Biomedical subjects
Publications and source records attributed to L N Johnson.
Three cases of cranial nerve paresis following epidural and spinal anesthesia are presented. One patient had combined oculomotor and abducens nerve paresis, while the other two patients had solitary abducens nerve pareses. The authors postulate that mechanical traction is the cause of cranial nerve paresis following epidural and spinal anesthesia.
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The crystal structure of R-state glycogen phosphorylase b has been determined at 2.9 A resolution. A comparison of T-state and R-state structures of the enzyme explains its cooperative behaviour on ligand binding and the allosteric regulation of its activity. Communication between catalytic sites of the dimer is provided by a change in packing geometry of two helices linking each site with the subunit interface. Activation by AMP or by phosphorylation results in a quaternary conformational change that switches these two helices into the R-state conformation.
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The binding of beta-glycerophosphate (glycerol-2-P) to glycogen phosphorylase b in the crystal has been studied by X-ray diffraction at 3 A resolution. Glycerol-2-P binds to the allosteric effector site in a position close to that of AMP, glucose-6-P, UDP-Glc, and phosphate. In this position, glycerol-2-P is stabilized through interactions of its phosphate moiety with the guanidinium groups of Arg 309 and Arg 310 which undergo conformational changes, and the hydroxyl group of Tyr 75, while the same residues and solvent are involved in van der Waals interactions with the remaining part of the molecule. Kinetic experiments indicate that glycerol-2-P partially competes with both the activator (AMP) and the inhibitor (glucose 6-phosphate) of phosphorylase b. A comparison of the positions of glycerol-2-P, AMP, glucose 6-phosphate, UDP-Glc, and Pi at the allosteric site is presented.
An in vitro investigation was undertaken to evaluate the bonding of orthodontic appliances onto lingual surfaces; 53 maxillary premolars, 37 mandibular premolars, and 37 mandibular incisors were used. Brackets were bonded onto the lingual and labial surfaces and fractured with an Instron machine. Enamel damage associated with debonding also was assessed. Results indicated comparable bond strengths (t test) on lingual (Li) and labial (La) surfaces: maxillary premolars--Li-138.2 kg/cm2, La-127.7 kg/cm2; mandibular premolars--Li-136.2 kg/cm2, La-121.6 kg/cm2; and mandibular incisors--Li-166.3 kg/cm2, La-161.1 kg/cm2. Adaptation of lingual bracket bases resulted in significantly higher lingual bond strengths for maxillary premolars (166.9 kg/cm2) and mandibular premolars (180.4 kg/cm2), but not for mandibular incisors (149.2 kg/cm2). On debonding, the percentages of lingual surfaces exhibiting horizontal "crescent-shaped" fracture lines and enamel fragment fractures were significantly higher (x2 test) than the corresponding percentages for labial surfaces: maxillary premolars--Li-67.9%, La-5.7%; mandibular premolars--Li-62.2%, La-13.5%; and mandibular incisors--Li-43.2%, La-18.9%. Furthermore, an increase in vertical enamel fracture lines (cracks) subsequent to debonding was seen labially and lingually. Bonding procedures for lingual surfaces should be identical to those advocated for labial surfaces. Care during debonding must be exercised to eliminate possible enamel damage.
Lipoid proteinosis (Urbach-Wiethe disease) is a rare autosomal recessive disorder associated with deposition of hyalinised material in the skin, mucous membrane, and brain. Corectopia has not been described in this disorder. A case is presented of lipoid proteinosis with bilateral corectopia.
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Riddoch's phenomenon, in which a stimulus is perceived during movement but not with static presentation, has been reported for occipital lobe and anterior visual pathway disorders. To our knowledge, a threshold for movement detection in the affected hemifield has not been reported. A case of statokinetic dissociation from occipital lobe infarction is presented in which a threshold range for movement detection was identified. The possible role of separate channels in the visual cortex for analysis of kinetic and static stimuli is suggested.
A simplified algorithm is presented for determining the palsied cyclovertical muscle, once the three-step test is completed. The examiner is required only to remember the three-step test characteristics of two muscles (right superior oblique, left inferior oblique) and the Knight's move in chess.
Alginate impression materials are universally accepted for the fabrication of dental casts. Since casts reflect a direct link between the patient and the dentist, it is imperative that their surfaces be of highest quality. This study assesses the quality of dental cast surfaces when poured against alginate impression materials. A standardized system previously designed by the authors allowed the evaluations of 16 different gypsums poured against 32 different alginate surfaces, including a plexiglass control surface. Five hundred and twelve gypsum surfaces were produced and evaluated for quality in the following manner: 1) assessed visually with the aid of photomicrographs for surface smoothness; 2) evaluated by a modified Vickers scratch test for surface hardness. The results indicate that certain gypsum-alginate combinations provide superior surfaces with respect to hardness and smoothness, while other combinations suggest a high degree of incompatibility.
