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D J Field

Publications and source records attributed to D J Field.

67 records · Page 4Linked to original sources
PubMed1985

Equilibrium and rapid kinetic studies on nocodazole-tubulin interaction.

The interaction between nocodazole and calf brain tubulin in 10(-2) M sodium phosphate, 10(-4) M GTP, and 12% (v/v) dimethyl sulfoxide at pH 7.0 was studied. The number of binding sites for nocodazole was shown to be one per tubulin monomer of 50,000 as a result of equilibrium binding studies by gel filtration and spectroscopic techniques. The presence of microtubule-associated proteins did not significantly affect the binding of nocodazole to tubulin. The apparent equilibrium constant measured at 25 degrees C was (4 +/- 1) X 10(5) M-1. Temperature does not significantly affect the apparent equilibrium constant; hence, the binding of nocodazole to tubulin is apparently entropy driven. Stopped flow spectroscopy was employed to monitor the rate of nocodazole binding under pseudo first order conditions. The effects of temperature and nocodazole concentration were studied. The apparent rate constants were dependent on the concentration of nocodazole in a nonlinear manner. In conjunction with results from structural and thermodynamic studies the kinetic results were interpreted to suggest a mechanism of T + N in equilibrium with TN in equilibrium with T* N, where T and N are tubulin and nocodazole, respectively. T and T* represent two conformational states of tubulin. Furthermore, the kinetic data are consistent with the thermodynamic data only if a model of two parallel similar reactions were considered, one rapid and the other slow. The initial binding step for both the rapid and slow phases was characterized by identical binding constants; however, there was a significant difference in the rates of isomerization. Hence, nocodazole is potentially a useful probe for amplifying differences in solution properties of tubulin subspecies.

Animals↗
PubMed1985

Changing overall workload in neonatal units.

An epidemiological study was carried out in the current Nottingham health district to determine changes in both neonatal unit workload and in the infants requiring neonatal care during two study periods. All admissions for 1977 and again for the 12 months 1 April 1983 to 31 March 1984 were reviewed. Total numbers of admissions have shown roughly a 50% reduction because of a more rigorous admission policy locally. The admissions of infants of 33 weeks' gestation or less have shown a significant increase, while the overall survival of infants less than 33 weeks' gestation has improved.

Birth Weight↗
PubMed1985

Isoelectric focusing and two-dimensional electrophoresis of tubulin using immobilized pH gradients under denaturing conditions.

Modifications of the LKB Immobiline isoelectric focusing (IEF) technique are described for use under conditions that solubilize and denature most proteins (8 M urea and 2% Nonidet-P40). This procedure permits pH gradients that are four- to fivefold shallower than previously available with conventional ampholine-IEF procedures. It can also be used as a first dimension in two-dimensional gel electrophoresis. The advantage of the stable ultranarrow pH gradient is demonstrated by directly comparing the resolution of vertebrate brain tubulins using (i) denaturing conventional ampholine-IEF and (ii) denaturing Immobiline-IEF. Analysis of tubulin on the Immobiline-IEF gel increases the separation distance between the individual tubulins and distinguishes differences among tubulin samples that could not be resolved by conventional ampholine isoelectric focusing.

Animals↗
PubMed1984

Phase reversal discrimination.

In the Fourier representation of space, the parameter of phase plays a crucial role. In this study, several experiments were performed involving discrimination of various phase relations of fundamental (2 c/deg) to second harmonic (4 c/deg) at low contrast levels. The results were consistent with a model involving four "channels", each optimally sensitive to one of the following phase relations: + cosine (bright bar), -cosine (dark bar), +sine (left edge), and -sine (right edge).

Fourier Analysis↗
PubMed1984

Heterogeneity of vertebrate brain tubulins.

We have examined the extent of brain tubulin heterogeneity in six vertebrate species commonly used in tubulin research (rat, calf, pig, chicken, human, and lamb) using isoelectric focusing, two-dimensional electrophoresis, and peptide mapping procedures that provide higher resolution than previously available. The extent of heterogeneity is extremely similar in all of these organisms, as judged by number, range of isoelectric points, and distribution of the isotubulins. A minimum of 6 alpha and 12 beta tubulins was resolved from all sources. Even the pattern of spots on two-dimensional peptide maps is remarkably similar. These similarities suggest that the populations of tubulin in all of these brains should have similar overall physical properties. It is particularly interesting that chicken, which has only four or five beta-tubulin genes, contains approximately 12 beta tubulins. Thus, post-translational modification must generate at least some of the tubulin heterogeneity. Mammalian species, which contain 15-20 tubulin DNA sequences, do not show any more tubulin protein heterogeneity than does chicken. This suggests that expression of only a small number of the mammalian genes may be required to generate the observed tubulin heterogeneity.

Animals↗
PubMed1981

Polymorphism of brain tubulin.

