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Biomedical subjects

B M Brown

Publications and source records attributed to B M Brown.

At least 19 recordsLinked to original sources
PubMed2007

Induced smoothing for rank regression with censored survival times.

Adaptions of weighted rank regression to the accelerated failure time model for censored survival data have been successful in yielding asymptotically normal estimates and flexible weighting schemes to increase statistical efficiencies. However, for only one simple weighting scheme, Gehan or Wilcoxon weights, are estimating equations guaranteed to be monotone in parameter components, and even in this case are step functions, requiring the equivalent of linear programming for computation. The lack of smoothness makes standard error or covariance matrix estimation even more difficult. An induced smoothing technique overcame these difficulties in various problems involving monotone but pure jump estimating equations, including conventional rank regression. The present paper applies induced smoothing to the Gehan-Wilcoxon weighted rank regression for the accelerated failure time model, for the more difficult case of survival time data subject to censoring, where the inapplicability of permutation arguments necessitates a new method of estimating null variance of estimating functions. Smooth monotone parameter estimation and rapid, reliable standard error or covariance matrix estimation is obtained.

Data Interpretation, Statistical↗
PubMed1999

Tolerance of Arc repressor to multiple-alanine substitutions.

Arc repressor mutants containing from three to 15 multiple-alanine substitutions have spectral properties expected for native Arc proteins, form heterodimers with wild-type Arc, denature cooperatively with Tms equal to or greater than wild type, and, in some cases, fold as much as 30-fold faster and unfold as much as 50-fold slower than wild type. Two of the mutants, containing a total of 14 different substitutions, also footprint operator DNA in vitro. The stability of some of the proteins with multiple-alanine mutations is significantly greater than that predicted from the sum of the single substitutions, suggesting that a subset of the wild-type residues in Arc may interact in an unfavorable fashion. Overall, these results show that almost half of the residues in Arc can be replaced by alanine en masse without compromising the ability of this small, homodimeric protein to fold into a stable, native-like structure.

Alanine↗
PubMed1999

MR angiography and surgery for unruptured familial intracranial aneurysms in persons with a family history of cerebral aneurysms.

OBJECTIVE: We used MR angiography to determine prevalence of unruptured familial intracranial aneurysms in a prepaid medical care program. We compared surgical outcomes and the cost of treating unruptured versus ruptured aneurysms. We compared the cost of MR angiography with the cost of screening mammography and with the cost of surgically treating a ruptured aneurysm. SUBJECTS AND METHODS: During a 30-month period, we performed MR angiography to show cerebral aneurysms in 63 surgical candidates who had one or more first-degree relatives with an aneurysm. Unruptured aneurysms seen on MR angiography were evaluated by digital subtraction angiography (DSA) and treated surgically. RESULTS: MR angiography showed nine unruptured aneurysms in six patients. Eight aneurysms were seen on MR angiography and nine were seen on DSA. Seven unruptured aneurysms were treated surgically. The mean treatment cost was 50% lower for an unruptured aneurysm than that for a ruptured aneurysm. No patient surgically treated for an unruptured aneurysm required rehabilitation, unlike 25% of patients with ruptured aneurysms. The annual total cost of MR angiography was equivalent to 2.9% of the annual cost of screening mammography. The annual cost of MR angiography equaled half the cost of treating one patient after aneurysm rupture. CONCLUSION: MR angiography showed a 9.5% prevalence of unruptured aneurysms among persons who had one or more first-degree relatives with a cerebral aneurysm. DSA confirmed 88% of aneurysms found on MR angiography. Persons with unruptured aneurysms had better treatment outcomes at lower cost than did patients treated for aneurysm rupture. The annual MR angiography cost was low compared with the cost of screening mammography and with the cost of treating one patient with aneurysm rupture.

Adolescent↗
PubMed1997

The workup for bariatric surgery does not require a routine upper gastrointestinal series.

