interfaces (P < .001) and gray-white differentiation (P < .001). Acquisition times for half- and full-Fourier variable refocusing flip angle single-shot FSE were faster than for single-shot FSE (P < .001) with a 53% and 47% reduction, respectively. Intermodality agreement between full-Fourier variable refocusing flip angle single-shot FSE and T2-FSE findings was near-perfect (κ = 0.90, κ = 0.95), with an 8% discordance rate for ground truth lesion detection.

CONCLUSIONS

Variable refocusing flip angle single-shot FSE achieved 2× faster scan times than single-shot FSE with improved image sharpness at brain-fluid interfaces and gray-white differentiation. Such improvements are likely attributed to a combination of improved contrast, spatial resolution, SNR, and reduced T2-decay associated with blurring. While variable refocusing flip angle single-shot FSE may be a useful alternative to single-shot FSE and, potentially, T2-FSE when faster scan times are desired, motion artifacts were more common in variable refocusing flip angle single-shot FSE, and, thus, they remain an important consideration before clinical implementation.

MR Imaging-Based Radiomic Signatures of Distinct Molecular Subgroups of Medulloblastoma

BACKGROUND AND PURPOSE

Distinct molecular subgroups of pediatric medulloblastoma confer important differences in prognosis and therapy. Currently, tissue sampling is the only method to obtain information for classification. Our goal was to develop and validate radiomic and machine learning approaches for predicting molecular subgroups of pediatric medulloblastoma.

MATERIALS AND METHODS

In this multi-institutional retrospective study, we evaluated MR imaging datasets of 109 pediatric patients with medulloblastoma from 3 children's hospitals from January 2001 to January 2014. A computational framework was developed to extract MR imaging-based radiomic features from tumor segmentations, and we tested 2 predictive models: a double 10-fold cross-validation using a combined dataset consisting of all 3 patient cohorts and a 3-dataset cross-validation, in which training was performed on 2 cohorts and testing was performed on the third independent cohort. We used the Wilcoxon rank sum test for feature selection with assessment of area under the receiver operating characteristic curve to evaluate model performance.

RESULTS

Of 590 MR imaging-derived radiomic features, including intensity-based histograms, tumor edge-sharpness, Gabor features, and local area integral invariant features, extracted from imaging-derived tumor segmentations, tumor edge-sharpness was most useful for predicting sonic hedgehog and group 4 tumors. Receiver operating characteristic analysis revealed superior performance of the double 10-fold cross-validation model for predicting sonic hedgehog, group 3, and group 4 tumors when using combined T1- and T2-weighted images (area under the curve = 0.79, 0.70, and 0.83, respectively). With the independent 3-dataset cross-validation strategy, select radiomic features were predictive of sonic hedgehog (area under the curve = 0.70-0.73) and group 4 (area under the curve = 0.76-0.80) medulloblastoma.

CONCLUSIONS

This study provides proof-of-concept results for the application of radiomic and machine learning approaches to a multi-institutional dataset for the prediction of medulloblastoma subgroups.

Dose-dependent expression of claudin-5 is a modifying factor in schizophrenia

Schizophrenia is a neurodevelopmental disorder that affects up to 1% of the general population. Various genes show associations with schizophrenia and a very weak nominal association with the tight junction protein, claudin-5, has previously been identified. Claudin-5 is expressed in endothelial cells forming part of the blood-brain barrier (BBB). Furthermore, schizophrenia occurs in 30% of individuals with 22q11 deletion syndrome (22q11DS), a population who are haploinsufficient for the claudin-5 gene. Here, we show that a variant in the claudin-5 gene is weakly associated with schizophrenia in 22q11DS, leading to 75% less claudin-5 being expressed in endothelial cells. We also show that targeted adeno-associated virus-mediated suppression of claudin-5 in the mouse brain results in localized BBB disruption and behavioural changes. Using an inducible 'knockdown' mouse model, we further link claudin-5 suppression with psychosis through a distinct behavioural phenotype showing impairments in learning and memory, anxiety-like behaviour and sensorimotor gating. In addition, these animals develop seizures and die after 3-4 weeks of claudin-5 suppression, reinforcing the crucial role of claudin-5 in normal neurological function. Finally, we show that anti-psychotic medications dose-dependently increase claudin-5 expression in vitro and in vivo while aberrant, discontinuous expression of claudin-5 in the brains of schizophrenic patients post mortem was observed compared to age-matched controls. Together, these data suggest that BBB disruption may be a modifying factor in the development of schizophrenia and that drugs directly targeting the BBB may offer new therapeutic opportunities for treating this disorder.

Survivin as a therapeutic target in Sonic hedgehog-driven medulloblastoma

Medulloblastoma (MB) is a highly malignant brain tumor that occurs primarily in children. Although surgery, radiation and high-dose chemotherapy have led to increased survival, many MB patients still die from their disease, and patients who survive suffer severe long-term side effects as a consequence of treatment. Thus, more effective and less toxic therapies for MB are critically important. Development of such therapies depends in part on identification of genes that are necessary for growth and survival of tumor cells. Survivin is an inhibitor of apoptosis protein that regulates cell cycle progression and resistance to apoptosis, is frequently expressed in human MB and when expressed at high levels predicts poor clinical outcome. Therefore, we hypothesized that Survivin may have a critical role in growth and survival of MB cells and that targeting it may enhance MB therapy. Here we show that Survivin is overexpressed in tumors from patched (Ptch) mutant mice, a model of Sonic hedgehog (SHH)-driven MB. Genetic deletion of survivin in Ptch mutant tumor cells significantly inhibits proliferation and causes cell cycle arrest. Treatment with small-molecule antagonists of Survivin impairs proliferation and survival of both murine and human MB cells. Finally, Survivin antagonists impede growth of MB cells in vivo. These studies highlight the importance of Survivin in SHH-driven MB, and suggest that it may represent a novel therapeutic target in patients with this disease.

