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Biomedical subjects

A H Burr

Publications and source records attributed to A H Burr.

14 recordsLinked to original sources
PubMed2000

A hemoglobin with an optical function.

Hemoglobins are best known as oxygen transport proteins. Here we describe a hemoglobin from the parasitic nematode Mermis nigrescens (Mn-GLB-E) that has an optical, light shadowing function. The protein accumulates to high concentration as intracellular crystals in the ocellus of mature phototactic adult females while also being expressed at low concentration in other tissues. It differs in sequence and expression pattern from Mn-GLB-B, a second Mermis globin. It retains the structure and oxygen-binding and light-absorbing properties typical of nematode hemoglobins. As such, recruitment to a shadowing role in the eye appears to have occurred by changes in expression without modification of biochemistry. Both globins are coded by genes interrupted by two introns at the conserved positions B12.2 and G7.0, which is in agreement with the 3exon/2intron pattern model of globin gene evolution.

Amino Acid Sequence↗
PubMed2000

Ocellar pigmentation and phototaxis in the nematode Mermis nigrescens: changes during development.

After 1 or 2 years of dormancy in the soil, Mermis nigrescens females emerge to lay eggs on vegetation where their grasshopper hosts are likely to feed. Females collected at this life stage exhibit a strong positive phototaxis and have a tubular region of pigmentation near the anterior tip consisting of concentrated oxyhaemoglobin. A previous investigation of the scanning motion of the 'head' and orientation of the 'neck' has implicated the shadowing of a photoreceptor inside the tube as the mechanism for identifying the direction of light during phototaxis. Here, we describe the development of the pigment in young adult females and investigate phototaxis in early developmental stages that lack the pigment. The orientation of the neck to a horizontal 420 nm stimulus (intensity 10(13 )photons s(-)(1 )cm(-)(2)) was measured for unpigmented fourth-stage larvae and immature adult females as well as mature females with pigmented ocelli. The orientation of the larvae and immature adults was weakly negative, whereas that of the mature adults was strongly positive. Head and neck movements were otherwise the same in the three stages. Thus, the pigmentation appears to be required for positive phototaxis, and the results provide further support for the shadowing role of ocellar haemoglobin.

Animals↗
PubMed1998

Mechanical significance of obliquely striated architecture in nematode muscle.

In certain invertebrate muscles, adjacent narrow columns of sarcomeres are displaced along the fiber axis, providing an obliquely striated myofilament pattern in certain section planes. Although this architecture is described in many phyla and has been the subject of much discussion (1-12), its mechanical significance has yet to be resolved. In nematodes, where ultrastructural details of the obliquely striated muscle have long been known (12-19), another unique and prominent feature is the attachment of every sarcomere to the plasmalemma and basal lamina via dense bodies (Z-disc analogs). Unfortunately, the importance of this feature to the transmission of the contractile force to the cuticle is not understood outside the Caenorhabditis elegans literature: it was overlooked in recent reviews covering obliquely striated muscle (9-11). Here we consider transmission of force and oblique striation together. We compare the contractile architecture in C. elegans with that in the more complex muscle type of larger nematodes. Both types are designed to transmit the force of contraction laterally to the cuticle rather than longitudinally to the muscle ends. In the second type, folding of the contractile structure around an inward extension of the basal lamina enables a higher number of sarcomeres to be linked to cuticle per unit length. We suggest that the mechanical significance of the oblique arrangement of sarcomeres in both types is that it distributes the force application sites of the sarcomeres more evenly over the basal lamina and cuticle. With this muscle architecture, smooth bending of the nematode body tube would be possible, and kinking would be prevented.

Actin Cytoskeleton↗
PubMed1994

Response of fibroblast cultures from ataxia-telangiectasia patients to reactive oxygen species generated during inflammatory reactions.

