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Biomedical subjects

L N Johnson

Publications and source records attributed to L N Johnson.

At least 145 records · Page 8Linked to original sources
PubMed1988

The sequence around the phosphorylation site of the porcine heart type phosphorylase isoenzyme.

1. A tetradecapeptide containing the phosphorylation site was obtained from 32P-labelled pig heart phosphorylase a isoenzyme by alpha-chymotryptic digestion. 2. The peptide was purified by Mono S cation-exchange chromatography and reversed-phase HPLC. 3. The phosphorylated residue was identified as Ser and the sequence was determined: T D G E R R K Q I S V R G L. 4. The sequence was compared to the known sequences of muscle and liver type isophosphorylases and the structural consequences of the amino acid residue exchanges were predicted.

Amino Acid Sequence↗
PubMed1988

Correlation of afferent pupillary defect with visual field loss on automated perimetry.

The study was designed to ascertain the relationship between visual loss in the central 30 degrees of vision and the density of the relative afferent pupillary defect (APD). The APD of 26 patients was quantified using a neutral log density filter. The mean threshold light sensitivity on Humphrey automated perimetry (Program 30-1) of one eye was substracted from the fellow eye total to yield the interocular visual field difference (VFD). A direct correlation was noted such that the log density of the APD increased linearly with an increase in VFD (r = 0.69, P = 0.0001). In the absence of ptosis or ocular media opacification, a VFD greater than 8.7 that is not associated with an APD is suggestive of functional visual loss. Four patients had an APD despite normal static automated perimetry, indicating that an APD may be one of the earliest signs of retinal ganglion cell or axonal dysfunction.

Adolescent↗
PubMed1988

Contact-lens correction of aphakic infants and children: early behavioral and VEP results.

We studied 11 infants and young children fitted with contact lenses for correction of unilateral or bilateral aphakia. The visual-acuity estimates obtained from behavioral cues (alternate-cover test and central-fixation monitoring) were compared with results obtained from pattern-reversal visual-evoked potentials (VEP). We conclude that VEP may be more sensitive than behavioral observation for monitoring the visual progress in certain aphakic children.

Aphakia↗
PubMed1987

Pyridoxal phosphate site in glycogen phosphorylase b: structure in native enzyme and in three derivatives with modified cofactors.

The detailed environment of the essential cofactor pyridoxal 5'-phosphate in glycogen phosphorylase b, resulting from crystallographic refinement at 1.9-A resolution, is described. The pyridoxal ring is buried in a nonpolar site containing three aromatic rings while the 5'-phosphate group is highly solvated and makes only three direct contacts to the protein. The pyridine nitrogen interacts via a water with protein atoms [main chain carbonyl oxygen (Asn-133) and OH of tyrosine (Tyr-90)]. The crystal structures of three active derivatives of phosphorylase reconstituted with 5'-deoxypyridoxal 5'-methylenephosphonate (PDMP), 6-fluoropyridoxal 5'-phosphate (6-FPLP), and pyridoxal (PL) in place of the natural cofactor have been determined at 2.5-A resolution. The results for PDMP-phosphorylase show a closer proximity of the phosphonate group to the NZ atom of a lysine (Lys-574) than that observed in the native enzyme, consistent with 31P NMR studies that have shown a change in ionization state of the phosphonate group compared to the native cofactor phosphate. The replacement of the polar 5'-ester linkage by a CH2 group results in a small shift of a water and its hydrogen-bonded tyrosine (Tyr-648). In 6-FPLP-phosphorylase the fluorine is accommodated with no significant change in structure. It is suggested that substitution of the electronegative fluorine at the 6-position may result in lower activity of 6-FPLP-phosphorylase through a strengthening of hydrogen-bonded interactions to the pyridine nitrogen N1.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Catalysis in the crystal: synchrotron radiation studies with glycogen phosphorylase b.

