RESUMO
Monolayer cultures of thyroid cells lose their iodide organification capacity a few days before the disappearance of thyroid peroxidase (TPO) activity. The present studies were performed in order to clarify this point. The above mentioned difference was due to the presence of an inhibitor in the monolayer thyroid cells culture, given that total homogenate prepared from confluent cells caused a significant inhibition of activity of TPO from fresh tissue. The inhibitor was localized in the 105000g supernatant of the homogenate of the cell culture, but not in a similar preparation obtained from fresh thyroid. It is thermostable, dialyzable and has a molecular weight of less than 2 kDa. Addition of the inhibitor at the end of the reaction of tyrosine iodination failed to alter the results. This fact suggests that the compound does not destroy the iodinated product. The presence of the cytosolic inhibitor was observed in monolayer thyroid cell cultures of different species (bovine, porcine, rat and human) but not in free follicles cultures.
Assuntos
Inibidores Enzimáticos/análise , Iodeto Peroxidase/antagonistas & inibidores , Iodetos/metabolismo , Glândula Tireoide/química , Animais , Bovinos , Linhagem Celular , Células Cultivadas , Diálise , Estabilidade de Medicamentos , Inibidores Enzimáticos/química , Temperatura Alta , Humanos , Radioisótopos do Iodo , Peso Molecular , Ratos , Tirosina/metabolismoRESUMO
The response of Fuchsia magellanica Lam. - a shade-tolerant perennial - to white light photon flux density and end-of-day red and far-red light irradiation was studied to evaluate the role that different morphological and functional alterations play in whole-plant acclimation to shade. The estimated relative growth rate of plants growing under a photon flux density of 30µmol m2 s1 was 71 % of that of those growing under 450µmol m-2 s-1 . The proportion of total dry weight allocated to shoots was higher in low light plants, but the proportion of shoot dry weight allocated to leaf blades was not affected by any of the treatments. The estimated relative growth rate, specific leaf area and leaf conductance were not affected by end-of-day irradiation. In low light plants the specific leaf area was higher, and at saturating photon flux density photosynthesis was lower on a leaf area basis, but was not on a leaf dry weight basis. Dark respiration was higher in high light plants when expressed on an area basis, but not when expressed on a dry weight basis. During the normal photoperiod, leaf conductance measured in darkness was higher in high light plants than in low light ones, which is consistent with the higher stomatal density observed in these plants. The cross-sectional area of the petioles was slightly reduced in low light in spite of a large increase in the area of individual leaves. Low irradiance affected the display of leaves, decreased leaf thickness and increased chlorophyll content per unit dry weight. End-of-day far-red resulted in plants with more erect branches. Both low photon flux density and far-red enhanced internode elongation. The effect of end-of-day irradiations was bigger in low light than in high light, which is the opposite to what is observed in shade-avoiders. The relatively small difference in mean growth rate can be explained by changes in specific leaf area, shoot: root dry weight partition ratio, and by the shape of the photosynthetic light response curve. However when comparing photosynthetic rates on a dry weight basis low light plants do not seem to be at a disadvantage under high light.
RESUMO
Direct and indirect mechanisms underlying the light response of stomata were studied in variegated leaves of the juvenile phase of Hedera helix L. Dose response curves of leaf conductance were measured with blue and red light in leaves kept in normal or in an inverted position. In the green portions of the leaves, the sensitivity to blue light was nearly 100 times higher than that to red light. No response to red light was observed in the white portions of the leaves up to 90 micromoles per square meter per second. Red light indirectly affected leaf conductance while blue light had a direct effect. Leaf conductance was found to be more sensitive to drought stress and showed a more persistent aftereffect in the white portions of the leaves. A differential effect of drought stress on the responses to blue and red light was also observed.