Diurnal changes in the regulatory properties of PEP-carboxylase in Crassulacean Acid Metabolism (CAM)

Abstract. The CAM plants Kalanchoe tubiflora and K. blossfeldiana were grown under photoperiodically controlled conditions (short days). In these plants, phos-phoenolpyruvate carboxylase capacity and the sensitivity of the enzyme to the effectors L-malate (inhibitor) and glucose-6-phosphate (activator) were measured throughout the diurnal CAM cycle. In K. tubiflora , enzyme capacity was higher if measured at pH 7.0 than at pH 8.0 and displayed a rhythmical behavior with highest values at the end of the light period. As reported earlier, in K. blossfeldiana PEP-C capacity was higher during the night. It was more pronounced when plants were kept in CO₂-free air during the dark period. In both plants, the sensitivity of the enzyme to the effectors showed very clear diurnal changes: inhibition by malate and activation by glucose-6-phosphate were strikingly higher during the day than during the night; the effect depended on PEP concentration. The changing activation of the enzyme by glucose-6-phos-phate reflects diurnal changes of the Km for PEP which was found to be higher during the day than during the night. Manipulations of malate accumulation by nocturnal application of CO₂-free air did not influence these effects. The results are discussed in context with the metabolic control of CAM.     

Selenicereus wittii (Cactaceae): An epiphyte adapted to Amazonian Igapó inundation forests

The biology, ecology, and distribution ofSelenicereus (Strophocactus)wittii, one of the least known taxa ofCactaceae, are described. This epiphyte climbs appressed to tree trunks with leaf-like, flattened stems and is found exclusively along the high waterline of black water rivers (Rio Negro, Vaupés, Apaporis) in the Igapó inundation forests of Amazonia. Ecophysiologically,S. wittii is a crassulacean acid metabolism (CAM) plant. It bears white, nocturnal flowers 25 cm in length which emit a fragrance consisting mainly of benzylalcohol, benzyl benzoate, and benzyl salicylate. They exhibit an extreme sphingophilous syndrome as an adaptation to pollination by probably only two species of hawkmoth from the generaAmphimoena andCocytius. The seeds, aberrant for the family, contain air-filled chambers and are water-dispersed. Thus,S. wittii represents the paradoxical life form of an hydrochorous epiphytic cactus which withstands periodical inundation.Dedicated to emer. Univ.-Prof. DrFriedrich Ehrendorfer on the occasion of his 70th birthday     

Distribution and δ 13C Values of Portulacaceae Species of the High Andes in Northern Chile

The distribution of species of Portulacaceae in the northern Andes of Chile was recorded during the late growing season along altitudinal transects from 1500 m up to the vegetation limit. The transects were located towards the east of the towns of Arica (18 °S) and Vallenar (28 °S). Whole plants were analysed for δ ¹³C values to examine the occurrence of different photosynthetic pathways. Three species of Calandrinia (C. thyrsoidea, C. spicata and C. grandiflora) and two species of Philippiamra (P. amaranthoides and P. celosioides) were found to have δ ¹³C values indicative of CAM. Portulaca philippi was the only C4 plant in the sample. The CAM species tend to occupy the lower altitudinal levels, which are also relatively drier. All the C3 species occur in the Andean belt, with more favourable water supply. Their δ ¹³C values averaged ?25.1 ‰, a value significantly lower than the average of lowland C3 Calandrinia species reported elsewhere or the general average for lowland C3 plants of ?28.8 ‰. CAM species are also limited to the western slope of the Andean range, while C3 species occur at both sides, but often have discontinuous distributions north or south of the driest section of the Andes (21–24 °S) indicating that their migration along the western side has probably been prevented by extreme aridity. The C3 species occurring on both sides of the aridity barrier are also annual in habit.     

Chick embryo chorioallantoic membrane (CAM): an alternative predictive model in acute toxicological studies for anti-cancer drugs

The chick embryo chorioallantoic membrane (CAM) is a preclinical model widely used for vascular and anti-vascular effects of therapeutic agents in vivo. In this study, we examine the suitability of CAM as a predictive model for acute toxicology studies of drugs by comparing it to conventional mouse and rat models for 10 FDA-approved anticancer drugs (paclitaxel, carmustine, camptothecin, cyclophosphamide, vincristine, cisplatin, aloin, mitomycin C, actinomycin-D, melphalan). Suitable formulations for intravenous administration were determined before the average of median lethal dose (LD₅₀) and median survival dose (SD₅₀) in the CAM were measured and calculated for these drugs. The resultant ideal LD₅₀ values were correlated to those reported in the literature using Pearson’s correlation test for both intravenous and intraperitoneal routes of injection in rodents. Our results showed moderate correlations (r²=0.42 − 0.68, P<0.005–0.05) between the ideal LD₅₀ values obtained using the CAM model with LD₅₀ values from mice and rats models for both intravenous and intraperitoneal administrations, suggesting that the chick embryo may be a suitable alternative model for acute drug toxicity screening before embarking on full toxicological investigations in rodents in development of anticancer drugs.     