Direct observation of the progress of a catalysed reaction in crystals of glycogen phosphorylase b has been made possible through fast crystallographic data collection achieved at the Synchrotron Radiation source at Daresbury, UK. In the best experiments, data to 2.7 A resolution (some 108,300 measurements; 21,200 unique reflections) were measured in 25 min. In a series of time-resolved studies in which the control properties of the enzyme were exploited in order to slow down the reaction, the conversion of heptenitol to heptulose-2-phosphate, the phosphorylysis of maltoheptaose to yield glucose-1-phosphate and the oligosaccharide synthesis reaction involving maltotriose and glucose-1-phosphate have been monitored in the crystal. Changes in electron density in the difference Fourier maps are observed as the reaction proceeds not only at the catalytic site but also the allosteric and glycogen storage sites. Phosphorylase b is present in the crystals in the T state and under these conditions exhibits low affinity for both phosphate and oligosaccharide substrates. There are pronounced conformational changes associated with the formation and binding of the high-affinity dead-end product, heptulose-2-phosphate, which show that movement of an arginine residue, Arg 569, is critical for formation of the substrate-phosphate recognition site. The results are discussed with reference to proposals for the enzymic mechanism of phosphorylase. The feasibility for time-resolved studies on other systems and recent advances in this area utilizing Laue diffraction are also discussed.

Animals↗
PubMed1987

Alteration of the visual evoked potential by macular holes: comparison with optic neuritis.

Nine patients with maculopathy (macular holes, macular cysts, and lamellar holes) and ten patients with optic neuritis were examined in order to determine changes in the visual evoked potential (VEP) in response to pattern-reversal stimulation. Eyes with lamellar holes had normal P100 latency, but eyes with macular cysts and macular holes had prolonged P100 latency. Eyes with optic neuritis exhibited greater prolongation of the P100 latency than eyes with macular holes. In contrast, eyes with macular holes had a greater reduction in the steady-state VEP amplitude than eyes with optic neuritis. The prolonged latency occurring in maculopathy may be due to a peculiar amplitude summation noted with half-field VEP, rather than to a true conduction delay like that seen in eyes with optic neuritis. The amplitude slope, which is usually positive in normal controls, was negative for 85.7% of eyes with macular holes and 69.2% of eyes with optic neuritis. The negative amplitude slope may represent a subtle defect in retinal ganglion X cells. Eyes with significantly lower values for four or more of the nine central test points on quantitative automated perimetry had negative amplitude slopes and prolonged P100 latency.

Adult↗
PubMed1986

Magnetic resonance imaging of craniopharyngioma.

Craniopharyngiomas are common tumors located in the suprasellar region. Contrast enhancement, cyst formation, and calcification are the three characteristic features of craniopharyngiomas on computed tomographic scan. More than 90% of suprasellar craniopharyngiomas exhibit at least two of these three features, thus providing easy radiologic detection. We treated a 41-year-old man in whom a large suprasellar craniopharyngioma producing severe visual loss was not detected by computed tomography but was easily identified with magnetic resonance imaging. Thus, despite high-resolution computed tomographic scan, large suprasellar craniopharyngiomas can be missed. Magnetic resonance imaging may be superior to computed tomography in detecting these tumors.

Adult↗
PubMed1986

Sphenoid sinus mucocele (anterior clinoid variant) mimicking diabetic ophthalmoplegia and retrobulbar neuritis.

Two patients (two men, 56 and 59 years old) had sphenoid sinus mucocele originating in the anterior clinoid process. In one case the mucocele initially mimicked diabetic ophthalmoplegia with pupil-sparing palsy of the oculomotor (third) nerve. After resolution of the palsy, severe visual loss developed with minimal recovery of vision after surgery. The second patient had recurrent episodes of retrobulbar optic neuropathy with optic atrophy and decreased vision. Visual loss from sphenoid sinus mucoceles is usually associated with a poor prognosis if surgical treatment is delayed more than seven to ten days.

Diabetes Complications↗
PubMed1985

Crystallization of pig skeletal phosphorylase b. Purification, physical and catalytic characterization.