Calf brain tubulin was subjected to isoelectric focusing and tryptic peptide map analysis. Results from isoelectric focusing experiments showed a total number of 17 well-resolved protein peaks. The number of peaks and the mass distribution under each peak remained the same when the concentration of protein or ampholyte was altered. When the protein was subjected to two-dimensional isoelectric focusing, a diagonal pattern was observed, indicating that the multiple peaks observed are not a manifestation of tubulin- ampholyte interaction. Further investigation by isolating these individual subspecies and subjecting them to isoelectric focusing yielded single peaks corresponding to the original ones without generating the initial pattern of multiple peaks. Tryptic peptide maps showed that among the subspecies of the alpha subunit there are 26 spots that are common among them. There are, however, 7 +/- 1 spots that are unique in each subspecies. Similar observations were obtained for the subspecies of the beta subunit although there are only 2 +/- 1 unique spots in each subspecies. These results suggest that tubulin subunits probably consist of polypeptides with both constant and variable regions in their sequences. Identical results were obtained for canine and rabbit brain tubulin, indicating that tubulin polymorphism is common among brain tissues. Tubulin isolated by either the polymerization-depolymerization or the modified Weisenberg procedures yielded identical results. These results show that the same subspecies of tubulin are extracted by both isolation procedures.

Animals↗
PubMed1980

Effects of nocodazole on structures of calf brain tubulin.

The interaction of nocodazole with calf brain tubulin was studied to determine the effect of such interaction on the structure of tubulin. The effect of nocodazole on the self-association of tubulin was monitored by turbidity measurements and velocity sedimentation. Sedimentation patterns indicate that nocodazole neither induces tubulin to undergo self-association to form higher orders of aggregate nor does it perturb the equilibrium of the reaction leading to the formation of 42S double-ring structures although nocodazole binds to both the tubulin dimers and the polymeric form. Nocodazole does, however, inhibit the in vitro reconstitution of microtubules, and the presence of microtubule-associated proteins does not amplify the inhibitory effect of the drug. The conformational changes in tubulin upon binding of nocodazole were monitored by differential spectroscopy, circular dichroism, fluorescence, and chemical modification of sulfhydryl residues. Results from these studies show that the sulfhydryl residues become more accessible to chemical modification. In contrast, the binding of nocodazole does not significantly alter the net environment of tryptophan chromophores. These residues are apparently not all located on the surface of the tubulin molecule and at least some are partially buried.

Animals↗
PubMedDate not supplied

Changing patterns in neonatal respiratory diseases.

Using the current Nottingham Health District, England, as a geographically defined population we have investigated the changing patterns of neonatal respiratory diseases. We have categorized all newborns who developed a respiratory problem during either 1977 or between April 1, 1983, and March 31, 1984 using a system based on that of Hjalmarson (Acta Paediatr Scand 1981; 70:773-783). There has been no change in the incidence of any of the disease types. However, the gestation of babies in two groups, idiopathic respiratory distress syndrome (IRDS) and minimal respiratory disturbance (requiring added oxygen for less than 6 hours), has shown a significant reduction. Overall, mortality for infants developing IRDS has shown a slight decrease, and for infants between 29 and 32 weeks gestation, mortality has shown a significant reduction. Overall, requirements for oxygen therapy and ventilation have shown a threefold increase, while hours in receipt of continuous positive airways pressure (CPAP) have increased sevenfold.

England↗
PubMedDate not supplied

Contour integration and cortical processing.

Our understanding of visual processing in general, and contour integration in particular, has undergone great change over the last 10 years. There is now an accumulation of psychophysical and neurophysiological evidence that the outputs of cells with conjoint orientation preference and spatial position are integrated in the process of explication of rudimentary contours. Recent neuroanatomical and neurophysiological results suggest that this process takes place at the cortical level V1. The code for contour integration may be a temporal one in that it may only manifest itself in the latter part of the spike train as a result of feedback and lateral interactions. Here we review some of the properties of contour integration from a psychophysical perspective and we speculate on their underlying neurophysiological substrate.

Action Potentials↗
PubMedDate not supplied

Cardiovascular effects of carbon dioxide in ventilated preterm infants.

Sick preterm infants may, under certain conditions, demonstrate blood pressure passive cerebral blood flow in response to changes in arterial carbon dioxide tension. Blood pressure in turn depends on cardiac output and peripheral resistance. A Doppler technique for assessing cardiac output compared favourably in terms of reproducibility to a thermodilution technique in a group of infants undergoing cardiac catheterization for congenital heart disease. Doppler was subsequently used to monitor changes in cardiac output following an increase in arterial carbon dioxide tension of 1 kPa in 25 ventilated preterm infants. Blood pressure increased significantly (p = 0.006). However, heart rate did not change significantly (p = 0.16) and, in addition, both stroke and minute volume decreased (p = 0.023, p = 0.02, respectively). This suggests that accompanying changes in components of peripheral resistance exert important effects on blood pressure in the preterm neonate in response to changes in arterial carbon dioxide tension.

Blood Pressure↗