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George Wald - One of the best experts on this subject based on the ideXlab platform.

  • The interconversion of the Retinenes and vitamins A in vitro.
    Biochimica et Biophysica Acta, 2020
    Co-Authors: George Wald
    Abstract:

    The Retinene1 which results from the bleaching of rhodopsin now appears to be vitamin A1 aldehyde. Morton et al. have given the best evidence for this, and have shown that Retinene1 can be prepared by the mild oxidation of vitamin A1. A simple procedure is described for performing this process chromatographically on a column of manganese dioxide. In the retina, Retinene1 is converted irreversibly to vitamin A1 by an enzyme system in which reduced cozymase (reduced Coenzyme I, DPNH2 serves as coenzyme. The essential process is the transfer of two hydrogen atoms from DPNH2 to Retinene1, reducing its aldehyde group to the primary alcohol group of vitamin A1. The enzyme system which performs this reduction can be assembled in solution from the following components: the coenzyme, DPNH2; as substrate, synthetic Retinene1; and the apoenzyme extracted with dilute salt solutions from homogenized frog or cattle retinas. The apoenzyme is non-dialysable, is precipitated by half-saturated ammonium sulphate, and is destroyed by heating at 100°C within 30 seconds. Its pH optimum lies at about 6.5. In the rods of freshwater fishes, a parallel enzyme system reduces Retinene2 to vitamin A2. This can be assembled from the following components, all in true solution: the coenzyme, DPNH2; as substrate, synthetic Retinene2, prepared by the chromatographic oxidation of vitamin A2 on manganese dioxide; and the apoenzyme extracted with dilute salt solutions from freshwater fish retinas (sunfish, yellow perch). The apoenzyme from frog retinas reduces Retinene2 as effectively as Retinene1. Similarly the fish apoenzyme acts equally well upon both Retinenes. One need consider only one apoenzyme, Retinene reductase, which together with one coenzyme, DPNH2, reduces either of the Retinenes to the corresponding vitamin A. The Retinene reductase system brings a second vitamin into the chemistry of rod vision. It presents the novel phenomenon of one vitamin regenerating another, for the central component of DPNH2 is nicotinamide, the anti-pellagra factor of the vitamin B complex. Rhodopsin solutions and retinal homogenates rapidly lose their power to reduce the Retinenes, through destruction of their DPN by a nucleotidase. Rhodopsin solutions which have lost their activity in this way are re-activated by the addition of new DPNH2. The coenzyme can also be protected by the presence of free nicotinamide and of α-tocopheryl phosphate. On addition to the enzyme system, the synthetic Retinenes rapidly couple with other molecules, and primarily with protein. The normal state of the Retinenes in retinas and retinal extracts is a labile equilibrium between the free and the coupled condition. The Retinenes couple with a variety of retinal molecules, and migrate freely from one to the other.

Steward T. A. Pickett - One of the best experts on this subject based on the ideXlab platform.

  • Plant litter: Its dynamics and effects on plant community structure
    The Botanical Review, 1991
    Co-Authors: José M. Facelli, Steward T. A. Pickett
    Abstract:

