Errorless performance in a two-frame apparent motion task using high contrast stimuli, a failure to replicate Cleary (1990)
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Contrast Sensitivity↗
Biomedical subjects
M J Cox
Publications and source records attributed to M J Cox.
Computerized Arden grating technique for the measurement of the contrast sensitivity function.
We developed a computerized contrast sensitivity function (CSF) measurement technique using a computer graphics board to reproduce Arden grating type stimuli. The reliability of the procedure was examined by evaluating the standard deviation of repeated test differences. The validity of the procedure was tested by comparison with both a manual method of adjustment technique and with Regan low contrast letter charts. The procedure has been performed at a number of exposure speeds to account for the effect of reaction time and attention of the results. The optimum exposure time for the procedure has been found to be greater than or equal to 22 s. As a result of the benefits of automation the test is quick, produces test-retest correlation coefficients for the five spatial frequencies tested which are similar to those for the method of adjustment procedure (0.76 to 0.91 for the computerized test, 0.66 to 0.88 for the signal generator measurements), and produces values which correspond closely with the method of adjustment measurement. It has the advantages of utilizing hardware already available in many computer systems, having a constant mean luminance across the screen, and having a constant and optimized presentation time.
Adult↗
Cloning, sequencing, and enhanced expression of the dihydropteroate synthase gene of Escherichia coli MC4100.
The Escherichia coli gene coding for dihydropteroate synthase (DHPS) has been cloned and sequenced. The protein has 282 amino acids and a compositional molecular mass of 30,314 daltons. Increased expression of the enzyme was realized by using a T7 expression system. The enzyme was purified and crystallized. A temperature-sensitive mutant was isolated and found to express a DHPS with a lower specific activity and lower affinities for para-aminobenzoic acid and sulfathiazole. The allele had a point mutation that changed a phenylalanine codon to a leucine codon, and the mutation was in a codon that is conserved among published DHPS sequences.
Amino Acid Sequence↗
Comparison of computer-assisted versus manual optic nerve head pallor measurement.
Computerized methods of examining the optic nerve head offer the possibility of providing consistent, reliable, and accurate measurements of the nerve head. The reliability and accuracy of such instruments must be examined to confirm this. This research was undertaken to gain an insight into the accuracy of such a system, which takes its measurements by tracking the luminance boundaries of the pallor area and the optic nerve head after an observer marks the starting points. Measurements made using the computer-tracked boundary were compared with those made using a boundary tracked by an image-processing system of greater sophistication, the human visual system. The comparison was made with images from one eye from each of 30 subjects (10 normal, 10 ocular hypertensive, and 10 glaucomatous). The measurements were made by two observers. Each observer made each set of measurements twice, and the results were analyzed with a five-factor analysis of variance. The measurements made using the computerized method did not differ significantly from those made using the manual boundary tracking for the two observers, and they were highly correlated (r = 0.99 for the area pallor-disc ratio). Significant differences (P < 0.05) were found between the two observers in both the manual and computerized boundary tracking. The computerized method, however, did not find differences between the two sets of measurements made by each observer; the manual method did show such differences. The computerized method appeared to trace the luminance boundaries successfully in the image, and it might reduce errors related to the observer marking the starting points on different occasions.(ABSTRACT TRUNCATED AT 250 WORDS)
Analysis of Variance↗
The distribution of macular pigment in human albinos.
This study investigated the variation in density of macular pigment across the central retina in normal and albino subjects. Luminance profiles were measured using a fundus camera and digital video techniques. The normal group had pigment spatial distributions consistent with previous studies. The albinos had no variation in absorbance across the central retina.
Adolescent↗
Inter-observer variability in a computer-assisted optic nerve head assessment system.
A computer-assisted optic nerve head analysis system was used to measure parameters of the optic nerve head from one eye from each of 15 subjects. The subjects comprised three groups of five, being glaucomatous, ocular hypertensive, and normal. The 15 images produced were analysed in a randomized masked fashion by ten observers of varying levels of experience to test the inter-observer variability of the measurements. An analysis of variance demonstrated significant differences in the measurements made by the different observers. Fisher's F-test showed that all the parameters assessing the pallor area of the optic nerve head had significantly lower inter-image variance (reported previously) compared with inter-observer variance (at the 5% level). Variability in taking measurements of the pallor area of the optic nerve head is mainly due to observer variations rather than the image variations.
Glaucoma↗
Computer-assisted optic nerve head assessment.
An IBM-compatible microcomputer-based system has been developed for the assessment of pallor area size, shape and degree of pallor in the optic nerve head. The system allows the instantaneous capture of optic nerve head images, and software analysis of these images produces measurements of the pallor area in about 2 min. The various methods of image analysis are presented. These are enhancement, thresholding, and digital filtering for edge detection. The results of their application to the images of the optic nerve head are shown. The results of the system's analysis on a particular optic nerve head are also shown. The potential uses and difficulties of such a system are discussed.