BACKGROUND: Morbid obesity is a serious disease that afflicts over five million Americans, threatening their health with such co-morbidities as diabetes, arthritis, pulmonary failure and stroke. Surgery is the only effective therapy, providing long-term control of weight, diabetes, pulmonary failure, and hypertension for as long as 14 years. Because the operation presents a major expense, this study examined whether X-ray examination of the gut could be omitted safely as a cost-saving measure. METHODS: The records of 814 consecutive morbidly obese patients who underwent gastric bypass were reviewed to determine: (1) whether these individuals had undergone an upper gastro-intestinal (GI) series, and (2) if these studies influenced therapy or caused cancellation or postponement of surgery. RESULTS: Of the 814 patients, 657 (80.7%) underwent a preoperative GI radiography. Of these examinations, 393 (59.8%) were normal, with the following abnormalities in the remaining 264: hiatal hernia, 164; esophageal reflux, 39; Schatzki's ring, 18; small bowel diverticula, four; renal stones, four; malrotation, three; gall stones, two; pyloric ulcer, one; possible pelvic mass, one; calcified leiomyoma, one; and dysphagial lusoria, one. None of these findings resulted in cancellation or a delay in surgery. CONCLUSIONS: The upper GI series can be safely omitted from the routine preoperative evaluation of patients undergoing gastric bypass. At a cost of $741.00 per examination, this change represents significant potential savings. Similar evaluations of other routine preoperative tests may well provide a better basis for the evaluation of these complex patients.

Adult↗
PubMed1996

Sequence determinants of folding and stability for the P22 Arc repressor dimer.

The Arc repressor is a small, homodimeric protein. Studies of mutant proteins show that the side chains that form the hydrophobic core are the most important determinants of structure. A variety of hydrogen bonds and salt bridges also contribute to stabilization of the native structure, but these can often be replaced by hydrophobic interactions. The transition state for folding/unfolding is dimeric and contains a large amount of buried hydrophobic surface, but the beta-sheet of native Arc is not formed. Moreover, relatively little side chain information appears to be used in the transition state, suggesting that tight packing of the hydrophobic core and optimization of hydrogen-bond geometry are events that occur later in folding.

Amino Acid Sequence↗
PubMed1995

P22 Arc repressor: transition state properties inferred from mutational effects on the rates of protein unfolding and refolding.

The kinetics of unfolding and refolding have been measured for a set of Arc repressor mutants bearing single amino acid substitutions at 44 of the 53 residue positions. Roughly half of the mutations cause significant changes in the unfolding and/or refolding rate constants. These substitutions alter the hydrophobic core, tertiary hydrogen bonds and salt bridges, and glycines with restricted backbone conformations. Overall, the mutations cause larger changes in the unfolding rates than the refolding rates, indicating that significantly less side-chain information is used between the denatured state and transition state than between the transition state and native state. The set of mutants displays reasonable Brønsted behavior, suggesting that many native interactions are partially formed in the transition state. Taken together, these observations suggest that the overall structure of most of the protein must be somewhat native-like in the transition state but without close, complementary packing of the hydrophobic core or good hydrogen bond geometry. Such a transition state is inconsistent with a model in which monomers fold to their correct conformations and then dock to form the dimer but supports a model in which folding and dimerization are concurrent processes.

Bacteriophage P22↗
PubMed1995

Who would have thought it? An operation proves to be the most effective therapy for adult-onset diabetes mellitus.

OBJECTIVE: This report documents that the gastric bypass operation provides long-term control for obesity and diabetes. SUMMARY BACKGROUND DATA: Obesity and diabetes, both notoriously resistant to medical therapy, continue to be two of our most common and serious diseases. METHODS: Over the last 14 years, 608 morbidly obese patients underwent gastric bypass, an operation that restricts caloric intake by (1) reducing the functional stomach to approximately 30 mL, (2) delaying gastric emptying with a c. 0.8 to 1.0 cm gastric outlet, and (3) excluding foregut with a 40 to 60 cm Roux-en-Y gastrojejunostomy. Even though many of the patients were seriously ill, the operation was performed with a perioperative mortality and complication rate of 1.5% and 8.5%, respectively. Seventeen of the 608 patients (< 3%) were lost to follow-up. RESULTS: Gastric bypass provides durable weight control. Weights fell from a preoperative mean of 304.4 lb (range, 198 to 615 lb) to 192.2 lb (range, 104 to 466) by 1 year and were maintained at 205.4 lb (range, 107 to 512 lb) at 5 years, 206.5 lb (130 to 388 lb) at 10 years, and 204.7 lb (158 to 270 lb) at 14 years. The operation provides long-term control of non-insulin-dependent diabetes mellitus (NIDDM). In those patients with adequate follow-up, 121 of 146 patients (82.9%) with NIDDM and 150 of 152 patients (98.7%) with glucose impairment maintained normal levels of plasma glucose, glycosylated hemoglobin, and insulin. These antidiabetic effects appear to be due primarily to a reduction in caloric intake, suggesting that insulin resistance is a secondary protective effect rather than the initial lesion. In addition to the control of weight and NIDDM, gastric bypass also corrected or alleviated a number of other comorbidities of obesity, including hypertension, sleep apnea, cardiopulmonary failure, arthritis, and infertility. Gastric bypass is now established as an effective and safe therapy for morbid obesity and its associated morbidities. No other therapy has produced such durable and complete control of diabetes mellitus.

