Showing papers by "University of York published in 1977"

Mechanical Properties of Mother of Pearl in Tension

Abstract: Mother of pearl, or nacre, is one of a number of characteristic skeletal structures of molluscs, occurring in cephalopods, gastropods and bivalves. It consists of plates of aragonite, about 0.3 $\mu $m thick, arranged in sheets, with a tenuous protein matrix. Mechanical tests of nacre from all three classes show that it has a tensile strength of between 35 and 110 MN m$^{-2}$. It is slightly viscoelastic, and shows marked, though not extensive, plastic deformation. The maximum measured strain was 0.018. While undergoing plastic deformation the material shows considerable optical changes. The regions where plastic flow is occurring show 'tension lines', probably equivalent to similar lines in bone. These are probably caused by voids forming in the protein matrix. The work of fracture is very different in different loading directions, being about 1.65 $\times $ 10$^{3}$ J m$^{-2}$ when fractured across the grain, and 1.5 $\times $ 10$^{2}$ J m$^{-2}$ when fractured along it. Nacre shows considerable ability to stop cracks. An attempt is made to explain qualitatively the mechanical behaviour of nacre in terms of its submicroscopic structure. It is concluded that the precise geometric arrangement of the plates is most important, and that this constraint may make nacre less suitable for shells that must be built quickly.

Number of trophic levels in ecological communities

Abstract: ECOLOGICAL food chains are typically short, consisting of not more than four or five trophic levels. This is usually explained by a reduction in the energy which is available to successive links in the food chain1,2. In contrast, we believe that the number of trophic levels is constrained by population dynamics and not by ecological energetics.

Harvesting natural populations in a randomly fluctuating environment.

Abstract: As harvesting effort and yield are increased, animal populations that are being harvested for sustained yield will take longer to recover from environmentally imposed disturbances. One consequence is that the coefficient of variation (the relative variance) of the yield increases as the point of maximum sustained yield (MSY) is approached. When overexploitation has resulted in a population smaller than that for MSY, high effort produces a low average yield with high variance. These observations accord with observed trends in several fish and whaling industries. We expect these effects to be more pronounced for a harvesting strategy based on constant quotas than for one based on constant effort. Although developed in a MSY context, the conclusions also apply if the aim is to maximize the present value of (discounted) net economic revenue.

Effects of plant type, size of geographical range and taxonomic isolation on number of insect species associated with British plants

Abstract: IN this communication we analyse the insects associated with higher plants other than trees, which have already received attention1–5. We are concerned with two basic problems. First, we ask how various types of higher plants differ in the numbers of species of insects which they support; we do this by comparing standard species–area curves1,3,6,7 for five different sorts of plants. Then we consider how the “taxonomic isolation” of a plant, measured simply as the number of related species in the same genus and geographical region, might also influence the total number of insect species to be found on it. We find that while plant type and geographical range have a considerable effect, taxonomic isolation does not, except in the case of monocotyledonous herbs.

Sustained Attention in Children at Risk for Schizophrenia: Report on a Continuous Performance Test

Abstract: • A test of sustained attention (the Continuous Performance Test) was administered to a group of children at risk for the eventual manifestation of schizophrenia and to a group of "normal control" children. Differences in performance between the two groups were attributable to poorer discriminability on the part of the high-risk subjects.

Influence of mycorrhizal infection on competition for phosphorus and potassium by two grasses

Abstract: Summary Lolium perenne L. and Holcus lanatus L. were grown together in 125-mm pots in a factorial experiment combining mycorrhizal infection and root competition. Both root competition and mycorrhizal infection gave a slight advantage to H. lanatus but, in combination, these produced considerable suppression of Lolium perenne. Root length of L. perenne was reduced by mycorrhizal infection, which was itself increased by competition. The competitive effects can be explained solely in terms of mycorrhizal infection.

Testing of some azo dyes and their reduction products for mutagenicity using Salmonella typhimurium TA 1538.

Abstract: A series of ten azo dyes as well as various single ring aromatic amines substituted on the benzene ring were tested for bacterial mutagenicity with Salmonella typhimurium TA 1538 using a soft-agar overlay method. Two dyes, sudan 2 and chrysoidin induced mutation but only in the presence of a rat liver preparation. Chrysoidin was the more active. Testing of its reduction products, aniline and 1,2,4-triaminobenzene showed a liver metabolite of the latter compound could be responsible for the mutagenic effect, having a comparable mutagenicity with 1,2-diamino-4-nitro-benzene, one of the mutagenic constituents of hair dyes. Structure-activity studies on a series of ring-substituted anilines indicated that mutagenic activity required at least two positions to be substituted with either amino or nitro groups, or one of each. The bacteria as well as the liver enzyme preparation may partake in the activation of these chemicals. The correlation between mutagenicity and carcinogenicity for this group of compounds is discussed.

