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M F Brown

Publications and source records attributed to M F Brown.

124 records · Page 7Linked to original sources
PubMed1978

Influence of cholesterol on the polar region of phosphatidylcholine and phosphatidylethanolamine bilayers.

The structural changes in the polar head group region of unsonicated bilayer membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine produced by addition of cholesterol have been determined using deuterium and phosphorus-31 NMR. Incorportion of up to 50 mol percent cholesterol produces little change in the phosphorus-31 chemical shielding anisotropies, compared with the values in pure bilayers above the phase transition temperatures, while some of the deuterium quadrupole splittings are reduced by almost a factor of two. Adjustment of the head group torsion angles by only a few degrees accounts for the observed spectral changes. Addition of cholesterol therefore has opposite effects on the hydrocarbon and polar regions of membranes: although cholesterol makes the hydrocarbon region gel-like, with an increased probability of trans conformations, the conformation of the polar head groups is very similar to that found in the liquid crystalline phase of pure phospholipid bilayers.

Chemical Phenomena↗
PubMed1977

Resolution of independently titrating spectral components in the ultraviolet circular dichroism of subtilisin enzymes by matrix rank analysis.

The ultraviolet circular dichroism of di-isopropylphophoryl-subtilisins Carlsberg and Novo (EC 3.4.21.14) has been examined as a function of pH. The CD of these enzymes below 260 nm is invariant over the pH interval 4 to 12, below or above which spectral changes occur suggesting a transition to a random coil form. Above pH 8 contributions due to the ionization of tyrosyl residues appear in the CD above 260 nm as bands shifted to longer wavelengths. Three independently titratable components, obtained by matrix rank analysis, account for the observed CD spectral changes above 260 nm of Dip-subtilisin Carlsberg in the pH interval 8 to 12. By contrast, two components were derived for the Novo enzyme. The identities of the matrix rank components were surmised from their apparent pKa values. One component of both subtilisin enzymes corresponds to the CD of the "buried" or irreversibly titratable tyrosyl residues of the enzyme. The other matrix rank components correspond to the CD of the "exposed" or freely ionizable tyrosyl residues. These residues are optically active only in the ionized state. Two types of "exposed" tyrosyl residues, arising because of differing sensitivity to the ionization of the "partially buried" or abnormally titrating tyrosyl residues, are evident in Dip-subtilisin Carlsberg. A pH-induced local conformational change in this enzyme is proposed to account for this behavior. The "partially buried" tyrosyl residues of both subtilisins appear to be devoid of optical activity in either the tyrosyl or tyrosylate form.

Circular Dichroism↗
PubMed1977

Quenching of the tyrosyl and tryptophyl fluorescence of subtilisins Carlsberg and Novo by iodide.

The tyrosyl and tryptophyl fluorescence of diisopropylphosphorylsubtilisins Carlsberg and Novo, respectively, is quenched efficiently by I- but is not significantly affected by Cs+. The I-quenching data were analyzed using a modified Stern-Volmer treatment (Lehrer, S.S. (1971), Biochemistry 10, 3254), yielding values for the effective fraction of accessible protein fluorescence of 90-95 and 88-92% for the tyrosyl and tryptophyl emission of diisopropyl-phosphorylsubtilising Carlsberg and Novo, respectively. Similar values were obtained pH 5 and 7. The effective collisional quenching constant depends on pH in a manner suggesting the participation of protein surface charge in the quenching mechanism. Significant singlet energy transfer (efficiency = 0.52) from tyrosyl to tryptophyl residues was inferred from the excitation spectra of diisopropylphosphorylsubtilisn Novo. The very low efficiency of energy transfer to Trp-113 in diisopropylphorphorylsubtilisin Carlsberg suggests that Trp-105 and Trp-241 are the acceptors of tyrosyl emission in the homologous Novo enzyme. The unusually low quantum yield of Trp-113 in diisopropylphosphorylsubtilisin Carlsberg together with the tryptophyl fluorescence quenching behavior of the Novo enzyme suggests that this residue is "buried" and in accessible to quenching in both enzymes. The tyrosyl quenching behavior of diisopropylphorphorylsubtilisin Carlsberg is consistent with the high degree of solvent exposure of aromatic residues evident in the X-ray model of subtilisin Novo.

Energy Transfer↗
PubMed1977

Proton spin-lattice relaxation of retinal rod outer segment membranes and liposomes of extracted phospholipids.