    En ésta revisión bibliográfica analizamos la dinámica de la broza, sus efectos sobre el ambiente fisico y quimíco, y los efectos directos e indirectos de la acumulación de broza sobre la estructura y dinámica de las poblaciones y comunidades vegetales. Finalmente, analizamos distintas adaptaciones de las poblaciones vegetales relacionadas con la acumulación de broza. La cantidad de broza acumulada en un sitio depende primariamente de su productividad, pero varios otros factores ambientales, así como el azar, pueden introducir importantes variaciones. El lapso entre la formatión de un órgano y su muerte y caída, puede ser un factor de gran importancia en la dinámica de la broza. El consumo heterotrófico, así como otros factores fisicos que destruyen biomasa o reducen la productividad, también puede afectar su acumulación. La destructión de la broza constituye un proceso cuasi-sucesional. Los principales subprocesos involucrados—fragmentatión fisica, consumo por invertebrados y descomposición—son controlados por factores ambientales y por las propiedades de la broza misma. La acumulación de broza puede afectar profundamente el ambiente físico y químico. Los cambios fisicos que induce pueden alterar la actividad de los descomponedores, mientras que su descomposición libera nutrientes y compuestos fitotóxicos en el suelo. La broza intercepta luz, sombreando semillas y plántulas, y reduciendo la amplitud térmica del suelo. Al reducir la temperature del suelo, y al crear una barrera a la difusión del vapor de agua, la broza reduce la evaporación desde el suelo. Sin embargo, puede también disminuir la disponibilidad de agua, si retiene una proporción considerable de la lluvia. Además, la broza constituye un barrera fisica que puede impedir la llegada al suelo de algunas semillas, así como dificultar la emergencia de plántulas y brotes. Los patrones de acumulación de broza introducen heterogeneidad temporal y espacial, que puede afectar la estructura y dinámica de la comunidad. Sus efectos pueden ser directos (cuando la presencia de broza afecta el éxito de una población) o indirectos (cuando el efecto de la broza sobre una población altera el resultado de la interacción con una segunda). Adicionalmente, la broza puede afectar las comunidades de invertebrados, lo que puede a su vez repercutir en la comunidad vegetal. Varios autores han propuesto que la tolerancia a la presencia de broza, la regulación del ciclo de nutrientes mediante distintas estrategias foliares, y la producción de broza como un medio de combatir competidores mediante sus efectos fisicos o químicos, son componentes de estrategias adaptativas de distintas poblaciones. La evidencia disponible sugiere que sólo la tolerancia a la broza acepta valor adaptativo como explicación más verosímil. We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems. Litter alters the physical and chemical environment directly and indirectly. The decomposition of litter may release both nutrients and phytotoxic substances into the soil. The physical changes produced by litter also alter the activity of decomposers, resulting in an indirect effect on the chemical environment. The accumulated litter intercepts light, shading seeds and seedlings, and reduces the thermal amplitude in the soil. By reducing maximum soil temperatures, and creating a barrier to water vapor diffusion, litter reduces evaporation from the soil. However, litter may also diminish water availability when it retains a large proportion of rainfall. Litter creates a physical barrier for seedling and sprout emergence and to seeds reaching the soil. The heterogeneity introduced into the abiotic environment by the patchy accumulation of litter may affect community structure. This effect may be both direct (when the litter of one species affects the performance of a second species) or indirect (when litter produced by one species alters the outcome of the interaction between a second and a third species). Litter tolerance, timing of litterfall to optimize external nutrient recycling, and accumulation of litter to deter competitors (either through physical or chemical effects) have been postulated as strategies associated with litter accumulation. The existing evidence shows that only tolerance to litter accumulation admits adaptative value as the most likely explanation.

José M. Facelli - One of the best experts on this subject based on the ideXlab platform.

  • Plant litter: Its dynamics and effects on plant community structure
    The Botanical Review, 1991
    Co-Authors: José M. Facelli, Steward T. A. Pickett
    Abstract:

    En ésta revisión bibliográfica analizamos la dinámica de la broza, sus efectos sobre el ambiente fisico y quimíco, y los efectos directos e indirectos de la acumulación de broza sobre la estructura y dinámica de las poblaciones y comunidades vegetales. Finalmente, analizamos distintas adaptaciones de las poblaciones vegetales relacionadas con la acumulación de broza. La cantidad de broza acumulada en un sitio depende primariamente de su productividad, pero varios otros factores ambientales, así como el azar, pueden introducir importantes variaciones. El lapso entre la formatión de un órgano y su muerte y caída, puede ser un factor de gran importancia en la dinámica de la broza. El consumo heterotrófico, así como otros factores fisicos que destruyen biomasa o reducen la productividad, también puede afectar su acumulación. La destructión de la broza constituye un proceso cuasi-sucesional. Los principales subprocesos involucrados—fragmentatión fisica, consumo por invertebrados y descomposición—son controlados por factores ambientales y por las propiedades de la broza misma. La acumulación de broza puede afectar profundamente el ambiente físico y químico. Los cambios fisicos que induce pueden alterar la actividad de los descomponedores, mientras que su descomposición libera nutrientes y compuestos fitotóxicos en el suelo. La broza intercepta luz, sombreando semillas y plántulas, y reduciendo la amplitud térmica del suelo. Al reducir la temperature del suelo, y al crear una barrera a la difusión del vapor de agua, la broza reduce la evaporación desde el suelo. Sin embargo, puede también disminuir la disponibilidad de agua, si retiene una proporción considerable de la lluvia. Además, la broza constituye un barrera fisica que puede impedir la llegada al suelo de algunas semillas, así como dificultar la emergencia de plántulas y brotes. Los patrones de acumulación de broza introducen heterogeneidad temporal y espacial, que puede afectar la estructura y dinámica de la comunidad. Sus efectos pueden ser directos (cuando la presencia de broza afecta el éxito de una población) o indirectos (cuando el efecto de la broza sobre una población altera el resultado de la interacción con una segunda). Adicionalmente, la broza puede afectar las comunidades de invertebrados, lo que puede a su vez repercutir en la comunidad vegetal. Varios autores han propuesto que la tolerancia a la presencia de broza, la regulación del ciclo de nutrientes mediante distintas estrategias foliares, y la producción de broza como un medio de combatir competidores mediante sus efectos fisicos o químicos, son componentes de estrategias adaptativas de distintas poblaciones. La evidencia disponible sugiere que sólo la tolerancia a la broza acepta valor adaptativo como explicación más verosímil. We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems. Litter alters the physical and chemical environment directly and indirectly. The decomposition of litter may release both nutrients and phytotoxic substances into the soil. The physical changes produced by litter also alter the activity of decomposers, resulting in an indirect effect on the chemical environment. The accumulated litter intercepts light, shading seeds and seedlings, and reduces the thermal amplitude in the soil. By reducing maximum soil temperatures, and creating a barrier to water vapor diffusion, litter reduces evaporation from the soil. However, litter may also diminish water availability when it retains a large proportion of rainfall. Litter creates a physical barrier for seedling and sprout emergence and to seeds reaching the soil. The heterogeneity introduced into the abiotic environment by the patchy accumulation of litter may affect community structure. This effect may be both direct (when the litter of one species affects the performance of a second species) or indirect (when litter produced by one species alters the outcome of the interaction between a second and a third species). Litter tolerance, timing of litterfall to optimize external nutrient recycling, and accumulation of litter to deter competitors (either through physical or chemical effects) have been postulated as strategies associated with litter accumulation. The existing evidence shows that only tolerance to litter accumulation admits adaptative value as the most likely explanation.

George B. Wislocki - One of the best experts on this subject based on the ideXlab platform.

  • HISTOCHEMICAL OBSERVATIONS ON RODS AND CONES IN
    2020
    Co-Authors: Richard L. Sidman, George B. Wislocki
    Abstract:

    Biochemical studies of the retina have concentrated on the compounds most easily extracted-the carotenoids, the opsins, and the enzymes which catalyze their interreactions (Wald, ‘53). The chemical structure of the carotenoids has been studied extensively. They occur in alcoholic form as the vitamins A and in aldehyclic form as the Retinenes (Ball, Goodwin and Morton, ‘48). Further, several stereoisomers of the carotenoids have been described (Hubbard and Wald, ‘52). These aspects of molecular structure determine in part the binding of the carotenoids to the opsins and to the enzymes which participate in the visual cycle. In contrast to the carotenoids, the chemistry of the opsins, which are the protein components of the visual pigments, is poorly understood. Available data include a molecular weight of about 40,000 for cattle rod opsin (Hubbard, ‘54), and the observation that opsin has reactive sulfhydryl groups which probably represent the sites on opsin to which the Retinenes attach to form the several

Paul Zeller - One of the best experts on this subject based on the ideXlab platform.