Depressive mood and adolescent illicit drug use: A longitudinal analysis.

Abstract: Through the technique of Goodman's log linear method, longitudinal data are analyzed to clarify the relationship between depressive mood and illicit drug use among youths. A representative sample of adolescents (N = 8206) from New York State secondary schools was followed over one academic year. At one point in time, users of illicit drugs other than marihuana (multiple drug users) were significantly more depressed than either nonusers or users of marihuana only. Differing interactive relationships were found over time depending upon the drugs involved. Depressive mood was related to the onset of marihuana use among nonusers and to the termination of marihuana use among users. Depressive mood also predicted the use of other illicit drugs by marihuana users. While the beginning use of illicit drugs other than marihuana was positively associated with increased depressive mood, continued multiple drug use was related to reduced depressive mood.

Aflatoxin B -oxide generated by chemical or enzymic oxidation of aflatoxin B1 causes guanine substitution in nucleic acids.

Abstract: AFLATOXIN B1 (AFB1) has the highest biological activity of the four naturally occurring aflatoxins produced by the mould Aspergillus flavus1. Feeding studies have shown it to be the most potent liver carcinogen known for the rat2. Its ingestion by man is associated with the high primary liver cancer incidence in certain parts of Africa3. There is much evidence to suggest that AFB1 requires metabolism to exert both its carcinogenic and mutagenic effects (reviewed in ref. 4). Structure-activity5, as well as chemical studies6,7 indicate that the major route for activation proceeds through mixed function oxidase attack yielding the 8,9-oxide (previously called the 2,3-oxide but renumbered according to IUPAC recommendations). Attempts to chemically synthesise this metabolite have so far been unsuccessful. We report here that peracid oxidation of AFB1 generates AFB1-8,9-oxide, and that this latter compound reacts readily with nucleic acids to give adducts identical to those obtained after liver mixed function oxidase (MFO) activation in vitro and in vivo.

Synthesis of Exported Proteins by Membrane-Bound Polysomes from Escherichia coli

Abstract: A membrane-bound fraction of polysomes of Escherichia coli has been isolated after lysis of cells without the use of lysozyme. Protein-synthesis studies in vitro show that membrane-bound and free polysomes are different in the following respects. 1 Membrane-bound polysomes synthesize proteins which are exported from the cell. The products include proteins of the outer membrane and a secreted periplasmic protein, the maltose-binding protein. 2 The major product synthesized by free polysomes is elongation factor Tu, a soluble cyto-plasmic protein. 3 The activity of membrane-bound polysomes in vitro is more resistant to puromycin than is the activity of free polysomes. In addition, the mRNA associated with membrane-bound polysomes is more stable than the bulk of cellular mRNA as revealed by studies with rifampicin.

Mucosal melanomas of the head and neck

Abstract: Only 3 per cent of all melanomas arise in the mucosa of the head and neck region. Melanomas of mucosal origin in this series differed from their cutaneous counterpart in that size of the primary lesion did not influence prognosis. Regional lymph node metastases were infrequent and did not seem to influence prognosis. Nearly all the primary lesions on microscopic review showed the presence of intralesional lymphatic and blood vessel invasion. Local recurrence at the primary site was the most common cause of treatment failure, in spite of seemingly adequate surgical resection. Surgical treatment offers long-term cure to only a limited number of patients.

EEG alpha correlates of non-smokers, smokers, smoking, and smoking deprivation.

Abstract: Computer analysis of tonic EEG alpha amplitude and dominant alpha frequency was employed to test the hypotheses that relative to non-smokers, smokers deprived of cigarettes are in a state of cortical hypo-excitation, and that smoking induces a state of excitation comparable to that of non-smokers. Ten non-smokers, 17 deprived smokers and 13 non-deprived smokers were used to record alpha activity with a right occipital-right ear placement before and after smoking two cigarettes. Analysis of pre-smoking activity (employing a .05 rejection region) revealed a slower dominant alpha frequency in deprived smokers relative to non-smokers and non-deprived smokers. Smoking increased dominant alpha frequency in deprived smokers to a level comparable to non-smokers and non-deprived smokers. No significant results were obtained with alpha amplitude. The significance of increased alpha frequency is discussed both in relation to present results and comparable work on humans and animals, and a suggestion is forwarded regarding smoking-induced specific excitation (attention) and the maintenance of smoking behavior.