Adolescent↗
PubMed1994

Scanning mutagenesis of the Arc repressor as a functional probe of operator recognition.

Protein-DNA and protein-protein interactions are central to most biological regulation, and yet our understanding of these macromolecular recognition events is still incomplete. Both types of interactions are critical for the function of the Arc repressor. The functional importance of residues in or near its operator DNA-binding surface and dimer-dimer interaction surface has been probed by alanine-scanning mutagenesis. Mutations in three categories cause large binding defects: beta-sheet side chains that directly interact with DNA bases; side chains that link different DNA-binding regions of Arc, and side chains required to maintain the active DNA-binding conformation.

Bacteriophage P22↗
PubMed1994

Protein stability effects of a complete set of alanine substitutions in Arc repressor.

The equilibrium stabilities of a complete set of single alanine-substitution mutants of the Arc repressor of bacteriophage P22 have been determined by thermal and urea denaturation experiments. Only half the alanine substitutions cause significant changes in stability, with the most deleterious mutations affecting side chains in the hydrophobic core or in salt bridges and hydrogen bonds which are protected from solvent. The five mutations that are most destabilizing affect a cluster of core residues that seem to form a structural foundation for Arc.

Alanine↗
PubMed1993

Assembly of the Arc repressor-operator complex: cooperative interactions between DNA-bound dimers.

Arc repressor, a member of the beta-ribbon family of DNA binding proteins, binds to its 21-base-pair operator as a tetramer. Here, the Arc dimer is shown to bind specifically to DNA fragments containing operator half-sites, and the equilibrium and kinetic constants for these reactions are determined. DNA-bound dimers are also shown to be transient intermediates in association experiments, indicating that assembly of the Arc tetramer-operator complex occurs by sequential addition of dimers to operator half-sites. When the left or right operator half-site is occupied by an Arc dimer, cooperative interactions increase the affinity of the second dimer by approximately 5900-fold [delta delta G = -5.1 (+/- 0.5) kcal/mol]. This increase in affinity is largely caused by an increase in the half-life of the complex; "non-cooperatively" bound dimers dissociate with a half-life of a few seconds while "cooperatively" bound dimers have half-lives of more than 1 h.

Base Sequence↗
PubMed1993

P22 Arc repressor: enhanced expression of unstable mutants by addition of polar C-terminal sequences.

Many mutant variants of the P22 Arc repressor are subject to intracellular proteolysis in Escherichia coli, which precludes their expression at levels sufficient for purification and subsequent biochemical characterization. Here we examine the effects of several different C-terminal extension sequences on the expression and activity of a set of Arc mutants. We show that two tail sequences, KNQHE (st5) and H6KNQHE (st11), increase the expression levels of most mutants from 10- to 20-fold and, in some cases, result in restoration of biological activity in the cell. A third tail sequence, HHHHHH (st6), was not as effective in increasing mutant expression levels. All three tail sequences are functionally and structurally silent, as judged by their lack of effects on the DNA binding activity and stability of otherwise wild-type Arc. The properties of the st11 tail sequence make it an efficient system for the expression and purification of mutant Arc proteins, both because mutant expression levels are increased and because the proteins can be rapidly purified using nickel-chelate affinity chromatography. Arc mutants containing the EA28, RL31, and SA32 mutations were purified in the st11 background. The thermodynamic stability of the EA28 mutant (delta delta Gu approximately -0.4 kcal/mol) is reduced modestly compared to the st11 parent, whereas the RL31 mutant (delta delta Gu approximately -3.0 kcal/mol) and SA32 mutant (delta delta Gu approximately -3.3 kcal/mol) are substantially less stable.

