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Biomedical subjects

A L Davidson

Publications and source records attributed to A L Davidson.

At least 19 recordsLinked to original sources
PubMed2001

Role of breast magnetic resonance imaging in difficult diagnostic situations.

In some patients with breast disease, mammography and ultrasonography can provide only limited diagnostic information. Magnetic resonance imaging of the breast has high sensitivity and specificity and can play a significant diagnostic role in problem situations. Patients who are most likely to benefit are those with (i) axillary adenopathy of unknown primary origin, (ii) possible tumour recurrence after surgery or radiotherapy, (iii) lesions overlying implants, or (iv) those requiring staging of lobular or multifocal carcinoma.

Adult↗
PubMed2001

Trapping the transition state of an ATP-binding cassette transporter: evidence for a concerted mechanism of maltose transport.

High-affinity uptake into bacterial cells is mediated by a large class of periplasmic binding protein-dependent transport systems, members of the ATP-binding cassette superfamily. In the maltose transport system of Escherichia coli, the periplasmic maltose-binding protein binds its substrate maltose with high affinity and, in addition, stimulates the ATPase activity of the membrane-associated transporter when maltose is present. Vanadate inhibits maltose transport by trapping ADP in one of the two nucleotide-binding sites of the membrane transporter immediately after ATP hydrolysis, consistent with its ability to mimic the transition state of the gamma-phosphate of ATP during hydrolysis. Here we report that the maltose-binding protein becomes tightly associated with the membrane transporter in the presence of vanadate and simultaneously loses its high affinity for maltose. These results suggest a general model explaining how ATP hydrolysis is coupled to substrate transport in which a binding protein stimulates the ATPase activity of its cognate transporter by stabilizing the transition state.

ATP-Binding Cassette Transporters↗
PubMed2001

Demonstration of conformational changes associated with activation of the maltose transport complex.

In Escherichia coli, interaction of a periplasmic maltose-binding protein with a membrane-associated ATP-binding cassette transporter stimulates ATP hydrolysis, resulting in translocation of maltose into the cell. The maltose transporter contains two transmembrane subunits, MalF and MalG, and two copies of a nucleotide-hydrolyzing subunit, MalK. Mutant transport complexes that function in the absence of binding protein are thought to be stabilized in an ATPase-active conformation. To probe the conformation of the nucleotide-binding site and to gain an understanding of the nature of the conformational changes that lead to activation, cysteine 40 within the Walker A motif of the MalK subunit was modified by the fluorophore 2-(4'-maleimidoanilino)naphthalene-6-sulfonic acid. Fluorescence differences indicated that residues involved in nucleotide binding were less accessible to aqueous solvent in the binding protein independent transporter than in the wild-type transporter. Similar differences in fluorescence were seen when a vanadate-trapped transition state conformation was compared with the ground state in the wild-type transporter. Our results and recent crystal structures are consistent with a model in which activation of ATPase activity is associated with conformational changes that bring the two MalK subunits closer together, completing the nucleotide-binding sites and burying ATP in the interface.

ATP-Binding Cassette Transporters↗
PubMed2001

cpDNA-RFLP in Ceramium (Rhodophyta): intraspecific polymorphism and species-level phylogeny.

Restriction fragment length polymorphism (RFLP) analysis of chloroplast (cp) DNA is a powerful tool for the study of microevolutionary processes in land plants, yet has not previously been applied to seaweed populations. We used cpDNA-RFLP, detected on Southern blots using labeled total plastid DNA, to search for intraspecific and intrapopulational cpDNA RFLP polymorphism in two species of the common red algal genus Ceramium in Ireland and Britain. In C. botryocarpum one polymorphism was detected in one individual among 18 from two populations. Twenty-six individuals of C. virgatum from five populations at three locations exhibited a total of four haplotypes. One was frequent (80.8% of individuals); the others were rare (7.7, 7.7, and 4.2%) and were private to particular populations. Polymorphism was observed in two populations. The corrected mean was 2.26 ± 0.36 haplotypes per population, which was within the typical range determined for higher plants using similar techniques. The spatial distribution of haplotypes was heterogeneous, with highly significant population differentiation (P = 0.00018; Fisher's exact test). Intraspecific polymorphism in C. virgatum had no impact on species-level phylogenetic reconstruction. This is the first unequivocal report of both intraspecific and intrapopulational cpDNA-RFLP polymorphism in algae.