Sugar level, fermentability, and acceptability of straw treated with different acids

The sugar level, fermentability, and acceptability by voles of ryegrass straw treated with different acids were compared. Sugar release was highest with HCl, but acceptability was best with H(3)PO(4) treatment. Candida utilis grew about equally well on all of the treated samples, except the one treated with 0.5 N HCl. That sample supported no growth, possibly because the level of chloride or degradation products of monomeric sugars was too high. Fermentability and palatability of straw were highest when treated with a combination of 0.23 N HCl and 0.15 N H(3)PO(4). Fermentation with Aureobasidium pullulans further increased the palatability of the acid-treated straw.

Modification of cysteine

This unit describes a number of methods for modifying cysteine residues of proteins and peptides by reduction and alkylation procedures. A general procedure for alkylation of cysteine residues in a protein of known size and composition with haloacyl reagents or N-ethylmaleimide (NEM) is presented, and alternate protocols describe similar procedures for use when the size and composition are not known and when only very small amounts of protein are available. Alkylations that introduce amino groups using bromopropylamine and N-(iodoethyl)-trifluoroacetamide are also presented. Two procedures that are often used for subsequent sequence analysis of the protein, alkylation with 4-vinylpyridine and acrylamide, are described, and a specialized procedure for 4-vinylpyridine alkylation of protein that has been adsorbed onto a sequencing membrane is also presented. Reversible modification of cysteine residues by way of sulfitolysis is described, and a protocol for oxidation with performic acid for amino acid compositional analysis is also provided. Gentle oxidation of cysteine residues to disulfides by exposure to air is detailed. Support protocols are included for recrystallization of iodoacetic acid, colorimetric detection of free sulfhydryls, and desalting of modified samples.

Comparison of MALDI-TOF mass spectrometric to enzyme colorimetric quantification of glucose from enzyme-hydrolyzed starch

Successful quantification of the glucose produced by enzyme hydrolysis of starch was achieved by a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) protocol, using sorbitol as an internal standard. The starch contents measured by MALDI-TOF MS of corn starch, fiber-enriched oat flour derivatives, oat and barley flours, and barley flour/corn starch composites were evaluated in comparison to a widely accepted and validated method of starch determination, which relies on enzyme colorimetry (EC). The average starch content measured in a series of corn starch samples of different masses was 93 and 101% for EC and MALDI-TOF MS, respectively, values that represent the estimated purity of the sample. There was an agreement of 99% between the starch contents determined by the two analytical methods for complex flour-derived samples. Starch values estimated by MALDI-TOF MS consistently showed a greater degree of variability than those determined by EC, but this limitation was readily compensated by rapid acquisition of multiple mass spectra. This study is the first to report the quantification of glucose by MALDI-TOF MS, and it offers new perspectives into the potential utility of MALDI-TOF MS as a definitive tool for monosaccharide analysis and rapid starch determination in complex samples.

A sensitive method for detection of sulfamethazine and N4-acetylsulfamethazine residues in environmental samples using solid phase immunoextraction coupled with MALDI-TOF MS

Sulfamethazine (SMT) and its major metabolite, N(4)-acetylsulfamethazine (NA-SMT), were each recovered from spiked water (0.1 ppb) and 10% (w/v) aqueous suspensions of soil (1 ppb) or composted manure (1 ppb), by using a three-stage solid phase immunoextraction (SPIE) system, followed by detection with matrix-assisted laser/desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Sulfonamide recovery rates are reported for separate stages of the SPIE system and for trace-level sulfonamide SPIE extraction from the environmental samples. SPIE MALDI-TOF MS is a rapid and definitive technique with potentially better efficiency relative to other established trace-level sulfonamide analytical methods. SPIE MALDI-TOF MS required 1.5 h per batch (8-24 samples/batch) for sample enrichment, 5 min per batch for probe preparation, and 5 min per sample to acquire and process the spectrum. This is the first time MALDI-TOF MS has been reported as a potential means of detecting trace-level drug residues in complex environmental samples.

Amino acid residue mutations uncouple cooperative effects in Escherichia coli D-3-phosphoglycerate dehydrogenase

d-3-Phosphoglycerate dehydrogenase from Escherichia coli contains two Gly-Gly sequences that occur at junctions between domains. A previous study (Grant, G. A., Xu, X. L., and Hu, Z. (2000) Biochemistry 39, 7316-7319) determined that the Gly-Gly sequence at the junction between the regulatory and substrate binding domain functions as a hinge between the domains. Mutations in this area significantly decrease the ability of serine to inhibit activity but have little effect on the K(m) and k(cat). Conversely, the present study shows that mutations to the Gly-Gly sequence at the junction of the substrate and nucleotide binding domains, which form the active site cleft, have a significant effect on the k(cat) of the enzyme without substantially altering the enzyme's sensitivity to serine. In addition, mutation of Gly-294, but not Gly-295, has a profound effect on the cooperativity of serine inhibition. Interestingly, even though cooperativity of inhibition can be reduced significantly, there is little apparent effect on the cooperativity of serine binding itself. An additional mutant, G336V,G337V, also reduces the cooperativity of inhibition, but in this case serine binding also is reduced to the point at which it cannot be measured by equilibrium dialysis. The double mutant G294V,G336V demonstrates that strain imposed by mutation at one hinge can be relieved partially by mutation at the other hinge, demonstrating linkage between the two hinge regions. These data show that the two cooperative processes, serine binding and catalytic inhibition, can be uncoupled. Consideration of the allowable torsional angles for the side chains introduced by the mutations yields a range of values for these angles that the glycine residues likely occupy in the native enzyme. A comparison of these values with the torsional angles found for the inhibited enzyme from crystal coordinates provides potential beginning and ending orientations for the transition from active to inhibited enzyme, which will allow modeling of the dynamics of domain movement.