Amino Acid Sequence↗
PubMed1993

Arterial thrombosis: putative role of lipoprotein(a).

Lipoprotein(a) has been associated with several arterial thrombotic disorders. This article presents evolving concepts of Lp(a) and its potential role in coagulation. An overview of lipoprotein structure and function provides background information for further understanding of the pathophysiologic role of Lp(a) in atherosclerosis and thrombosis. Further research is needed to clarify the function of Lp(a) in humans and its role in arterial thrombosis.

Arterial Occlusive Diseases↗
PubMed1992

Single-plane transesophageal echocardiography for assessing function of mechanical or bioprosthetic valves in the aortic valve position.

To assess the value and limitations of single-plane transesophageal echocardiography in the evaluation of prosthetic aortic valve function, 89 patients (69 mechanical and 20 bioprosthetic aortic valves) were studied by combined transthoracic and transesophageal 2-dimensional and color flow Doppler echocardiography. In the assessment of aortic regurgitation, the transthoracic and transesophageal echocardiographic findings were concordant in 71 of 89 patients (80%). In 8 patients, the degree of aortic regurgitation was underestimated by the transthoracic approach; in each case the quality of the transthoracic echocardiogram was poor. In 10 patients, transesophageal echocardiography failed to detect trivial aortic regurgitation due to acoustic shadowing of the left ventricular outflow tract from a mechanical valve in the mitral valve position. Transesophageal echocardiography was superior to transthoracic echocardiography in diagnosing perivalvular abscess, subaortic perforation, valvular dehiscence, torn or thickened bioprosthetic aortic valve cusps, and in clearly distinguishing perivalvular from valvular aortic regurgitation. Transesophageal echocardiography correctly diagnosed bioprosthetic valve obstruction in 1 patient, but failed to diagnose mechanical valve obstruction in another. In conclusion, transesophageal echocardiography offers no advantage over the transthoracic approach in the detection and quantification of prosthetic aortic regurgitation unless the transthoracic image quality is poor. Transesophageal echocardiography is limited in detecting mechanical valve obstruction and in detecting aortic regurgitation in the presence of a mechanical prosthesis in the mitral valve position. However, it is superior to transthoracic echocardiography in identifying perivalvular pathology, differentiating perivalvular from valvular regurgitation and in defining the anatomic abnormality responsible for the prosthetic valve dysfunction. Combined transthoracic and transesophageal examination provides complete anatomic and hemodynamic assessment of prosthetic aortic valve function.

Adolescent↗
PubMed1990

Arc repressor is tetrameric when bound to operator DNA.

The Arc repressor of bacteriophage P22 is a member of a family of DNA-binding proteins that use N-terminal residues in a beta-sheet conformation for operator recognition. Here, Arc is shown to bind to its operator site as a tetramer. When mixtures of Arc (53 residues) and an active variant of Arc (78 residues) are used in gel retardation experiments, five discrete protein-DNA complexes are observed. This result is as expected for operators bearing heterotetramers containing 4:0, 3:1, 2:2, 1:3, and 0:4 ratios of the two proteins. Direct measurements of binding stoichiometry support the conclusion that Arc binds to a single 21-base-pair operator site as a tetramer. The Arc-operator binding reaction is highly cooperative (Hill constant = 3.5) and involves at least two coupled equilibria. In the first reaction, two unfolded monomers interact to form a folded dimer (Bowie & Sauer, 1989a). Rapid dilution experiments indicate that the Arc dimer is the kinetically significant DNA-binding species and allow an estimate of the equilibrium dissociation constant for dimerization [K1 = 5 (+/- 3) x 10(-9) M]. The rate of association of Arc-operator complexes shows the expected second-order dependence on the concentration of free Arc dimers, with k2 = 2.8 (+/- 0.7) x 10(18) M-2 s-1. The dissociation of Arc-operator complexes is a first-order process with k-2 = 1.6 (+/- 0.6) x 10(-4) s-1. The ratio of these kinetic constants [K2 = 5.7 (+/- 2.3) x 10(-23) M2] provides an estimate for the equilibrium constant for dissociation of the DNA-bound tetramer to two free Arc dimers and the operator. An independent determination of this complex equilibrium constant [K2 = 7.8 (+/- 4.8) x 10(-23) M2] was obtained from equilibrium binding experiments.