Journal Article↗
PubMed2000

Vanadate-induced trapping of nucleotides by purified maltose transport complex requires ATP hydrolysis.

The maltose transport system in Escherichia coli is a member of the ATP-binding cassette superfamily of transporters that is defined by the presence of two nucleotide-binding domains or subunits and two transmembrane regions. The bacterial import systems are unique in that they require a periplasmic substrate-binding protein to stimulate the ATPase activity of the transport complex and initiate the transport process. Upon stimulation by maltose-binding protein, the intact MalFGK(2) transport complex hydrolyzes ATP with positive cooperativity, suggesting that the two nucleotide-binding MalK subunits interact to couple ATP hydrolysis to transport. The ATPase activity of the intact transport complex is inhibited by vanadate. In this study, we investigated the mechanism of inhibition by vanadate and found that incubation of the transport complex with MgATP and vanadate results in the formation of a stably inhibited species containing tightly bound ADP that persists after free vanadate and nucleotide are removed from the solution. The inhibited species does not form in the absence of MgCl(2) or of maltose-binding protein, and ADP or another nonhydrolyzable analogue does not substitute for ATP. Taken together, these data conclusively show that ATP hydrolysis must precede the formation of the vanadate-inhibited species in this system and implicate a role for a high-energy, ADP-bound intermediate in the transport cycle. Transport complexes containing a mutation in a single MalK subunit are still inhibited by vanadate during steady-state hydrolysis; however, a stably inhibited species does not form. ATP hydrolysis is therefore necessary, but not sufficient, for vanadate-induced nucleotide trapping.

ATP-Binding Cassette Transporters↗
PubMed2000

Neuropsychological effects of interferon beta-1a in relapsing multiple sclerosis. Multiple Sclerosis Collaborative Research Group.

Cognitive dysfunction is common in multiple sclerosis (MS), yet few studies have examined effects of treatment on neuropsychological (NP) performance. To evaluate the effects of interferon beta-1a (IFNbeta-1a, 30 microg administered intramuscularly once weekly [Avonex]) on cognitive function, a Comprehensive NP Battery was administered at baseline and week 104 to relapsing MS patients in the phase III study, 166 of whom completed both assessments. A Brief NP Battery was also administered at 6-month intervals. The primary NP outcome measure was 2-year change on the Comprehensive NP Battery, grouped into domains of information processing and learning/memory (set A), visuospatial abilities and problem solving (set B), and verbal abilities and attention span (set C). NP effects were most pronounced in cognitive domains vulnerable to MS: IFNbeta-1a had a significant beneficial effect on the set A composite, with a favorable trend evident on set B. Secondary outcome analyses revealed significant between-group differences in slopes for Brief NP Battery performance and time to sustained deterioration in a Paced Auditory Serial Addition Test processing rate, favoring the IFNbeta-1a group. These results support and extend previous observations of significant beneficial effects of IFNbeta-1a for relapsing MS.

Adolescent↗
PubMed1997

Compliance with universal precautions among physicians.

This study characterized and assessed self-reported levels of compliance with universal precautions (UP) among hospital-based physicians, and determined significant factors associated with both compliance and noncompliance. The physicians (n = 322) were a subgroup of a larger study population of hospital-based health care workers recruited from three geographically distinct locations (n = 1746), and were surveyed using a detailed confidential questionnaire that assessed personal, work-related, and organizational factors. Compliance with UP was measured through 11 items that examined how often physicians followed specific recommended work practices. Compliance was found to vary among the 11 items: they were high for certain activities (eg, glove use, 94%; disposal of sharps, 92%) and low for others (eg, wearing protective clothing, 55%; not recapping needles, 56%). Compliance with all items was low (31% to 38%). Stepwise logistic regression revealed that noncompliant physicians were likely to be age 37 or older, to report high work stress, and to perceive a conflict of interest between providing patient care and protecting themselves. Compliant physicians were more likely to be knowledgeable and to have been trained in universal precautions, to perceive protective measures as being effective, and to perceive an organizational commitment to safety.

Adult↗
PubMed1997

Mutation of a single MalK subunit severely impairs maltose transport activity in Escherichia coli.