Treatment of recurrent peripheral nerve entrapment problems: role of scar formation and its possible treatment

Surgical management of peripheral nerve entrapment syndromes is usually successful, but the recurrence of symptoms after initial improvement can and does occur. Extraneural fibrosis is one possible cause of recurrent peripheral nerve problems as a result of nerve compression or tethering. Several approaches to prevent extraneural scarring after surgery have been studied, including wrapping the involved nerve with a graft, the application of various chemical compounds, and radiation. ADCON-T/N, an antiscar bioabsorbable gel device was evaluated in a retrospective clinical review. Sixty-seven percent of patients treated with ADCON-T/N after reoperation of a peripheral nerve experienced prolonged clinical improvement compared with 50% of patients who did not receive ADCON-T/N. These preliminary results suggest that ADCON-T/N may prove to be clinically useful in the surgical treatment of peripheral nerve problems. Additional more rigorous clinical studies are necessary, however.

Specific interactions at the regulatory domain-substrate binding domain interface influence the cooperativity of inhibition and effector binding in Escherichia coli D-3-phosphoglycerate dehydrogenase

The crystal structure of d-3-phosphoglycerate dehydrogenase reveals a limited number of contacts between the regulatory and substrate binding domains of each subunit in the tetrameric enzyme. These occur between the side chains of Arg-339, Arg-405, and Arg-407 in the regulatory domain and main chain carbonyls in the substrate binding domain. In addition, Arg-339 participates in a hydrogen bonding network within the regulatory domain involving Arg-338 and Tyr-410, the C-terminal residue of the enzyme subunit. Mutagenic analysis of these residues produce profound effects on the enzyme's sensitivity to serine, the cooperativity of serine inhibition, and in some cases, the apparent overall conformation of the enzyme. Mutations of Arg-405 and Arg-407, which span the interface where the two domains come together, reduce the cooperativity of inhibition and increase the sensitivity of the enzyme to serine concentration. Serine binding studies with Arg-407 converted to Ala demonstrate that cooperativity of serine binding is also significantly reduced in a manner similar to the reduction in the cooperativity of inhibition. Mutations of Tyr-410 and Arg-338 decrease the sensitivity to serine without an appreciable effect on the cooperativity of inhibition. In the case of Tyr-410, a deletion mutant demonstrates that this effect is due to the loss of the C-terminal carboxyl group rather than the tyrosine side chain. All mutations of Arg-339, with the exception of its conversion to Lys, had profound effects on the stability of the enzyme. In general, those mutants that decrease sensitivity to serine are those that participate mainly in intradomain interactions and may also directly affect the serine binding sites themselves. Those mutants that decrease cooperativity are those that participate in interdomain interaction within the subunit. The observation that the mutants that decrease cooperativity also increase sensitivity to serine suggests a potential separation of pathways between how the simple act of serine binding results in noncooperative active site inhibition in the first place and how serine binding also leads to cooperativity between sites in the native enzyme.

Role of an interdomain Gly-Gly sequence at the regulatory-substrate domain interface in the regulation of Escherichia coli. D-3-phosphoglycerate dehydrogenase

The regulatory and substrate binding domains of D-3-phosphoglycerate dehydrogenase (PGDH, EC 1.1.1.95) from Escherichia coli are connected by a single polypeptide strand that contains a Gly-Gly sequence approximately midway between the domains. The potential flexibility of this sequence and its strategic location between major domain structures suggests that it may function in the conformational change leading from effector binding to inhibition of the active site. Site-directed mutagenesis of this region (Gly-336-Gly-337) supports this hypothesis. When bulky side chains were substituted for the glycines at these positions, substantial changes in the ability of serine to inhibit the enzyme were seen with little effect on the activity of the enzyme. The effect of these substitutions could be alleviated by placing a new glycine residue at position 335, immediately flanking the original glycine pair. On the other hand, substituting a glycine at position 338 revealed a critical role for the side chain of Arg-338. This residue may function in stabilizing the conformation about the Gly-Gly turn, resulting in a specific orientation of the adjacent domains relative to each other. Rotation about the phi or psi bonds of either Gly-336 or Gly-337 would have a profound effect on this orientation. The data are consistent with this as a role for the Gly-Gly sequence between the regulatory and substrate binding domains of PGDH.

Removal of the tryptophan 139 side chain in Escherichia coli D-3-phosphoglycerate dehydrogenase produces a dimeric enzyme without cooperative effects

Escherichia coli d-3-phosphoglycerate dehydrogenase (PGDH) is a homotetrameric enzyme whose activity is allosterically regulated by l-serine, the end-product of its metabolic pathway. Previous studies have shown that PGDH displays two modes of cooperative interaction. One is between the l-serine binding sites and the other is between the l-serine binding sites and the active sites. Tryptophan 139 participates in an intersubunit contact near the active site catalytic residues. Site-specific mutagenesis of tryptophan 139 to glycine results in the dissociation of the tetramer to a pair of dimers and in the loss of cooperativity in serine binding and between serine binding and inhibition. The results suggest that the magnitude of inhibition of activity at a particular active site is primarily dependent on serine binding to that subunit but that activity can be modulated in a cooperative manner by interaction with adjacent subunits. The disruption of the nucleotide domain interface in PGDH by mutating Trp-139 suggests the potential for a critical role of this interface in the cooperative allosteric processes in the native tetrameric enzyme.

A new model of the blood--brain barrier: co-culture of neuronal, endothelial and glial cells under dynamic conditions

Developing in vitro blood-brain barrier (BBB) models that closely mimic the natural state is important for theoretical and practical applications, including drug development. We previously developed an in vitro BBB model based on co-culturing endothelial cells with glia in the presence of flow on hollow fiber tube culture substrates. We now report that this dynamic in vitro BBB (DIV-BBB) can be successfully used to co-culture differentiated serotonergic neurons in the presence of a BBB. These neurons demonstrated fluoxetine-sensitive serotonin (5HT) uptake and depolarization-induced release of [3H]5HT. Our results demonstrate that the DIV-BBB is a suitable model for culturing of neurons in a quasi-physiological microenvironment and in the presence of a high-resistance, stereoselective BBB.