A new method for purification and crystallization of pig skeletal muscle phosphorylase b is presented. The ease of crystallization in the presence of 1 mM AMP and 1 mM spermine has permitted the study of some physical, chemical and enzymatic properties of the enzyme. The crystalline pig phosphorylase b gave a single band on SDS polyacrylamide gels of the same mobility as rabbit muscle phosphorylase subunit. Ultracentrifugation experiments showed that pig phosphorylase b exists in a dimeric form (S20,w = 8.4 S). No association occurred at 20 degrees C under conditions where rabbit phosphorylase b can be tetramerized; pig phosphorylase b was only 30% associated from dimer to tetramer at 13 degrees C. Pig phosphorylase b is highly stable to freezing and its specific activity did not change appreciably upon prolonged storage in the cold. Pig and rabbit phosphorylases b have comparable Vmax and Km values towards the substrate and the activator. However, there is an essential difference between the two enzymes in that pig phosphorylase b is not significantly inhibited by glucose 6-phosphate, which is a powerful inhibitor of the rabbit enzyme. Two different crystal forms of pig phosphorylase b were obtained which are small for X-ray diffraction studies. Diffusion of spermine into tetragonal crystals of rabbit phosphorylase b resulted in a difference Fourier synthesis at 3 A resolution that showed no strong indication of specific binding.

Animals↗
PubMed1985

Impaired cardiopulmonary baroreflex control of renal nerves in renal hypertension.

We recently reported that arterial baroreflex control of renal nerve traffic is impaired in renal hypertensive rabbits. The purpose of this study was to determine if vagal cardiopulmonary baroreflex control of renal nerve traffic is also impaired. Experiments were performed in 10 hypertensive (mean arterial pressure +/- SE in conscious state, 110 +/- 3 mm Hg) and 10 normotensive (79 +/- 1 mm Hg) chloralose-anesthetized rabbits. Responses to graded blood volume expansion (+5, +10, +15 ml/kg) with dextran in saline were recorded with all baroreflexes intact, after sinoaortic baroreceptor denervation, and after vagotomy. With arterial and cardiopulmonary baroreflexes intact, volume expansion resulted in decreases in renal nerve traffic of -12 +/- 2%/mm Hg increase in left atrial pressure in normotensive rabbits, but of only -5 +/- 2%/mm Hg in the hypertensive rabbits (P less than 0.05). This difference is particularly striking in view of the larger maximum increases in arterial (25 +/- 7 vs. 12 +/- 3 mm Hg) and left atrial pressure (9 +/- 1 vs. 6 +/- 1 mm Hg) during volume expansion in hypertensive vs. normotensive rabbits. After sinoaortic baroreceptor denervation, the responses of normotensive rabbits were preserved (-11 +/- 3%/mm Hg), while those of hypertensive rabbits were impaired further (-2 +/- 1%/mm Hg). Vagotomy abolished responses of renal nerves to volume expansion in both groups. These data demonstrate striking impairment of vagal cardiopulmonary baroreflex control of renal nerve traffic in renal hypertension. Even though arterial baroreflexes have been shown to be abnormal in renal hypertension, they still may partially compensate for markedly impaired cardiopulmonary baroreflex control of the renal nerves.

Animals↗
PubMed1985

Central retinal artery occlusion following transfemoral cerebral angiography.

Whereas most ocular complications following cerebral angiography are benign and transient, central retinal artery occlusion following cerebral angiography produces severe permanent visual deficits in nearly 40% of patients. Physicians ordering and performing cerebral angiography should be aware of this complication, since immediate attention to patients with visual disturbance from central retinal artery occlusion may save useful vision. All previously reported cases of central retinal artery occlusion following cerebral angiography have occurred after direct percutaneous carotid angiography. We report a case of central retinal artery occlusion following transfemoral cerebral angiography using a #5 French catheter and meglumine iothalamate (Conray-60) contrast.

Arterial Occlusive Diseases↗
PubMed1984

Substrate-cofactor interactions for glycogen phosphorylase b: a binding study in the crystal with heptenitol and heptulose 2-phosphate.