The maltose transport system of Escherichia coli, a member of the ABC transport superfamily of proteins, consists of a periplasmic maltose binding protein and a membrane-associated translocation complex that contains two copies of the ATP-binding protein MalK. To examine the need for two nucleotide-binding domains in this transport complex, one of the two MalK subunits was inactivated by site-directed mutagenesis. Complexes with mutations in a single subunit were obtained by attaching a polyhistidine tag to the mutagenized version of MalK and by coexpressing both wild-type MalK and mutant (His)6MalK in the same cell. Hybrid complexes containing one mutant (His)6MalK subunit and one wild-type MalK subunit were separated from those containing two mutant (His)6MalK proteins based on differential affinities for a metal chelate column. Purified transport complexes were reconstituted into proteoliposome vesicles and assayed for maltose transport and ATPase activities. When a conserved lysine residue at position 42 that is involved in ATP binding was replaced with asparagine in both MalK subunits, maltose transport and ATPase activities were reduced to 1% of those of the wild type. When the mutation was present in only one of the two subunits, the complex had 6% of the wild-type activities. Replacement of a conserved histidine residue at position 192 in MalK with arginine generated similar results. It is clear from these results that two functional MalK proteins are required for transport activity and that the two nucleotide-binding domains do not function independently to catalyze transport.

ATP-Binding Cassette Transporters↗
PubMed1996

Maltose-binding protein containing an interdomain disulfide bridge confers a dominant-negative phenotype for transport and chemotaxis.

Bacterial substrate-binding proteins exist in an equilibrium among four forms: open/substrate-free, open/substrate-bound, closed/substrate-free, and closed/substrate-bound. Ligands stabilize the closed conformation, whereas the open conformation predominates in the substrate-free species. In its closed form, the NH2-terminal and COOH-terminal domains of maltose-binding protein (MBP) are proposed to be aligned to allow residues in both domains to interact simultaneously with complementary sites on the MalF and MalG proteins of the maltodextrin uptake system or with the Tar chemotactic signal transducer. However, the initial interaction might occur with an open/substrate-bound form of the binding protein, which would then close in contact with MalFG or Tar. Ligand would help stabilize this complex. We introduced cysteines (G69C and S337C) by site-directed mutagenesis into each domain of MBP and found that they formed an interdomain disulfide cross-link that should hold the protein in a closed conformation. This mutant MBP confers a dominant-negative phenotype for growth on maltose, for maltose transport, and for maltose chemotaxis. The growth and transport defects are partially reversed when the cells are exposed to the reducing agent dithiothreitol. We conclude that the cross-linked form of MBP competes with wild-type MBP in vivo for interaction with MalFG and Tar.

ATP-Binding Cassette Transporters↗
PubMed1996

The maltose transport system of Escherichia coli displays positive cooperativity in ATP hydrolysis.

Maltose transport across the cytoplasmic membrane of Escherichia coli is catalyzed by a periplasmic binding protein-dependent transport system and energized by ATP. The maltose system, a member of the ATP-binding cassette or ABC transport family, contains two copies of an ATP-binding protein in a complex with two integral membrane proteins. ATP hydrolysis by the transport complex can be assayed following reconstitution into proteoliposomes in the presence of maltose binding protein and maltose. Mutations in the transport complex that permit binding protein-independent transport render ATP hydrolysis constitutive so that hydrolysis can also be assayed with the transport complex in detergent solution. We have used both of these systems to study the role of two ATP binding sites in ATP hydrolysis. We found that both the wild-type and the binding protein-independent systems hydrolyzed ATP with positive cooperativity, suggesting that the two ATP binding sites interact. Vanadate inhibited the ATPase activity of the transport complex with 50% inhibition occurring at 10 mum vanadate. In detergent solution, the degree of cooperativity in the binding protein-independent complex decreased with increasing pH. The loss of cooperativity was accompanied by a decrease in ATPase activity and a decrease in sensitivity to vanadate. Because reconstitution of the complex into a lipid bilayer prevented the loss of cooperativity, we expect that ATP hydrolysis is cooperative in vivo. The mutations leading to binding protein-independent transport do not significantly alter the affinity, cooperativity, vanadate sensitivity, or substrate specificity of the ATP binding sites during hydrolysis. These results justify the use of the binding protein-independent system to investigate the mechanism of transport and hydrolysis.