Phosphate ion partially relieves the cooperativity of effector binding in D-3-phosphoglycerate dehydrogenase without altering the cooperativity of inhibition

The binding of L-serine to phosphoglycerate dehydrogenase from E. coli displays elements of both positive and negative cooperativity. In addition, the inhibition of enzymatic activity by L-serine is also cooperative with Hill coefficients greater than 1. However, phosphate buffer significantly reduces the cooperative effects in serine binding without affecting the cooperativity of inhibition of activity. The maximal degree of inhibition and fluorescence quenching in Tris buffer occurs when an average of two serine binding sites out of four are occupied. This value increases to three out of the four sites at maximal levels of inhibition and quenching in phosphate buffer. The increase from two to three sites appears to be due to the ability of phosphate to reduce the site to site cooperative effects and render each ligand binding site less dependent on each other. The correlation between the level of inhibition and the fractional site occupancy indicates that in Tris buffer, one serine is bound to each interface at maximal effect. In the presence of phosphate, the order of binding appears to change so that both sites at one interface fill before the first site at the opposite interface is occupied. In each case, there is a good correlation between serine binding, conformational change at the regulatory site interfaces, and inhibition of enzyme activity. The observation that phosphate does not appear to have a similar effect on the cooperativity of inhibition of enzymatic activity suggests that there are two distinct cooperative pathways at work: one path between the four serine binding sites, and one path between the serine binding sites and the active sites.

The relationship between effector binding and inhibition of activity in D-3-phosphoglycerate dehydrogenase

The binding of L-serine to phosphoglycerate dehydrogenase from Escherichia coli displays elements of both positive and negative cooperativity. At pH 7.5, approximately 2 mol of serine are bound per mole of tetrameric enzyme. A substantial degree of positive cooperativity is seen for the binding of the second ligand, but the binding of the third and fourth ligand display substantial negative cooperativity. The data indicate a state of approximately 50% inhibition when only one serine is bound and approximately 80-90% inhibition when two serines are bound. This is consistent with the tethered domain hypothesis that has been presented previously. Comparison of the data derived directly from binding stoichiometry to the binding constants determined from the best fit to the Adair equation, produce a close agreement, and reinforce the general validity of the derived binding constants. The data also support the conclusion that the positive cooperativity between the binding to the first and second site involves binding sites at opposite interfaces over 110 A apart. Thus, an order of binding can be envisioned where the binding of the first ligand initiates a conformational transition that allows the second ligand to bind with much higher affinity at the opposite interface. This is followed by the third ligand, which binds with lesser affinity to one of the two already occupied interfaces, and in so doing, completes a global conformational transition that produces maximum inhibition of activity and an even lower affinity for the fourth ligand, excluding it completely. Thus, maximal inhibition is accomplished with less than maximal occupancy of effector sites through a mechanism that displays strong elements of both positive and negative cooperativity.

Solitary eosinophilic granuloma of the temporal lobe: case report and review of the literature

A solitary eosinophilic granuloma of the central nervous system is an unusual manifestation of histiocytosis X. A unique case of a solitary eosinophilic granuloma of the right temporal lobe without osseous involvement is described. A 20-year-old man presented with a grand mal seizure. Magnetic resonance imaging demonstrated an intraaxial enhancing mass in the right temporal lobe with marked vasogenic edema. A right temporal craniotomy was performed for resection of the lesion and the diagnosis of an eosinophilic granuloma was confirmed by histopathology. Follow-up MR imaging obtained 5 years following resection demonstrated no recurrence. Solitary eosinophilic granuloma should be considered in the differential diagnosis of enhancing mass lesions affecting the central nervous system. Although the natural history of solitary eosinophilic granulomas remains poorly defined, surgical treatment still remains the mainstay of therapy for these unifocal cerebral lesions.

Management of intrinsic gliomas of the tectal plate in children. A ten-year review

The natural history, management, and long-term outcome for patients with benign, intrinsic tectal plate gliomas remain controversial in spite of their propensity to cause late-onset hydrocephalus. A 10-year retrospective review has identified 11 consecutive children with tectal plate lesions. Headache, vomiting, a decline in school performance, tremor, and complex partial seizures were common presenting symptoms. All patients presented with signs and symptoms of hydrocephalus. Magnetic resonance (MR) imaging delineated an intra-axial mass lesion of the midbrain primarily localized to the tectal plate which uniformly was hyperintense on T2-weighted imaging and had a more variable appearance on T1-weighted imaging and rare enhancement with gadolinium. No patient underwent surgical resection, chemotherapy, or radiotherapy. Three of 11 patients (27%) showed evidence of progression in size or a new focus of enhancement on MR imaging, which was clinically asymptomatic. In this series, no patient with a tectal plate lesion less than 1.5 cm in maximal diameter and without gadolinium enhancement showed any evidence of clinical or radiological progression. Although intrinsic tectal lesions in children are clinically indolent and the initial management consists of CSF diversion, these lesions may eventually progress and still warrant long-term follow-up with serial MR imaging.

Recurrent intracranial Masson's vegetant intravascular hemangioendothelioma. Case report and review of the literature

In the central nervous system, recurrence of intracranial Masson's vegetant intravascular hemangioendothelioma (MVIH) is rare. To the authors' knowledge, only three recurrent intracranial cases have been reported. The authors report the case of a 75-year-old woman with a recurrent left-sided cerebellopontine angle and middle cranial fossa MVIH. When the patient was 62 years of age, she underwent preoperative embolization and subtotal resection of the intracranial lesion followed by postoperative radiotherapy. She was well and free from disease until 9 years postoperatively when she became symptomatic. At 71 years of age, the patient again underwent preoperative embolization and near-gross-total resection of the lesion. Follow-up imaging performed 15 months later revealed tumor recurrence, and she underwent stereotactic gamma knife radiosurgery. At a 2.75-year follow-up review, the patient's imaging studies revealed stable residual tumor. This case report is unique in that it documents the clinical and pathological features, surgical and postoperative treatment, and long-term follow-up review of a patient with recurrent intracranial MVIH and suggests that this unusual vascular lesion is a slow-growing benign tumor rather than a reactive process. Because the pathological composition of the lesion may resemble an angiosarcoma, understanding this benign vascular neoplasm is crucial so that an erroneous diagnosis of malignancy is not made and unnecessary adjuvant therapy is not given.

