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Biomedical subjects

H Hebert

Publications and source records attributed to H Hebert.

At least 37 records · Page 2Linked to original sources
PubMed1990

Coexistence of different forms of Na,K-ATPase in two-dimensional membrane crystals.

Two-dimensional membrane crystals of renal Na,K-ATPase were analyzed by electron microscopy and image processing. The particular property of the crystals in this work was that they showed unit cell parameters similar to the previously studied p21 crystals but lacked the dyad axis as observed in nominal 0 degrees-projections. A three-dimensional reconstruction revealed that structural differences between alpha beta-units of the enzyme gave rise to the asymmetry. A high degree of two-fold rotational symmetry was observed in the middle of the structure while the protein units had different three-dimensional shapes at levels above and below the central sections. The simultaneous coexistence of different forms of Na,K-ATPase suggests that the conformational flexibility of the enzyme plays an important role in the pumping process.

Animals↗
PubMed1990

Crystalline layers and three-dimensional structure of Staphylococcus aureus alpha-toxin.

Interaction of the pore-forming protein alpha-toxin from Staphylococcus aureus with lipid components from platelet membranes induces crystal formation of the toxin oligomers. Structure analysis of crystalline areas in either sodium phosphotungstic acid or a sodium phosphotungstic acid/glucose mixture has been performed with electron microscopy and image processing. Ordered domains extending up to a few micrometers were observed, particularly after application of alpha-toxin to pre-formed lipid layers. The crystals, showing tetragonal symmetry, formed either separate two-dimensional sheets or three-dimensional piles of layers. The corresponding unit cell parameter of the single layer was a = b = 109.4 A (standard deviation 2.1 A, n = 21). Incubation of the toxin with intact membranes or extracted lipids as well as application of the lipid layer technique resulted in congruous crystalline properties. The projected averaged alpha-toxin oligomer shows cyclic symmetry with a stain-filled space in the centre. The bulk of the three-dimensional model consists of four asymmetric protein units forming a ring. In addition, a small domain covers the central cavity at the face of the protein opposite to the underlying lipid. The conditions under which the tetragonal arrays are formed on the lipid layers suggest that the alpha-toxin molecule is in a conformation binding to a hydrophobic surface rather than fully inserted into a lipid bilayer.

Bacterial Toxins↗
PubMed1989

Two-dimensional crystalline arrays of Na,K-ATPase with new subunit interactions induced by cobalt-tetrammine-ATP.

Purified membrane-bound Na,K-ATPase incubated with cobalt-tetrammine-ATP [Co(NH3)4ATP], which is a stable MgATP complex analog, shows two new types of membrane crystals, a new p21 form and a p4 form. The building blocks of the crystalline arrays correspond to (alpha beta)2 dimers of the enzyme protein suggesting that alpha-alpha interaction may be important in the pumping process.

Adenosine Triphosphate↗
PubMed1988

Digitization of electron micrographs: a comparison of three different types of scanners.

Some aspects of digitization of electron micrographs have been investigated. The performances of a flat-bed, a rotating drum, and a diode array scanner have been evaluated. Estimates have been achieved for resolution, mechanical and optical stability, and optical density response. It is concluded that for routine transmission electron microscopy of, for example, negatively stained biologic specimens, a diode array scanner produces data good enough to obtain resolutions at a level normally expected. High speed is the major advantage with this type of equipment. However, for high-resolution work it is necessary to use a conventional scanner with a relatively slow scan speed.

Analog-Digital Conversion↗
PubMed1988

The projection structure of alpha-toxin from Staphylococcus aureus in human platelet membranes as analyzed by electron microscopy and image processing.

Most strains of Staphylococcus aureus produce alpha-toxin, a 33-kDa membrane active protein which is considered to be an important virulence factor of this bacterium. When alpha-toxin interacts with membranes an oligomeric from of the toxin can be seen by electron microscopy as characteristic ring structures in the membrane. A two-dimensional study of these annular structures, incorporated in membranes of human platelets, was performed, introducing a partly new method for rotational alignment of individual particles. It is shown that the averaged oligomer consists of six subunits. At neutral pH the outer diameter of the ring is about 75 A. The stain-filled pore or cavity in the center has a diameter of about 25 A. The size of the hexamer is increased if the pH is lowered.

Blood Platelets↗
PubMed1988

Three-dimensional structure of renal Na,K-ATPase determined from two-dimensional membrane crystals of the p1 form.

Electron microscopy and image processing were used to reconstruct a three-dimensional model of membrane-bound monomeric renal Na,K-ATPase from negatively stained two-dimensional crystals of the p1 type. Correlation methods were applied to obtain projection averages which were aligned by a phase difference minimization procedure. The self-consistency of the reconstruction process was high as determined by correlation between experimental projections and projections of the calculated model. The three-dimensional model of the Na,K-ATPase promoter in the p1 crystal form contains three characteristic domains, a protein dense ellipsoid, a small globular stain deficient domain, and a connecting low-contrast region. The latter is thought to correspond to the lipid-penetrating part of the Na,K-ATPase promoter. The location of this domain gives the protein an asymmetric distribution in the bilayer so that it is exposed primarily on one side proposed to correspond to the intracellular face.