Amino Acid Sequence↗
PubMed1990

Protein kinase A phosphorylates retinal phosducin on serine 73 in situ.

Photoreceptors of vertebrate retinas contain a 33,000-dalton phosphoprotein, phosducin, which complexes with the beta, gamma subunits of the photoreceptor G-protein (guanine nucleotide-binding protein), transducin. In situ, the retinal content of phosphorylated phosducin is modulated by light in conjunction with light-triggered changes in intracellular cyclic nucleotide concentration. In vitro, phosducin is phosphorylated by either exogenous or endogenous protein kinase A. 32P-Labeled rat retina phosducin was isolated by immunoprecipitation either after phosphorylation by protein kinase A in the presence of [gamma-32P]ATP or after incubation of retinas in darkness with 32Pi. In either case, phosphoamino acid analysis showed that greater than 98% of 32P was linked to serine, with less than 2% to threonine. Two-dimensional peptide mapping showed that [32P]phosphoserine was associated with the same characteristic set of tryptic peptides. Furthermore, Cleveland peptide analysis using four different proteases showed that either sample exhibited identical patterns of phosphopeptides which were characteristic of the protease used. Identical phosphopeptide maps were also obtained from 32P-labeled bovine retina phosducin, indicating that the serine phosphorylation site for protein kinase A is conserved between rat and bovine. Edman degradation of phosphopeptides derived from 32P-labeled bovine phosducin showed that radioactive phosphate was incorporated into serine residue 73 which is located within a consensus phosphorylation sequence for protein kinase A (-R-K-M-S73(P)-). These observations are uniformly in agreement with protein kinase A being the endogenous kinase that phosphorylates phosducin in vivo.

Adenosine Triphosphate↗
PubMed1989

Chromosomal aberrations and sister chromatid exchange tests in Chinese hamster ovary cells in vitro. IV. Results with 15 chemicals.

The National Toxicology Program has undertaken a study to assess the ability of four genetic toxicology assays to predict the carcinogenicity of chemicals in 2-year rodent studies [Tennant et al.: Science 236:933-941, 1987]. Two of the assays, used for evaluating in vitro cytogenetic damage, were the SCE and chromosome aberration assays in Chinese hamster ovary cells. The results and data for 15 of the chemicals tested in these two assays are presented here. Each chemical was tested with and without exogenous metabolic activation. The chemicals tested were bisphenol A, 2-chloroethanol, C.I. acid orange 10, C.I. disperse yellow 3, C.I. solvent yellow 14, cytembena, D&C red 9, 1,2-dibromoethane, FD&C yellow 6, malaoxon, D,L-menthol, phenol, sulfisoxazole, titanium dioxide, and tris(2-ethylhexyl)phosphate. In vitro cytogenetic results from the other chemicals presented by Tennant et al. (Science 236:933-941, 1987) have been published by Galloway et al. (Environmental and Molecular Mutagenesis 10(Suppl 10): 1-175, 1987), Gulati et al. (Environmental and Molecular Mutagenesis 13:133-193, 1989), and Love-day et al. (Environmental Mutagenesis 13:60-94).

Animals↗
PubMed1989

Asymptomatic fusiform aneurysm of the petrous carotid artery in a patient with von Recklinghausen's neurofibromatosis.

Fusiform aneurysmal dilatation of the petrous portion of the right carotid artery was found in a young patient with von Recklinghausen's neurofibromatosis. The patient presented with sudden left-sided hearing loss, and, during the evaluation of the hearing deficit, an asymptomatic right-sided vascular abnormality was identified. This vascular abnormality is unusual because those abnormalities commonly associated with von Recklinghausen's neurofibromatosis include occlusion of the cerebral arteries, arteriovenous malformations, and intracranial aneurysms. The pathophysiology and natural history of vascular abnormalities accompanying von Recklinghausen's neurofibromatosis are discussed.

Adolescent↗