Evaluation and surgical management of peripheral nerve problems

OBJECTIVE

To illustrate how an understanding of the basic biological responses of peripheral nerves to injury is important in formulating a rational treatment plan.

METHODS

Peripheral nerve anatomy and physiology are described in a context that is relevant to understanding the different grades of peripheral nerve injury. Methods of evaluating and treating peripheral nerve injuries both medically and surgically are reviewed. Relevant scientific studies with potential clinical impact are also discussed.

RESULTS

The clinical symptoms, physical findings, and electrodiagnostic and imaging test results relevant to the diagnosis of peripheral nerve problems are reviewed. Conventional and new medical or surgical strategies in the management of peripheral nerve injuries and mass lesions are described.

CONCLUSION

The diagnosis and treatment of peripheral nerve injuries follow logically from an understanding of the biological responses of peripheral nerves after injury and during recovery.

The contribution of adjacent subunits to the active sites of D-3-phosphoglycerate dehydrogenase

D-3-Phosphoglycerate dehydrogenase (PGDH) from Escherichia coli is allosterically inhibited by L-serine, the end product of its metabolic pathway. Previous results have shown that inhibition by serine has a large effect on Vmax and only a small or negligible effect on Km. PGDH is thus classified as a V-type allosteric enzyme. In this study, the active site of PGDH has been studied by site-directed mutagenesis to assess the role of certain residues in substrate binding and catalysis. These consist of a group of cationic residues (Arg-240, Arg-60, Arg-62, Lys-39, and Lys-141') that potentially form an electrostatic environment for the binding of the negatively charged substrate, as well as the only tryptophan residue found in PGDH and which fits into a hydrophobic pocket immediately adjacent to the active site histidine residue. Interestingly, Trp-139' and Lys-141' are part of the polypeptide chain of the subunit that is adjacent to the active site. The results of mutating these residues show that Arg-240, Arg-60, Arg-62, and Lys-141' play distinct roles in the binding of the substrate to the active site. Mutants of Trp-139' show that this residue may play a role in stabilizing the catalytic center of the enzyme. Furthermore, these mutants appear to have a significant effect on the cooperativity of serine inhibition and suggest a possible role for Trp-139' in the cooperative interactions between subunits.

Risk of early closed reduction in cervical spine subluxation injuries

OBJECT

The authors retrospectively reviewed 121 patients with traumatic cervical spine injuries to determine the risk of neurological deterioration following early closed reduction.

METHODS

After excluding minor fractures and injuries without subluxation, the medical records and imaging studies (computerized tomography and magnetic resonance [MR] images) of 82 patients with bilateral and unilateral locked facet dislocations, burst fractures, extension injuries, or miscellaneous cervical fractures with subluxation were reviewed. Disc injury was defined on MR imaging as the presence of herniation or disruption: a herniation was described as deforming the thecal sac or nerve roots, and a disruption was defined as a disc with high T2-weighted signal characteristics in a widened disc space. Fifty-eight percent of patients presented with complete or incomplete spinal cord injuries. Thirteen percent of patients presented with a cervical radiculopathy, 22% were intact, and 9% had only transient neurological deficits in the field. Early, rapid closed reduction, using serial plain radiographs or fluoroscopy and Gardner-Wells craniocervical traction, was achieved in 97.6% of patients. In two patients (2.4%) closed reduction failed and they underwent emergency open surgical reduction. The average time to achieve closed reduction was 2.1+/-0.24 hours (standard error of the mean). The incidence of disc herniation and disruption in the 80 patients who underwent postreduction MR imaging was 22% and 24%, respectively. However, the presence of disc herniation or disruption did not affect the degree of neurological recovery, as measured by American Spinal Injury Association motor score and the Frankel scale following early closed reduction. Only one (1.3%) of 80 patients deteriorated, but that occurred more than 6 hours following closed reduction.

CONCLUSIONS

Although disc herniation and disruption can occur following all types of traumatic cervical fracture subluxations, the incidence of neurological deterioration following closed reduction in these patients is rare. The authors recommend early closed reduction in patients presenting with significant motor deficits without prior MR imaging.

Destructive tophaceous calcium hydroxyapatite tumor of the infratemporal fossa. Case report and review of the literature

Tophaceous pseudogout is one of the rarest forms of crystal deposition disease, typically presenting as a destructive and invasive mass involving the temporomandibular joint or the infratemporal fossa region in the absence of any other articular manifestations. Previous cases have been assumed to be caused by calcium pyrophosphate dihydrate (CPPD) crystal deposition, based on finding weakly birefringent crystals in the involved tissues. The authors present the unique case of a 65-year-old woman with a destructive and invasive facial mass extending to the middle cranial fossa with microscopic and clinical features consistent with tophaceous pseudogout. High-resolution x-ray crystallographic powder diffraction and Fourier transformed infrared spectroscopy subsequently revealed that the crystals were composed of calcium hydroxyapatite without CPPD. The patient was later found to have primary hyperparathyroidism and mild hypercalcemia. This case demonstrates that tissue deposits of calcium hydroxyapatite can cause a destructive and invasive mass containing weakly birefringent crystals and raises the question of whether previous cases attributed to tophaceous pseudogout resulting from CPPD actually were composed of birefringent calcium hydroxyapatite.