Algorithms↗
Inter- and intra-image variability in computer-assisted optic nerve head assessment.
A computer-assisted system for optic nerve head analysis was used to measure one eye from each of 15 subjects. These comprised three groups of five, being glaucomatous, ocular hypertensive, and normal subjects. Each eye was imaged with the system 10 times. The 150 images produced were analysed in a randomized masked fashion to test the inter-image variability of the measurements. Linear pallor: disc ratios had mean coefficients of variation of 6.6% (normal), 4.8% (ocular hypertensive) and 2.6% (glaucomatous). This trend of reducing coefficient of variation from normal to ocular hypertensive to glaucomatous was reflected in other parameters also. Intra-image variability was assessed by analysing one image from each subject 10 times, in a randomized masked fashion. The mean coefficients of variation of the linear pallor: disc ratios were 6.3% (normal), 3.6% (ocular hypertensive) and 3.2% (glaucomatous). Fisher's F-test showed that no parameters had significantly lower intra-image rather than inter-image variance (at the 5% level). Variability is apparently due to operator variations rather than the image variations.
Glaucoma↗
Seroepidemiological study of the transmission of the mumps virus in St. Lucia, West Indies.
A seroepidemiological study of the prevalence of mumps virus specific antibodies reveals a pattern of endemic persistence on the island of St Lucia in the West Indies. In the unvaccinated population the proportion seropositive rose rapidly in the child age classes to attain a stable plateau close to unity in value in the teenage and adult age groups. The average age at infection was estimated to be between 3 and 4 years of age and the average duration of detectable levels of maternally derived antibodies was approximately 3 months. Analyses based on mathematical models of the transmission dynamics of the virus suggest that in excess of 75% of each cohort of 1- to 2-year-old children must be effectively immunized to eliminate mumps virus transmission. A mumps radial haemolysis test, developed for quantitative measurements of antibody, is discussed.
Adolescent↗
Marriage, adult adjustment, and early parenting.
The impact of parents' marriages, measured prenatally, on their parenting of firstborn, 3-month-old infants was assessed. Though the association between marriage and parenting was the focus, adult psychological adjustment was measured also to rule out the alternative hypothesis that psychological adjustment relates to both marital quality and parenting quality and accounts for any association between them. Hierarchical multiple regression analyses in which parental adjustment was entered first as a covariate were used to test the relation between close/confiding marriages and parenting of 3-month-old infants. From the findings, it was concluded that even when differences in individual psychological adjustment are taken into account, mothers are warmer and more sensitive with their infants and fathers hold more positive attitudes toward their infants and their roles as parents when they are in close/confiding marriages. It is asserted that qualities of marriage play an important part in the development of parent-child relationships.
Female↗
Immobilized fluorogenic substrates for proteinases: a method for visualization and quantitation of elastase release from human monocytes.
An elastase-specific fluorogenic substrate, 6-(N-carbo-benzoxy-L-alanyl-L-alanyl-L-alanylamido)-qu inoline, was synthesized and immobilized via the fluorophoric group to an alkylatable derivative of polyacrylamide microspheres. Upon hydrolysis by elastase, the proteolytic product of the reaction fluorescences with a characteristic greenish-yellow light corresponding to the presence of the 1-alkyl-6-aminoquinolinium ion. This method has been applied to detect the elastase activity released from monocytes grown on the microspheres. Because the fluorescent product is covalently attached to the microsphere and cannot diffuse away from the site of reaction, it is possible to identify individual cells releasing the proteinase molecules. These experiments demonstrate that covalently immobilized fluorogenic substrates can be used for direct visualization and quantitation of proteinase activity from individual cells in culture.
Acrylic Resins↗
Role of ubiquitin conformations in the specificity of protein degradation: iodinated derivatives with altered conformations and activities.
Three iodinated derivatives of ubiquitin have been synthesized and these derivatives have been characterized in the ubiquitin-dependent protein degradation system. With chloramine-T as the oxidant, a derivative containing monoiodotyrosine is formed in the presence of 1 M KI and a derivative containing diiodotyrosine is produced in the presence of 1 mM KI. These derivatives exhibit phenolate ionizations at pH 9.2 and 7.9 with absorbance maxima at 305 and 314 nm, respectively. In addition to modification of the tyrosine residue, these conditions lead to the oxidation of the single methionine residue and iodination of the single histidine residue [M.J. Cox, R. Shapira, and K.D. Wilkinson (1986) Anal. Biochem. 154, 345-352]. Iodination of ubiquitin under these conditions renders the protein sensitive to hydrolysis by trypsin and results in an enhanced susceptibility to alcohol-induced helix formation. When the derivatives are tested in the ATP: pyrophosphate exchange reaction catalyzed by the ubiquitin adenylating enzyme, they are found to exhibit activity comparable to the native protein. When these derivatives are tested for the ability to act as a cofactor in the ubiquitin-dependent protein degradation system, they are both found to support a rate of protein degradation that is twice that of native ubiquitin. At high concentrations of derivatives, the rate of protein degradation is inhibited, while the steady state level of conjugates increases. Thus, the free derivatives inhibit the protease portion of the reaction, but are fully active in the activation and conjugation portions of the reaction. With iodine as the modification reagent, monoiodination of tyrosine is the predominant reaction. This derivative exhibits activity similar to native ubiquitin. Thus, it appears that modification of the histidine residue is responsible for the increased activity of the more highly iodinated derivatives. The enzymes of the system must recognize different portions of the ubiquitin structure, or different conformations of ubiquitin that are affected by the iodination of the histidine residue. These results suggest a conformational change of the ubiquitin molecule may be important in determining the rate and specificity of proteolysis.