Cells from patients with ataxia-telangiectasia (AT) are more sensitive than cells from normal individuals to a number of compounds which induce DNA damage via oxygen-derived free radical attack. We tested the hypothesis that AT cells would show a sensitivity to reactive oxygen species (ROS) generated by activated inflammatory cells. AT cells were exposed to neutrophils activated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or to xanthine/xanthine oxidase (X/XO), an enzyme system which generates superoxide and hydrogen peroxide. Induced micronuclei (MN) frequencies (corrected for spontaneous MN frequencies) were significantly higher in AT cell cultures than in cultures from normal individuals (comparison of MN frequencies of AT vs. normal cultures: for treatment with activated neutrophils, P = 0.003; for X/XO, P = 0.05). The comet assay was used to determine whether the elevated chromosomal damage in the treated AT cells was due to a difference in strand breakage or its rejoining. X/XO treatment was used in studies of single-stranded (SS) DNA breakage, and X-ray treatment for double-stranded (DS) DNA damage. AT and normal cells showed no significant differences in the initial levels of SS (P = 0.29) or DS (P = 0.91) DNA damage. Likewise, they exhibited similar rejoining kinetics (rejoining half-time for SS = 10 min, for DS = 30 min). These data support the involvement of the AT loci in determining a cell's ability to deal with oxidative stress, although the mechanism underlying this effect has yet to be resolved. The data also suggest that AT patients are at elevated risk of sustaining DNA damage in tissues undergoing inflammatory reactions.

Ataxia Telangiectasia↗
PubMed1993

A sensitivity to oxidative stress is linked to chromosome 11 but is not due to a difference in single strand DNA breakage or repair.

Defects in loci on chromosome 11 have been associated with tumourigenicity, anchorage-independent growth, metastasis and radiosensitive DNA repair in tumour cells. The introduction of normal chromosome 11 into these cells suppresses these responses. In the present study we tested two hypotheses: (1) that microcell fusion of normal chromosome 11 into bladder-carcinoma cells (A1698) can protect the cells against chromosomal damage by oxidative stress; and (2) that insertion of normal chromosome 11 corrects a single-strand (SS) DNA-repair defect. Cultures of A1698 (termed parent) and its microcell-mediated hybrid (termed hybrid) were exposed for 1 h to xanthine/xanthine oxidase (X/XO) or co-incubated with human neutrophils activated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Micronucleus frequencies (an indication of chromosomal damage) were significantly higher in parent cultures after treatment than in hybrid (P < 0.0001). The level of single-strand DNA breakage and its repair was assayed in X/XO-treated cultures with the alkaline comet assay. There was no significant difference between parent and hybrid in the amount of SS DNA breakage at treatment (P > 0.1) or after 20 min of repair (P > 0.1). The data support the involvement of a defect in chromosome 11 leading to sensitivity to oxidative stress and suggest this defect is not in the initial amount or rate of rejoining of SS DNA breakage.

Cell Line↗
PubMed1985

Kinetics and mechanism of hemoglobin denaturation in alkali.

The denaturation of oxy, deoxy, CO and met derivatives of human hemoglobin A at pH 11.7 and 25 degrees C was followed by three assay methods: chromophore absorbance (which indicates changes in the heme), and the amount of precipitation in 24% ammonium sulfate neutral buffer or 0.1 M NaCl neutral buffer (which indicates degree of destabilization of the protein structure). We find that oxyhemoglobin denatures in two parallel reaction sequences. The rate of sequence I is increased when the sulfhydryl groups in the alpha 1 beta 1 subunit interface have been modified by binding p-hydroxymercuribenzoate (which forces monomer formation) and is decreased by the binding of -HgOH (which, unlike the sulfhydryls, is not ionized at pH 11.7). These results support a mechanism in which the net rate of monomer formation is rate limiting and is enhanced in alkali by the ionization of sulfhydryls in the alpha 1 beta 1 subunit interface. In subsequent rapid reactions, ferric hemoglobin and low-salt-precipitable protein are formed. The formation of an oxidant, such as superoxide, is indicated by the kinetics of sulfhydryl oxidation. The same oxidant would be available to initiate the second sequence by oxidizing some of the unreacted oxyhemoglobin to methemoglobin. During methemoglobin denaturation, as in sequence II, a low-salt-soluble ferric hemochrome is formed. In both reactions, this intermediate becomes low-salt-precipitable at the same rate. Deoxyhemoglobin denatures to ferrous hemochrome at the same rate as oxyhemoglobin denaturation in sequence I, and provided oxygen is excluded, the denaturation is fully reversible on neutralization. In the absence of oxygen, CO-hemoglobin does not denature to any detectable extent. The destabilization of hemoglobin structure that was indicated by precipitability occurred only for ferriheme derivatives and was independent of disulfide formation.

Alkalies↗
PubMed1985

Naturally crystalline hemoglobin of the nematode Mermis nigrescens. An in situ microspectrophotometric study of chemical properties and dichroism.