Differential roles for disulfide bonds in the structural integrity and biological activity of kappa-Bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist

kappa-Bungarotoxin, a kappa-neurotoxin derived from the venom of the banded Krait, Bungarus multicinctus, is a homodimeric protein composed of subunits of 66 amino acid residues containing five disulfide bonds. kappa-Bungarotoxin is a potent, selective, and slowly reversible antagonist of alpha3 beta2 neuronal nicotinic acetylcholine receptors. kappa-Bungarotoxin is structurally related to the alpha-neurotoxins, such as alpha-bungarotoxin derived from the same snake, which are monomeric in solution and which effectively antagonize muscle type receptors (alpha1 beta1 gamma delta) and the homopentameric neuronal type receptors (alpha7, alpha8, and alpha9). Like the kappa-neurotoxins, the long alpha-neurotoxins contain the same five conserved disulfide bonds, while the short alpha-neurotoxins only contain four of the five. Systematic removal of single disulfide bonds in kappa-bungarotoxin by site-specific mutagenesis reveals a differential role for each of the disulfide bonds. Removal of either of the two disulfides connecting elements of the carboxy terminal loop of this toxin (Cys 46-Cys 58 and Cys 59-Cys 64) interferes with the ability of the toxin to fold. In contrast, removal of each of the other three disulfides does not interfere with the general folding of the toxin and yields molecules with biological activity. In fact, when either C3-C21 or C14-C42 are removed individually, no loss in biological activity is seen. However, removing both produces a polypeptide chain which fails to fold properly. Removal of the C27-C31 disulfide only reduces the activity of the toxin 46.6-fold. This disulfide may play a role in specific interaction of the toxin with specific neuronal receptors.

Probing the regulatory domain interface of D-3-phosphoglycerate dehydrogenase with engineered tryptophan residues

D-3-Phosphoglycerate dehydrogenase from Escherichia coli is a homotetrameric enzyme which is allosterically regulated by the end product of its pathway, L-serine. The enzyme binds 4 L-serine molecules at two interfaces formed by the noncovalent association of the regulatory domains. The two domains that comprise each interface are related by an approximately 180 degrees axis of symmetry, and two serine molecules bind at each interface by forming a hydrogen bond network between the domains. A model has been proposed that suggests that serine functions by drawing adjacent domains together and that this in turn translates a conformational change to the active site. A tryptophan residue has been engineered into the helices flanking the regulatory interfaces that displays significant quenching in response to serine binding. Residues on the adjacent subunit appear to be primarily responsible for the tryptophan quenching and thus support the hypothesis that serine binding leads to an increase in the proximity between residues on neighboring subunits. Serine binding studies show that this quenching, as well as inhibition of enzymatic activity, are essentially complete when only two of the four serine binding sites are occupied. The requirement for only one serine per interface is consistent with the notion that the interface is formed by relatively rigid domains and that hydrogen bonding at only a single site is all that is required to substantially close the interface. The fluorescence quenching in response to L-serine binding generally correlates with enzymatic inhibition, but there appears to be a slight lag in inhibition relative to quenching at low serine concentrations. The observed fluorescence quenching of residues in the regulatory domains of D-3-phosphoglycerate dehydrogenase provide the first direct evidence for a conformational change in response to effector binding and provide a means to monitor the first step in the allosteric mechanism.

Accuracy of continuous jugular bulb oximetry in the intensive care unit

OBJECTIVE

To address the accuracy of a bedside jugular bulb oxygen saturation (SjO2) catheter monitor (Baxter-Edwards, Santa Ana, CA) versus in vitro co-oximetry measurements in the intensive care unit (ICU).

METHODS

By prospective protocol, we compared blood gas measurements with simultaneously recorded continuous bedside oximetric monitor values for 31 ICU patients with traumatic brain injury undergoing jugular bulb catheter monitoring. For suboptimal fiberoptic light signal quality indices, the catheter was repositioned, flushed, or both before drawing the sample for in vitro measurement. Laboratory and bedside monitor data were examined for association using the chi2 and paired t tests and a linear regression model.

RESULTS

We assessed 195 samples (median, 5 per patient; range, 1-14) who were monitored an average of 3.4 (range, 1-6) days. The in vivo monitor (range, 32-94%) and in vitro co-oximetry (range, 38-93%) values had acceptable correlation (y = 0.94x + 4.4, r2 = 0.80). For bedside monitor detection of jugular bulb desaturation (SjO2 < 50% for 10 min), the kappa statistic was 0.35, the sensitivity was 45 to 50%, and the specificity was 98 to 100%.

CONCLUSION

Continuous ICU SjO2 monitoring correlates significantly with in vitro values, but less so than previously described during intracranial surgery. Although sensitivity of the bedside monitor to detect confirmed desaturations remains an issue, the high specificity indicates that it is less of a concern that patients may be misdiagnosed as having desaturations resulting in unnecessary interventions. Nonetheless, suspected jugular bulb desaturation should be verified before taking therapeutic actions.

Modification of the NADH of the isoniazid target (InhA) from Mycobacterium tuberculosis

The preferred antitubercular drug isoniazid specifically targets a long-chain enoyl-acyl carrier protein reductase (InhA), an enzyme essential for mycolic acid biosynthesis in Mycobacterium tuberculosis. Despite the widespread use of this drug for more than 40 years, its precise mode of action has remained obscure. Data from x-ray crystallography and mass spectrometry reveal that the mechanism of isoniazid action against InhA is covalent attachment of the activated form of the drug to the nicotinamide ring of nicotinamide adenine dinucleotide bound within the active site of InhA.

Edman sequencing as tool for characterization of synthetic peptides

Sequence analysis of synthetic peptides using Edman chemistry can be very useful for the elucidation of certain types of synthetic problems, such as residue deletions and the presence of common stable derivatives, and for following the progress of the synthesis itself. However, it can also be a relatively poor technique for assessing quantitative aspects and the type and degree of adduct formation that arise from the synthetic chemistry. For these latter considerations, techniques such as mass spectrometry can often give more precise and informative data about the integrity of a synthetic peptide. Thus, sequence analysis is best applied judiciously and then used in combination with other methods. Furthermore, proper interpretation of the results of sequence analysis of synthetic peptides relies on a thorough knowledge of the sequencing process.