ATP-Binding Cassette Transporters↗
PubMed1994

Diurnal and gestational patterns of uterine activity in normal human pregnancy. The Uterine Activity in Pregnancy Working Group.

OBJECTIVES: To define the profile of 24-hour uterine activity in normal pregnancy and to correlate contraction frequency with physical activity and emotional stress diaries. METHODS: One hundred nine low-risk pregnant women who delivered at term recorded uterine contractions for 24 hours twice weekly from 20-40 weeks' gestation using an ambulatory monitor and kept a physical activity and emotional stress diary. Contractions per hour were calculated for each hour of the day and week of gestation, and related to physical activity and emotional stress. RESULTS: We analyzed 71,683 hours. No contractions were recorded in 73% of the hours, and fewer than four contractions per hour occurred in 96%. Significant inter-individual variability was noted. Contractions increased markedly with gestational age: The 95th percentile was 1.3 contractions per hour at 21-24 weeks, 2.9 at 28-32 weeks, and 4.9 at 38-40 weeks. A strong clustering of contractions occurred at night, which became pronounced after 24 weeks (night:day ratio 2:1 at 28-32 weeks). To adjust for the effects of gestational age and time of day, contractions per hour were converted to gestation- and hour-specific percentiles ("contraction percentiles"). Rest was associated with a fall in contraction percentile by 1.25, whereas coitus increased the contraction percentile by 5.52 (P < .05). No changes were noted with emotional stress. CONCLUSIONS: This study provides normative contraction data in uncomplicated pregnancy. A strong diurnal rhythm is present from 24 weeks onward, with 67% of contractions occurring at night. Contractions per hour increase with gestational age but rarely exceed three per hour before term. Rest and sexual activity have small but measurable effects on contraction frequency.

Activities of Daily Living↗
PubMed1992

Mechanism of maltose transport in Escherichia coli: transmembrane signaling by periplasmic binding proteins.

Maltose transport across the cytoplasmic membrane of Escherichia coli is dependent on the presence of a periplasmic maltose-binding protein (MBP), the product of the malE gene. The products of the malF, malG, and malK genes form a membrane-associated complex that catalyzes the hydrolysis of ATP to provide energy for the transport event. Previously, mutants were isolated that had gained the ability to grow on maltose in the absence of MBP. After reconstitution of the transport complex into proteoliposomes, measurement of the ATPase activity of wild-type and mutant complexes in the presence and absence of MBP revealed that the wild-type complex hydrolyzed ATP rapidly only when MBP and maltose were both present. In contrast, the mutant complexes have gained the ability to hydrolyze ATP in the absence of maltose and MBP. The basal rate of hydrolysis by the different mutant complexes was directly proportional to the growth rate of that strain on maltose, a result indicating that the constitutive ATP hydrolysis and presumably the resultant cyclic conformational changes of the complex produce maltose transport in the absence of MBP. These results also suggest that ATP hydrolysis is not directly coupled to ligand transport even in wild-type cells and that one important function of MBP is to transmit a transmembrane signal, through the membrane-spanning MalF and MalG proteins, to the MalK protein on the other side of the membrane, so that ATP hydrolysis can occur.

ATP-Binding Cassette Transporters↗
PubMed1991

Purification and characterization of the membrane-associated components of the maltose transport system from Escherichia coli.

Maltose is transported across the cytoplasmic membrane of Escherichia coli by a binding protein-dependent transport system. The three membrane-associated components of the transport system, the MalK, MalF, and MalG proteins, have been solubilized from the membrane and maltose transport activity has been reconstituted in proteoliposome vesicles (Davidson, A. L., and Nikaido, H. (1990) J. Biol. Chem. 265, 4254-4260). A modification of the reconstitution technique is presented which permits reconstitution from the detergent dodecyl maltoside. Utilizing reconstitution of maltose transport as an assay, we have purified these proteins in the presence of n-dodecyl-beta-D-maltoside. The purified proteins catalyze both maltose transport activity and ATP hydrolysis. In all experiments, the MalF, MalG, and MalK proteins behaved as a multiprotein complex; all three proteins were immunoprecipitated using antibody prepared against MalF, and they copurified, eluting from a gel filtration column between markers of Mr 160,000 and 200,000. Each complex contains two MalK, one MalF, and one MalG proteins, providing two putative sites for ATP hydrolysis. Chemical cross-linking detected specific interactions between MalF and MalG and between MalF and MalK.