A comparison of the refined crystal structures of dimeric glycogen phosphorylase b and a reveals structural changes that represent the first step in the activation of the enzyme. On phosphorylation of serine-14, the N-terminus of each subunit assumes an ordered helical conformation and binds to the surface of the dimer. The consequent structural changes at the N- and C-terminal regions lead to strengthened interactions between subunits and alter the binding sites for allosteric effectors and substrates.
Regions of low packing density in the vicinity of the catalytic site of glycogen phosphorylase b are described with the aid of a computer program that generates a contour map in which the contour level is inversely proportional to the packing density in the protein. It is shown that, although there is no direct route from the catalytic site to the surface, there are two possible channels that could allow access for substrates following conformational changes in the enzyme. The first channel, channel 1, leads from the catalytic site to the surface close to the nucleoside inhibitor site and requires movements of residues 280-285 and Arg 569 in order to obtain access. Previous crystallographic experiments have shown that in the presence of substrates or R-state inhibitors these parts of the polypeptide chain undergo large conformational changes. The properties of the second channel (channel 2), which is the more extensive channel, have been investigated with the potent beta-glycosidase inhibitor D-gluconohydroximo-1,5-lactone N-phenylurethane (PUG). Crystallographic binding studies at 2.4-A resolution show that the compound binds neatly at the catalytic site of phosphorylase b. The glucopyranosylidene ring, in the half-chair conformation, occupies a similar but not identical position (shift about 0.6 A) to that occupied by other glucosyl compounds bound at the catalytic site.(ABSTRACT TRUNCATED AT 250 WORDS)
UDP-glucose is an R-state inhibitor of glycogen phosphorylase b, competitive with the substrate, glucose 1-phosphate and noncompetitive with the allosteric activator, AMP. Diffusion of 100 mM UDP-glucose into crystals of phosphorylase b resulted in a difference Fourier synthesis at 0.3-nm resolution that showed two peaks: (a) binding at the allosteric site and (b) binding at the catalytic site. At the allosteric site the whole of the UDP-glucose molecule can be located. It is in a well defined folded conformation with its uracil portion in a similar position to that observed for the adenine of AMP. The uracil and the glucose moieties stack against the aromatic side chains of Tyr-75 and Phe-196, respectively. The phosphates of the pyrophosphate component interact with Arg-242, Arg-309 and Arg-310. At the catalytic site, the glucose-1-P component of UDP-glucose is firmly bound in a position similar to that observed for glucose 1-phosphate. The pyrophosphate is also well located with the glucose phosphate interacting with the main-chain NH groups at the start of the glycine-loop alpha helix and the uridine phosphate interacting through a water molecule with the 5'-phosphate of the cofactor pyridoxal phosphate and with the side chains of residues Tyr-573, Lys-574 and probably Arg-569. However the position of the uridine cannot be located although analysis by thin-layer chromatography showed that no degradation had taken place. Binding of UDP-glucose to the catalytic site promotes extensive conformational changes. The loop 279-288 which links the catalytic site to the nucleoside inhibitor site is displaced and becomes mobile. Concomitant movements of residues His-571, Arg-569, and the loop 378-383, together with the major loop displacement, result in an open channel to the catalytic site. Comparison with other structural results shows that these changes form an essential feature of the T to R transition. They allow formation of the phosphate recognition site at the catalytic site and destroy the nucleoside inhibitor site. Kinetic experiments demonstrate that UDP-glucose activates the enzyme in the presence of high concentrations of the weak activator IMP, because of its ability to decrease the affinity of IMP for the inhibitor site.
The diagnosis of pituitary apoplexy, an often-fatal disorder, is frequently delayed. Computed tomographic (CT) scanning has been shown to be useful in the detection of pituitary apoplexy; however, the value of magnetic resonance imaging (MRI) is yet to be determined. The MRI and CT scans of three consecutive and histopathologically proved cases of pituitary apoplexy were reviewed. The MRI scans obtained at least five days after the onset of symptoms suggested pituitary apoplexy (hemorrhage) in all three cases, while CT scanning indicated pituitary hemorrhage in only one case. Increased signal on the T1-weighted image was the hallmark on MRI scans in all three cases. These findings suggest that MRI scanning may be superior to CT scanning in identifying pituitary apoplexy, at least in the subacute phase.
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