Recovery of membrane micro-vesicles from human erythrocytes stored for transfusion: a mechanism for the erythrocyte discocyte-to-spherocyte shape transformation.

Abstract: It has been shown for fresh human erythrocytes that when intracellular concentrations of calcium are elevated a form of echinocytosis is induced (Allan & Michell, 1975; White, 1974). The resulting echinocytes then lose cytosol-filled vesicles to the medium (Allan et al., 1976). It therefore seemed possible that the more slowly developing echinocytosis which occurs in human erythrocytes during storage for transfusion at 4°C (Rumsby er al., 1974) might also be followed by the loss of membrane microvesicles into the surrounding plasma and that this process might provide the mechanism by which lipid is lost from aging erythrocytes undergoing the disc-to-sphere shape change, in which some 20% or more of the erythrocyte total phospholipid is lost (Fig. 1). This would also explain the irreversible nature of the disc-to-sphere transformation shown by aged cells. Therefore a search was made for membrane micro-vesicles in human blood aged in vitro in citric acid/sodium citrate/dextrose at 4°C. Donations of blood (Group 0 and A, Rhesus positive) in acid/citrate/dextrose were stored under Blood Transfusion Service conditions at 4°C for up to 8 weeks. Periodically, samples of blood were taken and tested for bacterial contamination (none found throughout the study) before the erythrocytes and leucocytes were sedimented by centrifugation at l000g for l0min. The plasma was withdrawn and centrifuged at 7000g for 15min to ensure the removal of leucocyte fragments and platelets. Subsequently the 7000g supernatant was centrifuged at 20000g for 30min and the supernatant from this step was then centrifuged at l00000g for 30min. Membrane microvesicles would be expected to sediment under these conditions (Allan et al., 1976). The pellets obtained by this procedure were washed by resuspension in 0 . 1 5 ~ NaCl at 4°C and recovered by re-centrifugation. With this procedure no pellets were obtained with either freshly drawn blood or with blood stored for 1 week. However, from week 2 to week 8 increasing amounts of a red pellet were obtained at 20000g and at 100000g, with about twice as much material in the lower-speed pellet. Electron microscopy of both pellets showed that they consisted of membrane-bounded micro-vesicles. These were approx. 100 nm in diameter, had electrondense interiors and appeared essentially identical with the cytosol-aled membrane micro-vesicles which are released from echinocytes induced by high intracellular calcium concentrations (Allan ef al., 1976). During the aging of human erythrocytes in vitro the molar cholesterol/phospholipid ratio of the membrane decreases slightly from the normal value of about 0.8 to around 0.65 (M. G. Rumsby, unpublished work). Analysis of the lOOOOOg micro-vesicles from blood aged in uitro showed that these had a molar cholesterol/phospholipid ratio similar to that of the parent erythrocytes. The phospholipid composition of the microvesicles was similar to that of the erythrocytes, except that they were slightly enriched in sphingomyelin. The micro-vesicles were enriched in 1 ,2-diacylglycerol, a lipid which also accumulates to a marked extent in the microvesicles released by erythrocytes having an elevated intracellular calcium content (Allan et al., 1976). Diacylglycerol concentrations were about four times as high in the micro-vesicles as in theerythrocytes from which they were derived. Analyses of erythrocyte ‘ghosts’ and of the micro-vesicle fractions from whole blood by gel electrophoresis show (Fig. 2) that the polypeptides of the spectrin complex (bands 1 and 2) are almost absent from the micro-vesicle membranes, as they were from micro-vesicles formed from erythrocytes with high internal calcium concentrations.

Plastic deformation during freeze-cleavage: a review.