ATP-Binding Cassette Transporters↗
PubMed1990

Overproduction, solubilization, and reconstitution of the maltose transport system from Escherichia coli.

Maltose is transported across the cytoplasmic membrane of Escherichia coli by a binding protein-dependent transport system. We observed a 10-fold increase in the level of transport activity in assays with membrane vesicles when the three membrane-associated components of the transport system (the MalF, MalG, and MalK proteins) were overproduced. In addition, we have successfully reconstituted maltose transport activity in proteoliposome vesicles from solubilized proteins using a detergent dilution procedure. The addition of ATP as an energy source was sufficient to obtain transport, and this activity was dependent on the presence of maltose binding protein and was not seen in proteoliposomes prepared from a strain with a deletion of the maltose genes. We determined that hydrolysis of ATP was directly coupled to maltose uptake. In the majority of these experiments, an average of 1.4 mol of ATP was hydrolyzed for each mole of maltose accumulated. However, in the remaining experiments, ATP hydrolysis was observed to be much higher and averaged 17 mol of ATP hydrolyzed per mol of maltose transported. Possible explanations for a variable stoichiometry are discussed. These results provide strong evidence that it is the hydrolysis of ATP by a component of the transport complex that provides the energy required for active maltose transport.

ATP-Binding Cassette Transporters↗
PubMed1989

Maltose transport in membrane vesicles of Escherichia coli is linked to ATP hydrolysis.

We examined the energy requirement for maltose transport in right-side-out membrane vesicles derived from Escherichia coli. When membrane vesicles were made from strains producing tethered maltose-binding proteins by dilution of spheroplasts into phosphate buffer, those from an F0F1 ATPase-containing (unc+) strain transported maltose in the presence of an exogenous electron donor, such as ascorbate/phenazine methosulfate, at a rate of 1-5 nmol/min per mg of protein, whereas those from an isogenic unc- strain failed to transport maltose. Transport in vesicles obtained from the latter strain could be restored in the presence of electron donors if the vesicles were made to contain NAD+ and either ATP or an ATP-regenerating system. ATP hydrolysis was apparently required for transport, since nonhydrolyzable ATP analogues did not sustain transport. Maltose transport significantly increased ATP hydrolysis in ATP-containing vesicles from unc- cells. Finally, ATP-containing vesicles from unc- strains producing normal maltose-binding proteins could accumulate maltose in the absence of electron donors. These results provide convincing evidence that it is the hydrolysis of ATP that drives maltose transport, and probably also other periplasmic-binding-protein-dependent transport systems.

Adenosine Triphosphate↗
PubMed1988

Relationship of estrogen and progesterone receptors to prognosis in breast cancer.

To ascertain the role of estrogen (ER) and progesterone (PR) receptors as prognostic indicators of resectable breast cancer, the records of 204 patients were analyzed whose receptor studies were done at the Maimonides Medical Center from 1975 to 1983. All patients had radical or modified radical mastectomies and did not show any evidence of distant metastases at the time of operation. Median follow-up was 37 months. An additional 117 patients received some form of adjuvant therapy, mainly chemotherapy, and were analyzed separately. Life table analysis using the log rank test for measuring significance, and a Cox multivariate analysis was performed. At 48 months, 22% of the ER positive (ER+) group versus 33% of the ER negative (ER-) group had recurred as compared to 16% and 35% for the PR+ versus PR- groups, respectively. Life table analysis of the disease free interval (DFI) showed that the difference between the ER+ and ER- groups was not significant (p greater than 0.1), while the difference in DFI between the PR+ and PR- groups was significant (p less than 0.05). Multivariate analysis revealed that the most important factors in predicting the DFI were nodal status (p less than 0.001), tumor size (p less than 0.025), and progesterone receptor status (p less than 0.05). Estrogen receptor status was not found to be significant. In conclusion, PR- patients have a shorter DFI than PR+ patients and that PR status is a more valuable predictor of DFI than ER status.

Aged↗
PubMed1988

Stercoral perforation of the colon: report of a new location.

Stercoral perforations of the colon unassociated with obstructive lesions are rarely reported. With the exception of one case report which described a stercoral perforation of the cecum, all the reported perforations occurred at the sigmoid or rectosigmoid colon. This report describes a new location of perforation which hitherto has not been reported.

Aged↗