Animals↗
PubMed1988

Assembly of two-dimensional membrane crystals of Na,K-ATPase.

The assembly of vanadate-induced two-dimensional membrane crystals of Na,K-ATPase was analyzed by electron microscopy and image processing. Electron micrographs of negatively stained linear arrays of protein molecules were recorded and processed by correlation averaging methods. The arrays were compared with fully developed p21 crystals of the enzyme. On the basis of similarity in protein form, symmetry, and packing arrangement it was concluded that the fully developed crystals are built of tightly packed ribbons. Assembly pathways for two-dimensional membrane crystals of Na,K-ATPase are proposed.

Animals↗
PubMed1988

Structure analysis of fibrinogen by electron microscopy and image processing.

Human fibrinogen was observed by electron microscopy following rotary shadowing with tungsten. Structure analysis of the molecules was performed by image processing of electron micrographs. A method is described for selection, alignment, and classification of molecules. The widely accepted overall trinodular structure of the protein was observed. The flexibility about the central domain of the molecule was quantitatively analyzed. A Gaussian distribution of this conformational parameter was obtained having an average corresponding to a maximally extended structure. Correspondence analysis applied to the aligned images showed that the degree of folding of the molecule was continuously distributed. The averaged structure of fibrinogen was estimated to be 450 A long. The central domain had a diameter of 50 A and the peripheral domains were 90 A long and 50 A wide. The latter regions had two separated maxima of scattering density.

Fibrinogen↗
PubMed1984

[Metastatic malignant thymoma].

Three cases of metastatic malignant thymoma are reported. In all three cases the tumour was invasive and excision was only partial or impossible. Histologically, these tumours were of epithelial origin with a variable lymphocytic component. Metastases were initially present in one case and in the other two developed within the first year. Partial remission was obtained with different drug combinations in only one case, and it was of short duration. The authors emphasize the increased frequency and poor prognosis of these tumours and the need for multiple chemotherapy and loco-regional treatment in the management of invasive thymomas.

Adolescent↗
PubMed1983

Prognostic index for clinical Stage I cutaneous malignant melanoma.

Thirteen variables were evaluated for their significance in predicting the survival of 148 patients in a retrospective study with clinical Stage I cutaneous malignant melanoma. The variables studied were histological type, tumor thickness, level of invasion, mitotic activity, pigmentation type, existence of ulceration, presence of lymphocytic infiltration, cell type, sex and age of the patient, site of melanoma, and wide local excision preceded or not by contact radiotherapy and associated or not with lymphadenectomy. When these variables were individualized, only seven were significantly related to survival: histological type; tumor thickness; level of invasion; mitotic activity; pigmentation type; existence of ulceration; and sex of the patient. Multivariate analysis based on Breslow's version of the Cox proportional-hazards model was performed on a group of 110 patients. This analysis demonstrated that 5 of the original 13 variables (i.e., mitotic activity, tumor thickness, sex, lymphadenectomy, and site of primary melanoma) could be used to develop a prognostic model. A Gompertz distribution which provided for an appropriate smoothing of the Breslow model estimates was used to derive a simple prognostic index and to predict the survival of individual patients. Fifty-four patients in a prospective study were subsequently evaluated with the Gompertz model in order to test the prognostic accuracy of the model for the five variables.

Adult↗
PubMed1982

Crystallization patterns of membrane-bound (Na+ +K+)-ATPase.

Extensive formation of two-dimensional crystals of the proteins of the pure membrane-bound (Na+ +K+)-ATPase is induced during prolonged incubation with vanadate and magnesium. Some membrane crystals are formed in medium containing magnesium and phosphate. Computer-averaged images of the two-dimensional crystals show that the unit cell in vanadate-induced crystals contains a protomeric alpha beta-unit of the enzyme protein. In phosphate-induced crystals an (alpha beta) 2-unit occupies one unit cell suggesting the interactions between alpha beta-units can be of importance in the function of the Na+, K+ pump.

Animals↗
PubMed1982

Ultrastructure of the Na, K-ion pump.

Na, K-ATPase has been analysed by electron microscopy to obtain information about the structure of the enzyme and its organization within the membrane. Following negative staining the membrane-bound enzyme was observed as surface particles which on the basis of their size and frequency and the enzymatic and chemical composition of the membranes are interpreted as protomers (alpha beta-units). Freeze-fracture electron microscopy revealed the enzyme as intramembrane particles. Quantitative electron microscope studies suggested that the intramembrane particles are oligomers of the protein unit that forms the surface particles. Following reconstitution of the enzyme into phospholipid vesicles it was demonstrated that similar intramembrane particles represent a protein unit which transports sodium and potassium. Vanadate and magnesium induced the formation of two-dimensional crystals in the membrane fragments of the purified Na, K-ATPase. Further information regarding the shape and dimensions of the protomer was obtained through analysis of electron micrographs of negatively stained crystals with optical diffraction and image reconstruction methods.

Animals↗