The structural relationships between substrate and pyridoxal phosphate in glycogen phosphorylase b (EC 2.4.1.1) have been studied by X-ray diffraction experiments at 3-A resolution. Recent work [Klein, H. W., Im, M. J., & Helmreich, E. J. M. (1984) in Chemical and Biological Aspects of Vitamin B6 Catalysis (Evangelopoulos, A. E., Ed.) pp 147-160, Liss, New York] has shown that phosphorylase in the presence of inorganic phosphate catalyzes the conversion of heptenitol to heptulose 2-phosphate. The latter compound is a dead-end product and a most potent inhibitor (Ki = 14 microM). The X-ray diffraction studies show that heptenitol binds at the catalytic site of phosphorylase in a position essentially identical with that observed for the glucopyranose moiety of glucose 1-phosphate. Incubation of a phosphorylase b crystal for 50 h in a solution containing the substrates heptenitol and inorganic phosphate and the activators AMP and maltohetaose resulted in the formation of a phosphorylated product bound at the active site. The structure of this product, as analyzed by a difference Fourier synthesis at 3 A, is consistent with that of heptulose 2-phosphate. Analysis of the surrounding soak solution by thin-layer chromatography showed that heptulose 2-phosphate was produced under these conditions. Heptulose 2-phosphate binds with its glucopyranose moiety in the same position as that for glucose 1-phosphate, but there is a marked difference in phosphate positions. The presence of the methyl group in the beta-configuration in heptulose 2-phosphate forces a change in the torsion angle O5-C1-O1-P from 117 degrees as observe in glucose 1-phosphate to -136 degrees in heptulose 2-phosphate. The "down" position of the phosphate (with respect to the crystallographic z axis) results in a change in the distance between the 5'-phosphorus atom of the pyridoxal phosphate and the phosphorus atom of the substrate from 6.8 (with glucose 1-phosphate) to 4.5 A (with heptulose 2-phosphate). The closest distance between the phosphate oxygen of the cofactor and a phosphate oxygen of heptulose 2-phosphate is 2.7 A, and it is assumed that there must be a hydrogen bond between them. These observations are consistent with the NMR experiments reported in the preceding paper in which sharing of a proton between heptulose 2-phosphate and pyridoxal 5'-phosphate is observed [Klein, H.W., Im, M. J., Palm, D., & Helmreich, E. J. M. (1984) Biochemistry (preceding paper in this issue)].(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Monophosphate↗
PubMed1984

Allosteric interactions of glycogen phosphorylase b. A crystallographic study of glucose 6-phosphate and inorganic phosphate binding to di-imidate-cross-linked phosphorylase b.

The binding to glycogen phosphorylase b of glucose 6-phosphate and inorganic phosphate (respectively allosteric inhibitor and substrate/activator of the enzyme) were studied in the crystal at 0.3 nm (3A) resolution. Glucose 6-phosphate binds in the alpha-configuration at a site that is close to the AMP allosteric effector site at the subunit-subunit interface and promotes several conformational changes. The phosphate-binding site of the enzyme for glucose 6-phosphate involves contacts to two cationic residues, Arg-309 and Lys-247. This site is also occupied in the inorganic-phosphate-binding studies and is therefore identified as a high-affinity phosphate-binding site. It is distinct from the weaker phosphate-binding site of the enzyme for AMP, which is 0.27 nm (2.7A) away. The glucose moiety of glucose 6-phosphate and the adenosine moiety of AMP do not overlap. The results provide a structural explanation for the kinetic observations that glucose 6-phosphate inhibition of AMP activation of phosphorylase b is partially competitive and highly co-operative. The results suggest that the transmission of allosteric conformational changes involves an increase in affinity at phosphate-binding sites and relative movements of alpha-helices. In order to study glucose 6-phosphate and phosphate binding it was necessary to cross-link the crystals. The use of dimethyl malondi-imidate as a new cross-linking reagent in protein crystallography is discussed.

Adenosine Monophosphate↗
PubMed1984

Sinus tumors invading the orbit.

Forty -seven of 79 patients with sinus and paranasal tumors had clinical, radiographic or operative evidence of orbital involvement. Seventy percent of those patients with orbital extension had clinical or radiographic involvement of the orbit at the time of initial presentation. Common presenting signs and symptoms included proptosis, nasal obstruction or discharge, nasal mass, facial and/or eye pain, visual loss, facial and/or lid edema and diplopia. The most common tumor seen was squamous cell carcinoma. The maxillary sinus was the most frequent site of origin. Three patients were misdiagnosed as having sinusitis on initial evaluation. A diagnosis of sinusitis is tentative and should be reevaluated early with repeat roentgenographic studies and biopsy, especially in the presence of protracted facial and eye pain. When ordering CT scans, one must specifically request cuts of the base of the sinuses and skull as routine brain CT scans do not evaluate those regions. Two of four patients with intractable pain unrelieved by narcotics obtained pain relief with cisplatinum.

Adolescent↗