The function of the aerenchyma in arborescent lycopsids: evidence of an unfamiliar metabolic strategy

Most species of the modern family Isoëtaceae (Quillworts) some other modern hydrophytes, use a metabolic pathway for carbon fixation that involves uptake of sedimentary carbon and enrichment of CO₂ in internal gas spaces as a carbon-concentrating mechanism. This metabolism, which is related to ‘aquatic CAM’, is characterized by morphological, physiological and biochemical adaptations for decreasing photorespirative loss, aerating roots and maintaining high growth rates in anoxic, oligotrophic, stressed environments. Some of the closest relatives of the Isoëtaceae were the ‘arborescent lycopsids’, which were among the dominant taxa in the coal swamps found in lowland ecosystems during the Carboniferous and Permian periods (approx. 300 Ma). Morphological, ecological and geochemical evidence supports the hypothesis that the arborescent lycopsids had an unusual metabolism similar to that of modern Isoëtaceae and processed a biogeochemically significant proportion of organically fixed carbon over a period of about 100 million years in the late Palaeozoic. The temporal coincidence between the dominance of plants with this metabolism and an anomalous global atmosphere (high O₂; low CO₂) supports the idea that biosphere feedbacks are important in regulating global climatic homeostasis. The potential influence of this metabolism on the global carbon cycle and its specific adaptive function suggest that it should perhaps be considered a fourth major photosynthetic pathway.     

Signaling Pathways of the F11 Receptor (F11R; a.k.a. JAM-1, JAM-A) in Human Platelets: F11R Dimerization,Phosphorylation and Complex Formation with the Integrin GPIIIa

The F11 receptor (F11R) (a.k.a. Junctional Adhesion Molecule, JAM) was first identified in human platelets as a 32/35 kDa protein duplex that serves as receptor for a functional monoclonal antibody that activates platelets. We have sequenced and cloned the F11R and determined that it is a member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. The signaling pathways involved in F11R-induced platelet activation were examined in this investigation. The binding of M.Ab.F11 to the platelet F11R resulted in granule secretion and aggregation. These processes were found to be dependent on the crosslinking of F11R with the FcγRII by M.Ab.F11. This crosslinking induced actin filament assembly with the conversion of discoidal platelets to activated shapes, leading to the formation of platelet aggregates. We demonstrate that platelet secretion and aggregation through the F11R involves actin filament assembly that is dependent on phosphoinositide-3 kinase activation, and inhibitable by wortmannin. Furthermore, such activation results in an increase in the level of free intracellular calcium, phosphorylation of the 32 and 35 kDa forms of the F11R, F11R dimerization coincident with a decrease in monomeric F11R, and association of the F11R with the integrin GPIIIa and with CD9. On the other hand, F11R-mediated events resulting from the binding of platelets to an immobilized surface of M.Ab.F11 lead to platelet adhesion and spreading through the development of filopodia and lammelipodia. These adhesive processes are induced directly by interaction of M.Ab.F11 with the platelet F11R and are not dependent on the FcγRII. We also report here that the stimulation of the F11R in the presence of nonaggregating (subthreshold) concentrations of the physiological agonists thrombin and collagen, results in supersensitivity of platelets to natural agonists by a F11R-mediated process independent of the FcγRII. The delineation of the two separate F11R-mediated pathways is anticipated to reveal significant information on the role of this cell adhesion molecule in platelet adhesion, aggregation and secretion, and F11R-dependent potentiation of agonist-induced platelet aggregation. The participation of F11R in the formation and growth of platelet aggregates and plaques in cardiovascular disorders, resulting in enhanced platelet adhesiveness and hyperaggregability, may serve in the generation of novel therapies in the treatment of inflammatory thrombosis, heart attack and stroke, and other cardiovascular disorders.