A large fraction of the phospholipid protons of bovine retinal rod outer segment (ROS) disc membrane vesicles yield well-resolved nuclear magnetic resonance lines near physiological temperature. The spin-lattice (T1) relaxation rates of the resolved sharp resonance of ROS disc membranes appear biphasic above 10 degrees C. The rate of the more rapidly relaxing component of each resonance matches closely the relaxation rate of the corresponding resonances of liposomes of purified ROS phospholipids. The slowly relaxing component of each disc membrane resonance is most likely due to phospholipids whose motion is affected by rhodopsin. The primary difference in the relaxation behavior of phospholipids in the ROS membrane vesicles and ROS liposomes appears to be in T1, rather than T2, since the corresponding sharp resonances of both preparations have similar linewidths. These observations suggest that the interaction of rhodopsin with the more fluid membrane phospholipids predominantly affects relatively high frequency segmental motions, which determine T1, while having minimal effects on the lower frequency segmental motions, which influence T2. This conclusion can be rationalized by assuming that a substantial fraction of the interacting phospholipids are relatively fluid with respect to less frequent, larger amplitude segmental motions, but that the more frequent segmental motions (such as beta-coupled trans-gauche isomerizations) are significantly restricted by interaction with protein.

Animals↗
PubMed1976

Electron microscopy of vesicular-arbuscular mycorrhizae of yellow poplar. IV. Host-endophyte interactions during arbuscular deterioration 1,2.

Scanning electron stereoscopy and transmission electron microscopy were used to correlate morphological alterations and cytological phenomena associated with deterioration of arbuscules in yellow poplar mycorrhizae. Arbuscular degradation was initiated at the tips of the finest branches and progressed basipetally. Cytoplasm in arbuscular hyphae progressively deteriorated and was followed by collapse of the fungal walls. Degraded portions of the arbuscules aggregated into clumps comprised of host wall material and the distorted fungal walls. Host nuclei, abundant mitochondria, and proplastids were closely associated with arbuscular branches undergoing cytoplasmic deterioration and with clumped portions of the arbuscule which contained degraded hyphal branches. Most of the arbuscules observed had deteriorated to the clumped stage. Some cortical cells contained several clumped arbuscules and nearly mature, intact arbuscules which indicated that reinfection occurs even as degradative phenomena are in progress. It is suggested that substantial quantities of mineral nutrients may be made available to the host via degradation of fungal cytoplasm in the arbuscular hyphae preceding aggregation of degraded hyphae into discrete clumps.

Cell Nucleus↗
PubMed1976

Procedures for obtaining sectional views of fungal fructifications by scanning electron microscopy.

Procedures for sectioning fungal fructifications in host tissues or on artificial media are described, which allow observation of internal structures by scanning electron microscopy. Perithecia of Ceratocystis fimbriata and Phyllachora graminis, and telia of Puccinia xanthii showed excellent preservation of exposed structures in sections which were osmium-coated before being dried. While similar preservation was obtained in sectioned acervuli of Lecanosticta acicola and Marssonia juglandis and in pycnidia of Dothiorella ribis and Phomopsis occulta, the mucilaginous substances produced in these fructifications precluded observation of conidiophores. Extraction of mucilage from these sections was accomplished by periodic acid and dilute KOH treatments, followed by an osmium-coating procedure. In such preparations, mucilage was removed, internal structures were preserved, and pertinent characteristics of conidiogenous cells were resolved.

Agar↗
PubMed1976

Use of a tissue sectioner to expose internal structures of biological samples for scanning electron microscopy.

A procedure yielding sections of unembedded biological samples for observation by scanning electron microscopy is described. Sections of samples, fixed and hardened in OsO4, were obtained in quantity with a tissue sectioner. Subsequent treatments to osmium-coat cut surfaces were employed prior to critical point drying. The procedure yields cleanly cut surfaces through cells and cytoplasmic organelles which are retained in their normal position. Sections of apple leaf and mouse kidney are illustrated. Sections can be readily cut in a desired plane with less structural damage than is typically encountered by other sectioning or dissection techniques.

Animals↗
PubMed1975

Circular dichroism and gel filtration behavior of subtilisin enzymes in concentrated solutions of guanidine hydrochloride.