Circular Dichroism↗
Synthesis and characterization of ubiquitin ethyl ester, a new substrate for ubiquitin carboxyl-terminal hydrolase.
A new substrate for ubiquitin carboxyl-terminal hydrolase, the carboxyl-terminal ethyl ester of ubiquitin, has been synthesized by a trypsin-catalyzed transpeptidation. In the presence of 1.6 M glycylglycine ethyl ester, trypsin removes the carboxyl-terminal glycylglycine of ubiquitin and replaces it with the dipeptide ester. The equilibrium mixture under these conditions contains 30% ubiquitin ethyl ester and 70% hydrolysis product, the 74-residue fragment of ubiquitin. Ubiquitin ethyl ester can be purified by gel filtration and ion-exchange chromatography. The structure of this product has been verified by identification of the products of base hydrolysis, tryptic cleavage in aqueous solution, and peptide mapping. When ubiquitin ethyl ester is incubated with purified ubiquitin carboxyl-terminal hydrolase, specific cleavage of the ester linkage is observed. A rapid, sensitive assay is described utilizing high-performance liquid chromatography. By use of this assay, it has been shown that ubiquitin carboxyl-terminal hydrolase is inactivated in the absence of thiols. Optimal protective effects are seen with 10 mM dithiothreitol. The rate of catalysis is maximal at pH 8.5, with evidence for catalytically important groups with pK values of 5.2, 7.6, and 9.5. These findings are consistent with the participation of a thiol group in the active site. Native ubiquitin is a competitive inhibitor of ubiquitin ethyl ester hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)
Erythrocytes↗
Structure and activities of a variant ubiquitin sequence from bakers' yeast.
Ubiquitin is an extremely conserved protein, with an identical sequence throughout the animal kingdom. However, the gene sequence of the yeast protein [Ozkaynak, E., Finley, D., & Varshavsky, A. (1984) Nature (London) 312, 663-666] predicts three amino acid differences. This implies that some functions or binding interactions of ubiquitin are different in yeast and animal cells. In an effort to define these differences, ubiquitin has been purified to homogeneity from bakers' yeast and characterized. Amino acid analysis of the protein and the isolated tryptic peptides confirms the primary structure of this protein as predicted from the gene sequence. This result indicates that the gene sequenced is the transcriptionally active gene from yeast. The conformation of yeast ubiquitin is similar to human ubiquitin as judged by circular dichroism, sensitivity to trypsin, and Stokes radius. Yeast and animal ubiquitins show identical activities in supporting ubiquitin-dependent protein degradation and in the ATP-pyrophosphate exchange reaction catalyzed by the purified ubiquitin-adenylating enzyme. Thus, the three conservative amino acid differences between yeast and animal ubiquitins have very little effect on the structure of ubiquitin or its activity in the ubiquitin-dependent proteolytic system. These results suggest that at least some of the evolutionary pressure preventing sequence variation among animal ubiquitins stems from one or more of its nonproteolytic functions.
Amino Acid Sequence↗
Tryptic peptide mapping of ubiquitin and derivatives using reverse-phase high performance liquid chromatography.
The conditions for tryptic digestion and subsequent peptide mapping of the ATP-dependent proteolysis cofactor ubiquitin and its derivatives are described. In aqueous solution, the native ubiquitin which is composed of 76 amino acids undergoes only a single cleavage at arginine-74. Full digestion of ubiquitin was obtained in 6.5 M urea, although cleavages at lysine-33 and arginine-74 were slow. Peptide mapping was achieved by reverse-phase high-performance liquid chromatography with a C18 column using a trifluoroacetic acid/triethylamine buffer system and acetonitrile as eluants. The peptides, separated using a linear gradient, were identified by amino acid analysis. Derivatives analyzed by this method include oxidized, monoiodotyrosyl, and diiodotyrosyl ubiquitin. This technique will be useful in examining peptides of chemically modified ubiquitin with respect to extent and specificity of modification. In addition, this technique will be useful in comparing ubiquitin peptides of different organisms.
Amino Acid Sequence↗
125I-ubiquitin is kinetically invalid as a tracer radiolabel for studies of ATP-dependent proteolysis.
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ATP-Dependent Proteases↗