Thrombotic, infectious, and procedural complications of the jugular bulb catheter in the intensive care unit

OBJECTIVE

An assessment of the thrombotic, infectious, and technical complications of continuous jugular bulb catheter monitoring in the intensive care unit (ICU) was made.

METHODS

Over a 1-year period, 44 patients suffering from traumatic brain injury, subarachnoid hemorrhage, or stroke received jugular bulb catheter monitoring in the ICU. They were followed for catheter insertion complications and the development of bacteremia. In 20 patients chosen randomly, an ultrasonographic evaluation was performed after removal of the catheter for an assessment of internal jugular vein thrombosis.

RESULTS

Of the 44 patients, 1 became bacteremic; the source was identified as a thoracostomy site. Among the complications related to the 44 catheter insertions, there were 2 instances of carotid artery puncture (4.5%), 1 misplaced catheter (thoracic placement), and 1 clinically insignificant hematoma. Of the 20 patients investigated with ultrasonography, 8 (40%) had nonobstructive, subclinical internal jugular vein thrombi after jugular bulb catheter monitoring (95% confidence interval, 19-61%). The median monitoring duration was 3 days (range, 1-6 d). No clinical factor was identified to be associated with thrombus formation.

CONCLUSION

We conclude the following: 1) the risk of bacteremia related to the jugular bulb catheter was negligible; 2) complications related to catheter insertion were rare and clinically insignificant; and 3) the incidence of subclinical internal jugular vein thrombosis after jugular bulb catheter monitoring is considerable. Although it is worthy to note this complication, no patient with a thrombus became symptomatic in the present series. The risk-benefit assessment of this monitoring technique must include consideration of subclinical thrombosis.

Critical interactions at the dimer interface of kappa-bungarotoxin, a neuronal nicotinic acetylcholine receptor antagonist

The alpha- and kappa-neurotoxins are polypeptide antagonists of nicotinic acetylcholine receptors derived from snake venom. They are structurally very similar but differ in their specificity for receptor subtype and in their native aggregation state. While the alpha-neurotoxins are monomeric, the kappa-neurotoxins occur as homodimers. The crystal structure shows that there is a correlation in the distance between essential arginine residues in the kappa-bungarotoxin dimer and the distance between the acetylcholine binding sites in the pentameric receptor. This has lead to an investigation of the critical interactions at the dimer interface of kappa-bungarotoxin. Mutations of residues that the crystal structure indicates participate in dimer interaction were found to fall into two general groups: those that do not affect the dimerization state or activity of kappa-bungarotoxin as single mutants, and those that interfere with it to such an extent that the protein is no longer able to fold properly. In general, those residues that fall into the latter group are found to be invariant in kappa-neurotoxins and not found in alpha-neurotoxins. The results suggest that the extent of both the main chain-main chain beta-sheet hydrogen bond interaction and van der Waals interactions between Phe 49 and Ile 20 are required for dimer formation. These studies provide a basis for understanding why the kappa-neurotoxins readily dimerize in solution and the alpha-neurotoxins do not and also suggest that there is a possible interrelationship between dimer formation and protein folding in kappa-bungarotoxin.

Cerebral autoregulation following minor head injury

The purpose of this study was to determine whether patients with minor head injury experience impairments in cerebral autoregulation. Twenty-nine patients with minor head injuries defined by Glasgow Coma Scale (GCS) scores of 13 to 15 underwent testing of dynamic cerebral autoregulation within 48 hours of their injury using continuous transcranial Doppler velocity recordings and blood pressure recordings. Twenty-nine age-matched normal volunteers underwent autoregulation testing in the same manner to establish comparison values. The function of the autoregulatory response was assessed by the cerebral blood flow velocity response to induced rapid brief changes in arterial blood pressure and measured as the autoregulation index (ARI). Eight (28%) of the 29 patients with minor head injury demonstrated poorly functioning or absent cerebral autoregulation versus none of the controls, and this difference was highly significant (p = 0.008). A significant correlation between lower blood pressure and worse autoregulation was found by regression analysis in head-injured patients (r = 0.6, p < 0.001); however, lower blood pressure did not account for the autoregulatory impairment in all patients. Within this group of head-injured patients there was no correlation between ARI and initial GCS or 1-month Glasgow Outcome Scale scores. This study indicates that a significant number of patients with minor head injury may have impaired cerebral autoregulation and may be at increased risk for secondary ischemic neuronal damage.

A recombinant adenovirus that directs secretion of biologically active kappa-bungarotoxin from mammalian cells

A novel Cre-lox system was used to construct an adenovirus encoding kappa-bungarotoxin (kappa-Bgt), modified to be secreted by attachment of a bovine prolactin signal sequence at the N-terminus of the toxin. Western blot of medium from HEK-293 cells infected with the virus demonstrated that recombinant kappa-Bgt (R-kappa-Bgt) was secreted. The biological activity of the secreted R-kappa-Bgt was investigated in Xenopus oocytes that expressed neuronal nicotinic acetylcholine receptor (nAChR) subtypes alpha3beta2 and alpha2beta2. The recombinant toxin inhibited the response of alpha3beta2 type AChRs to ACh, but did not inhibit the response of alpha2beta2 type AChRs. These data demonstrated that the recombinant adenovirus directs the secretion of biologically active kappa-Bgt from a mammalian cell line. Because adenovirus can be used to infect post-mitotic cells, recombinant adenoviruses encoding biologically active peptides may be of use as delivery vehicles for in vivo experiments where repeated application of the purified peptide is unfeasible.