Abstract: SUMMARY Examples of plastic deformation are illustrated from a wide range of specimens that have been prepared for electron microscopic examination by fracturing at low temperature prior to replication of the frozen surface. Deformation has been shown to occur in some specimens when fractured at temperatures as low as 4 K. Plastic deformation is recognized in non-biological polymers such as polystyrene and polyacrylate latex spheres, as well as in similar biological molecules such as poly-β-hydroxybutyrate (PHB). However, it is also demonstrated that plastic deformation occurs widely in more complex biological systems, including membranes and protein macromolecules. The interpretation of the structure of fracture faces of frozen membranes, and particularly the lack of complementarity on opposing fracture faces, is discussed in relation to deformation artifacts. It is concluded that very considerable energy must be dissipated as heat during the cleavage process. In the case of some of the latex spheres, the glass transition temperature (Tg) of the bulk polymer can be more than 200 K above the cleavage temperature, and yet plastic deformation still occurs. Once a molecule has deformed, its appearance in the final replica may be significantly changed by heating during evaporation of the replica. An empirical attempt has been made to define the factors leading to the ‘survival’ of a deformed particle. Although the evidence in this review has been drawn from freeze-fracture and freeze-etching studies, it is emphasized that the process of cleaving at low temperature—whether in freeze-fracture techniques or in cryoultramicrotomy—is essentially the same. Therefore the interpretation of structure in ultra-thin frozen sections will also need to allow for the possibility of deformation artefacts.

The Isolation and Characterization of Differentially Phosphorylated Fractions of Phosphofructokinase from Rabbit Skeletal Muscle

Abstract: A preparation of phosphofructokinase from rabbit skeletal muscle is described which exploits the association-dissociation properties of the enzyme. Phosphofructokinase so prepared is partially phosphorylated and may be fractionated into three distinct species with sedimentation coefficients of 30 S, 18 S and 13 S by chromatography on agarose gels, hydroxyaptite or DEAE-cellulose. Measurements of alkali-labile phosphate content (phosphoserine and/or phosphothreonine) show that fractions consisting almost exclusively of 30-S species and fractions consisting predominantly of 18-S and 13-S species contain approximately 0.15 and 0.29 mol of phosphate per phosphofructokinase monomer (Mr= 80 000) respectively. The results are interpreted in terms of at least two 13-S components which differ in their phosphate contents and also in their self-association properties. The possible significance of phosphorylation is discussed.

Development of Photosystem I and Photosystem II Activities in Leaves of Light-grown Maize (Zea mays).

Abstract: To compare chloroplast development in a normally grown plant with etiochloroplast development, green maize plants (Zea mays), grown under a diurnal light regime (16-hour day) were harvested 7 days after sowing and chloroplast biogenesis within the leaf tissue was examined. Determination of total chlorophyll content, ratio of chlorophyll a to chlorophyll b, and O2-evolving capacity were made for intact leaf tissue. Plastids at different stages of development were isolated and the electron-transporting capacities of photosystem I and photosystem II measured. Light saturation curves were produced for O2-evolving capacity of intact leaf tissue and for photosystem I and photosystem II activities of isolated plastids. Structural studies were also made on the developing plastids. The results indicate that the light-harvesting apparatus becomes increasingly efficient during plastid development due to an increase in the photosynthetic unit size. Photosystem I development is completed before that of photosystem II. Increases in O2-evolving capacity during plastid development can be correlated with increased thylakoid fusion. The pattern of photosynthetic membrane development in the light-grown maize plastids is similar to that found in greening etiochloroplasts.

Immunologic evaluation and prognosis in patients with head and neck cancer

Abstract: Patients with mucosal squamous cell carcinoma of the head and neck generally have suppressed T cell function, and this suppression tends to be more pronounced with progression of disease. The prognostic relationships of multiple tests of immune function were analyzed in 183 clinically staged patients. Correlation of immune parameters with prognosis was evident only with the DNCB response, which correlated with recurrence in stages I and II disease (but not in stages III and IV). There was no correlation of any of the in vitro tests of immuno-function with recurrence in any stage. There were no correlations between any of the immune parameters and age or sex nor between DNCB reactivity and any of the in vitro responses.

The Impact of Slavery on British Radical Politics: 1787?1838

Abstract: The economic and social forces that destroyed the British slave trade and slavery in the British West Indies have been the subject of detailed and controversial analysis in the past 30 years. Rarely, however, have the abolitionist campaigns between 1787-1 838 been seen as an element in a burgeoning, popular radical movment in Britain. These were the years of widespread agitation and extraparliamentary pressure for a range of reforms, and in the process an articulate and highly organized popular radical movement evolved. The purpose of this paper is to outline the role played by the campaigns against the slave trade and slavery within that popular movement; to suggest the importance of popular antagonism towards the slave trade and slavery within the wider abolitionist cause, and finally to argue that the experience and rhetoric of the antislavery cause were infused into other early nineteenth-century working class agitation.