A dichroic microspectrophotometer was used to measure isotropic and dichroic absorbance spectra of this unique cytoplasmic hemoglobin and its derivatives. A perfusion slide enabled changing the media bathing the Mermis head. The native spectrum, which has an exceptionally low alpha-band extinction, was shown to be entirely due to oxyhemoglobin. The CO-hemoglobin spectrum is more typical, however, the alpha- and beta-bands are unusually closely spaced. A ferric hemochrome was formed on oxidation with ferricyanide or hydroxylamine and was readily converted to ferric hemoglobin cyanide on adding cyanide. Aquoferric hemoglobin and ferric hemoglobin fluoride were not easily formed. Deoxyhemoglobin, identified by its typical absorption spectrum, was formed only under the extremely low O2 pressures attainable in the presence of dithionite. A glucose oxidase, catalase solution deoxygenated hemoglobin in human erythrocytes but not in adjacent Mermis preparations. The affinity for O2 is much greater than for CO. Also, spectral evidence points to an oxyheme environment that is different than in vertebrate hemoglobin and myoglobin. The polarization ratio (PR) magnitude and the PR spectrum were unaffected by perfusion with high refractive index solvents; therefore, form dichroism due to the rodlike crystals is negligible. Maximum extinction is approximately perpendicular to the long axis of the microscopic crystals, which are oriented parallel to the body axis within the hypodermal cells. The PR spectra of the hemoglobin derivatives strongly resemble the corresponding spectra previously reported of single crystals made of horse hemoglobin, whale myoglobin, or Aplysia myoglobin and change appropriately when the ligand is changed. This confirms that the intracellular crystals of Mermis are of oxyhemoglobin.

Animals↗
PubMed1978

The pumping mechanism of the nematode esophagus.

The radial orientation of the myofilaments in the nematode esophagus raises interesting questions as to how such a structure can function as a pump. A physical model of the esophagus of Ascaris lumbricoides was developed and the membrane theory of shells applied in order to relate the observed dimensional changes to myofilament force, pressure stresses, and membrane elastic constants. By stressing the excised esophagus passively with osmotic pressure, the esophagus was shown to be elastically anisotropic with the ratio of circumferential to longitudinal elastic constants, E(psi)/E(l) approximately 2.74. When this value was incorporated, the model predicted the ratio of the respective strains, epsilon(psi)/epsilon(l), to be 0.52 during an equilibrium contraction of the esophagus. This agreed with the experimental value, 0.46 +/- 0.10, measured during occasional, prolonged muscle contractions. When measured during normal pumping, on the other hand, the value of epsilon(psi)/epsilon(l) was 0 +/- 0.10. This indicated that a nonequilibrium condition normally occurs in which a greater myofilament force per unit area of lumen membrane is not balanced by internal pressure and therefore acceleration of the lumen contents and negative intraluminal pressure occurs.The pumping action of esophagi dissected from Ascaris was observed to be normally peristaltic and periodic. Contraction was initiated by a spontaneous depolarization that propagated at 4.0 +/- 0.20 cm/s along the esophageal membrane. A wave of localized increases in the internal pressure of the muscle and localized changes in external dimensions was observed. A subsequent spontaneous repolarization, which propagated at 5.8 +/- 0.23 cm/s, triggered relaxation of the muscle during which the localized pressure and dimensional changes returned to resting values. A mechanism was deduced in which fluid is drawn into and moved along the lumen by the wave of contraction. During the wave of relaxation, the lumen contents are pressurized and injected into the intestine by elastic restoring forces.

Animals↗
PubMed1975

Properties of a hemoglobin from the chromatrope of the nematode Mermis nigrescens.

The chromatrope pigment of Mermis nigrescens (Phylum: Aschelminthes, Class: Nematoda) was previously thought to have a role in photoreception. In this work it is shown to be an oxyhemoglobin whose absorption spectrum in vivo and in extracts resembles that of Ascaris oxyhemoglobins in having a weak alpha-band absorptivity. The extracted hemoglobin binds O2 and CO reversibly and with an affinity higher than that of the same concentration of horse hemoglobin. The Soret band of the deoxy derivative is unusually low and broad. Absorptivities and lambdamax for absorption bands of the oxy, deoxy and CO derivatives are tabulated for comparison with other hemoglobins. Microchemical procedures were developed which revealed that the chromatrope contains an average of 5-10(-12) mol of non-dialysable protoheme. Thus the hemoglobin concentration in the approx. 0.5 nl chromatrope volume is on the order of 10 mM (heme). The O2 binding ability and high in vivo concentration of this hemoglobin make possible a role in O2 storage or facilitation of O2 diffusion.

Animals↗