Binding of native kappa-neurotoxins and site-directed mutants to nicotinic acetylcholine receptors

The kappa-neurotoxins are useful ligands for the pharmacological characterization of nicotinic acetylcholine receptors because they are potent antagonists at only a subgroup of these receptors containing either alpha 3- or alpha 4-subunits (IC50 < or = 100 nM). Four of these highly homologous, 66 amino acid peptides have been purified from the venom of Bungarus multicinctus (kappa-bungarotoxin (kappa-Bgt), kappa 2-Bgt, kappa 3-Bgt] and Bungarus flaviceps [kappa-Fvt)]. Two approaches were taken to examine the binding of these toxins to nicotinic receptors. First, venom-derived kappa-Fvt and kappa-Bgt were radioiodinated and the specific binding was measured of these toxins to overlapping synthetic peptides (16-20 amino acids in length) prepared based on the known sequence of the nicotinic receptor alpha 3-subunit. At least two main regions of interaction between the toxins and the receptor subunit were identified, both lying in the N-terminal region of the subunit that is exposed to the extracellular space. The second approach examined the importance of several sequence position in kappa-Bgt for binding to alpha 3-containing receptors in autonomic ganglia and alpha 1-containing muscle receptors. This was done using site-directed mutants of kappa-Bgt produced by an Escherichia coli expression system. Arg-34 and position 36 were important for binding to both receptor subtypes, while replacing Gln-26 with Trp-26 (an invariant in alpha-neurotoxins) increased affinity for the muscle receptor by 8-fold. The results confirm that kappa-neurotoxins bind potently to the alpha 3-subunit and bind with considerably reduced affinity (Kd approximately 10 microM) to muscle receptors. Site-directed mutagenesis of recombinant kappa-Bgt is thus an important approach for the study of structure-function relationships between kappa-Bgt and nicotinic receptors.

The mechanism of velocity modulated allosteric regulation in D-3-phosphoglycerate dehydrogenase. Site-directed mutagenesis of effector binding site residues

D-3-Phosphoglycerate dehydrogenase (EC 1.1.1.95) from Escherichia coli catalyzes the first committed step in serine biosynthesis and is allosterically regulated by L-serine, the end product of the pathway. Each subunit of the homotetramer is made up of three distinct domains with one of the intersubunit contacts being between adjacent regulatory domains. Each regulatory domain interface contains two symmetrical serine binding sites such that serine forms hydrogen bonds to both domains across the interface. Previous work (Al-Rabiee, R., Lee, E. J., and Grant, G. A. (1996) J. Biol. Chem. 271, 13013-13017) demonstrated that when adjacent regulatory domains are covalently linked to one another by engineered disulfide bonds, the enzyme was inactivated. Breaking the disulfide bonds by reduction restored enzymatic activity. This study demonstrates that the complementary situation is also true. Site-directed mutagenesis of three residues at the effector binding site, His344, Asn346, and Asn364', render the enzyme increasingly less susceptible to inhibition by the effector. When mutations result in a situation where it is no longer possible to establish a stable hydrogen bonding network across the regulatory domain interface, the inhibitory capacity of the effector is lost. Furthermore, mutations that produce as much as 5 orders of magnitude decrease in the ability of L-serine to inhibit the enzyme have no appreciable effect on the Km or kcat of the enzyme. These observations support the model that predicts that catalytic activity in D-3-phosphoglycerate dehydrogenase is regulated by the movement of adjacent regulatory domains about a flexible hinge and that effector binding tethers the regulatory domains together producing a state that results in a stable, open active site cleft that is no longer able to promote catalysis.

The mechanism of velocity modulated allosteric regulation in D-3-phosphoglycerate dehydrogenase. Cross-linking adjacent regulatory domains with engineered disulfides mimics effector binding

D-3-Phosphoglycerate dehydrogenase (PGDH) (EC 1.1.1.95) from Escherichia coli is an allosterically regulated enzyme of the Vmax type. It is a tetramer of identical subunits and each subunit is made up of three identifiable domains, the cofactor binding domain, the substrate binding domain, and the regulatory domain. Each subunit contacts two other subunits through adjacent cofactor binding domains and through adjacent regulatory domains. L-Serine, the physiological effector, inhibits catalytic activity by apparently tethering regulatory domains from adjacent subunits together through the formation of hydrogen bonds to each subunit. This investigation demonstrates that cross-linking adjacent regulatory domains with engineered disulfides produces catalytic inhibition in the absence of inhibitor in a manner similar to that produced by the inhibitor. The inhibition due to cross-linking can be completely reversed in a concentration dependent manner by dithiothreitol. The active mutant enzyme, containing the engineered cysteines in the reduced state, retains its ability to be inhibited by L-serine, although at a 100-fold higher concentration. Hill plots of the serine inhibition of mutant and native enzyme indicate that the number of interacting sites remains at 2 in the mutant enzyme. The reversible inhibition of enzyme activity that results from tethering adjacent regulatory domains with engineered disulfides suggests that these domains move in some manner relative to one another during the active to inhibited state transition. These observations support the model which predicts that catalytic activity is regulated by the movement of rigid domains about flexible hinges and that effector binding prevents this by locking the regulatory domains in a state that produces an open active site cleft.

Facile production of native-like kappa-bungarotoxin in yeast: an enhanced system for the production of a neuronal nicotinic acetylcholine receptor probe

Research on the mammalian central nervous system had been hindered by the limited number and meager supply of naturally occurring toxins that can be used as pharmacological reagents. The kappa-neurotoxins in particular are not found abundantly in nature and are difficult to obtain and isolate in quantities sufficient for research purposes. Here we report the expression and isolation of relatively large quantities of the kappa-neurotoxin, kappa-bungarotoxin, in an active form using a yeast, Pichia pastoris, expression system. The resultant product of the expression system has a short amino-terminal amino acid extension relative to venom-derived kappa-bungarotoxin, but is equivalent to the native toxin in physical and biological properties, as judged by the CD spectra, the ability to form dimers in solution, and the activity on chick ciliary ganglia. The yeast system produces approximately 0.2 mg from a 2 liter culture and the purification takes approximately 2 days. In contrast, E. coli, the only other available expression system for this toxin, produces one-fifth to one-half as much active material from a 5 liter high-density fermentation and the resulting protein takes over a week to purify. No high mol. wt disulfide-bonded aggregates were found in the yeast expression system product, indicating that the product is that of a biologically assisted folding process. This has significant implications not only for the efficient production of native toxin but also for the production of mutant proteins to study the structure-function relationship in these proteins.