The circular dichroism of diisopropylphosphorylsubtilisins Novo and Carlsberg in both the near- and farultraviolet spectral regions is unaltered by concentrations of guanidine hydrochloride as high as 4 M at neutral pH. At concentrations of guanidine hydrochloride greater than 4 M slow irreversible time-dependent changes, apparently obeying second-order kinetics, are evident in both the near- and far-ultraviolet circular dichroism of these enzymes. Gel filtration studies of inactivated subtilisin enzymes reveal the circular dichroism changes to be accompained by the appearance of aggregated protein material. The changes in circular dichroism and the production of associated subtilisin species are sensitive to protein concentration, denaturant concentrations, and pH. The circular dichroism of active subtilisins Novo and Carlsberg in guanidine hydrochloride exhibits irreversible changes similar to those observed for the inactivated subtilisins. Aggregated protein material is also formed initially in the presence of guanidine hydrochloride, but is rapidly autolyzed to low molecular weight fragments.

Binding Sites↗
PubMed1975

Electron microscopy of vesicular-arbuscualr mycorrhizae of yellow poplar. I. Characterization of endophytic structures by scanning electron stereoscopy.

Vesicular-arbuscular mycorrhizae of yellow poplar (Liriodendron tulipifera L.) were examined by scanning electron microscopy. The morphology and spatial distribution of endophytic structures are illustrated. The use of stereoscopic techniques in conjunction with removal of host cytoplasm permits detection of subtle morphological alterations and progressive deterioration of the fungal component.

Cell Wall↗
PubMed1975

Electron microscopy of vesicular-arbuscular mycorrhizae of yellow poplar. II. Intracellular hyphae and vesicles.

Intracellular hyphae and vesicles in mycorrhizal roots of yellow poplar were examined by electron microscopy. An investing layer of host wall material and cytoplasm enclosed the endophyte within the cells. Young developing hyphae contained abundant cytoplasm and few vacuoles. As hyphae matured, they became highly vacuolated and accumulated carbohydrate (glycogen) and lipid reserves. Mature vesicles were engorged with lipid droplets, possessed a trilaminate wall and were also enclosed by host wall material and cytoplasm. Compared with uninfected cells, infected cortical cells showed an increase in cytoplasmic volume, enlarged nuclei, and a reduction of starch reserves. Host nuclei were always proximal to the hyphae during hyphal development and deterioration. While other cytoplasmic components of infected and uninfected cells were comparable large electron-dense bodies occurred in vacuoles of most cells containing hyphae. Deterioration of intracellular hyphae occurred throughout the samples examined. Septa separated functional and degenerating portions of the hyphae. Hyphal deterioration involved degeneration and ultimate disappearance of fungal cytoplasm as well as collapse of hyphal walls. Based on these observations, the authors hypothesize that deterioration of the endophyte may release significant quantities of mineral nutrients, via hyphal contents, which are absorbed by the host.

Cell Wall↗
PubMed1975

Electron microscopy of vesicular-arbuscular mycorrhizae of yellow poplar. III Host-endophyte interactions during arbuscular development.

Scanning- and transmission-electron microscopy were used to examine developing and mature functional arbuscules in mycorrhizal roots of yellow poplar. Arbuscules developed from intracellular hyphae which branched repeatedly upon penetration into the host cells. Intermediate and late stages of developemnt were characterized by the production of numerous, short, bifurcate hyphae throughout the arbuscule. Mature arbuscules exhibited a coralloid morphology which resulted in a considerable increase in the surface area of the endophyte exposed within the host cells. Distinctive ultrastructural features of arbuscular hyphae included osmiophilic walls, nuclei, abundant cytoplasm, glycogen, and numerous small vacuoles. All arbuscular components were enclosed by host wall material and cytoplasm during development and at maturity. In infected cells, host nuclei were enlarged and the cytoplasm associated with the arbuscular branches typically contained abundant mitochondria, endoplasmic reticulum, and proplastids. Ultrastructural observations suggested that nutrient transfer may be predominantly directed toward the fungal endophyte during arbuscular development and while mature arbuscules remain functional.

Cell Wall↗
PubMedDate not supplied

Predictive values of umbilical artery waveforms and repeat cardiotocography in pregnancies complicated by nonreactive cardiotocography.

Fifty patients with a nonreactive nonstress cardiotocograph were studied. All had measurement of the A/B ratio of the umbilical artery Doppler waveform and 39 also had a repeat nonstress test. The positive and negative predictive values of the repeat nonstress test were 61% and 94%, respectively, and those of the A/B ratio were 67% and 83%. A repeat nonreactive nonstressed cardiotocograph appeared to be a test of high sensitivity (92%) and the A/B ratio a test of high specificity (83%). It is suggested that the combination of the two tests may prove a powerful predictor of adverse perinatal outcome.

Blood Flow Velocity↗