RESUMEN
The effects of high concentrations of Hg (2+) (10 (-2) M and 10 (-3) M) were investigated on the ultrastructure and on the light-induced transformation of isolated prolamellar bodies (PLBs) of dark-grown wheat leaves. Our earlier work on wheat leaf homogenates ( , Plant Biology 6, 358 - 368) showed that, depending on the concentration, Hg (2+) reacts with protochlorophyllide, NADPH and the NADPH : protochlorophyllide oxidoreductase (POR, EC 1.3.1.33) enzyme and induces disaggregation of the macrodomain structure of this latter. Spectroscopic analyses confirmed that 15 min incubation with 10 (-2) M Hg (2+) at 4 degrees Celsius completely inhibited the activity of POR also in isolated PLBs. Ultrastructural investigations revealed the loosening of the PLB structure in the Hg (2+)-treated sample, i.e., intensive vesicle formation on the surface of the PLB membranes. The hexagonal geometry of the inner lattice was not disturbed, however, the unit cell size significantly increased. The disruption of the PLB membranes upon irradiation was studied after 40 min incubation with 10 (-3) M Hg (2+) at 4 degrees Celsius and a subsequent irradiation for 40 min at 20 degrees Celsius. Equimolar concentrations (10 (-3) M) of NADPH and Hg (2+) were added to the samples 10 min prior or after the addition of Hg (2+). Our results suggest that Hg (2+) accelerates the disruption of the PLB membranes and that NADPH can only partially prevent this process. These membrane transformations were similar to those observed in the initial steps of the Shibata shift of control samples.
Asunto(s)
Mercurio/toxicidad , Triticum/efectos de los fármacos , Triticum/ultraestructura , Relación Dosis-Respuesta a Droga , Hojas de la Planta/efectos de los fármacos , Hojas de la Planta/ultraestructuraRESUMEN
The molecular background of Hg (2+)-induced inhibition of protochlorophyllide (Pchlide) photoreduction was investigated in homogenates of dark-grown wheat leaves. Our earlier work showed that 15 min incubation with 10 (-2) M Hg (2+) completely inhibits the activity of NADPH : Pchlide oxidoreductase ( ). Detailed analysis of spectra recorded at 10 K indicated the appearance of emission bands at 638 and 650 nm, which are characteristic for NADP (+)-Pchlide complexes. Fluorescence emission spectra recorded with different excitation wavelengths, fluorescence lifetime measurements and the analysis of acetone extractions revealed that Hg (2+) can also react directly with Pchlide, resulting in protopheophorbide formation. At 10 (-3) M Hg (2+), the phototransformation was complete but the blue shift of the chlorophyllide emission band speeded up remarkably. This indicates oxidation of the NADPH molecules that have a structural role in keeping together the etioplast inner membrane components. We suggest a complex model for the Hg (2+) effect: depending on concentration it can react with any components of the NADPH : Pchlide oxidoreductase macrodomains.
Asunto(s)
Mercurio/farmacología , Oxidorreductasas actuantes sobre Donantes de Grupo CH-CH/metabolismo , Fotosíntesis/fisiología , Hojas de la Planta/enzimología , Triticum/enzimología , Contaminantes Ambientales/farmacología , Oxidorreductasas actuantes sobre Donantes de Grupo CH-CH/antagonistas & inhibidores , Fotosíntesis/efectos de los fármacos , Hojas de la Planta/efectos de los fármacos , Triticum/efectos de los fármacosRESUMEN
An antiserum to alpha-tocoquinone was prepared by immunization of rabbits. Immunization was obtained by injection of a conjugate consisting of the hapten alpha-tocoquinone attached to methylated ovalbumin into the rabbit. The antiserum recognizes the 3,4-dimethyl-p-benzoquinone group of the molecule as well as part of the immediate vicinity to the side chain. This is concluded from the fact that the antibody has some affinity also to plastoquinone. No reaction of the antibody is observed with alpha-tocopherol hydroquinone or alpha-tocopherol. Reaction of the antiserum to alpha-tocoquinone with photosystem II-particle preparations from tobacco affects the functionality of the preparation. Chlorophyll(a)-fluorescence emission is quenched without an alteration of the emission spectrum. Concomitant with this fluorescence quenching, the lifetime of two fluorescence components namely that of a fast and a slower component are shortened. By analogy with the literature the fast component is associated with chlorophyll(a) of the reaction center core and that of the slow component with the antenna system in which the lifetime parameter is shortened by the antibody from 3.42 ns to 1.795 ns. The action on the fast component is less and leads to a shortening of the lifetime parameter from 0.373 ns to only 0.249 ns. The effect is interpreted in terms of an enhancement of linear photosynthetic electron transport possibly due to an inhibition of the cyclic electron transport around PS II. discovered by Gruszecki et al. (1995), Z. Naturforsch. 50c, 61-68.