The impact of introduced ship rats (Rattus rattus) on seedling recruitment and distribution of a subantarctic megaherb (Pleurophyllum hookeri)

Abstract The impact of introduced ship rats (Rattus rattus) on recruitment of the megaherb Pleurophyllum hookeri Buchan. (Asteraceae) was examined on subantarctic Macquarie Island, an island with no extant native terrestrial vertebrates. Pleurophyllum hookeri (Asteraceae) forms a dominant component of the Macquarie Island vegetation and is restricted to the subantarctic. The Macquarie Island population of P. hookeri is the most extensive and intact. Introduced ship rats (Rattus rattus) are well established in tall tussock grassland of Macquarie Island. We detected rat activity for the first time within P. hookeri herbfields, in autumn 2000. We found rats were destroying up to 90% of racemes. By excluding rats from caches of inflorescences that they had formed, we found they were having a significant negative effect on initial recruitment and seedling survival within the caches. However, because of high seedling mortality after 1 year, there was no sustained impact of the exclosures on P. hookeri seedling density.     

Comparative partial purification of the active dicarboxylate transport system of rat liver, kidney and heart mitochondria

Hydroxylapatite chromatography of Triton-extracted inner-membrane proteins from rat liver mitochondria allowed a ten-fold purification of the dicarboxylate carrier. The purified system, reconstituted into liposomes, displayed all the properties of the dicarboxylate carrier and mediated malonate-malate and malonate-phosphate exchanges. Six protein bands of Mr ranging from 27,000 to 34,000 could be resolved by sodium dodecylsulfate-polyacrylamide gel electrophoresis. The purification of the dicarboxylate carriers of liver, kidney and heart mitochondria were carried out by this method and their properties were compared with respect to transport activity and electrophoresis patterns. Our results demonstrate that the dicarboxylate carrier of rat mitochondria can be obtained in an advanced state of purification and with a high specific activity.     

Sexual Dimorphism of the Feto-Placental Phenotype in Response to a High Fat and Control Maternal Diets in a Rabbit Model

Maternal environment during early developmental stages plays a seminal role in the establishment of adult phenotype. Using a rabbit model, we previously showed that feeding dams with a diet supplemented with 8% fat and 0.2% cholesterol (HH diet) from the prepubertal period and throughout gestation induced metabolic syndrome in adult offspring. Here, we examined the effects of the HH diet on feto-placental phenotype at 28 days post-coïtum (term = 31days) in relation to earlier effects in the blastocyst (Day 6). At 28 days, both male and female HH fetuses were intrauterine growth retarded and dyslipidemic, with males more affected than females. Lipid droplets accumulated in the HH placentas’ trophoblast, consistent with the increased concentrations in cholesteryl esters (3.2-fold), triacylglycerol (2.5-fold) and stored FA (2.12-fold). Stored FA concentrations were significantly higher in female compared to male HH placentas (2.18-fold, p<0.01), whereas triacylglycerol was increased only in HH males. Trophoblastic lipid droplet accumulation was also observed at the blastocyst stage. The expression of numerous genes involved in lipid pathways differed significantly according to diet both in term placenta and at the blastocyst stage. Among them, the expression of LXR-α in HH placentas was reduced in HH males but not females. These data demonstrate that maternal HH diet affects the blastocyst and induces sex-dependent metabolic adaptations in the placenta, which appears to protect female fetuses from developing severe dyslipidemia.     

Cyclooxygenases and lipoxygenases are used by the fungus Podospora anserina to repel nematodes

Oxylipins are secondary messengers used universally in the living world for communication and defense. The paradigm is that they are produced enzymatically for the eicosanoids and non-enzymatically for the isoprostanoids. They are supposed to be degraded into volatile organic compounds (VOCs) and to participate in aroma production. Some such chemicals composed of eight carbons are also envisoned as alternatives to fossil fuels. In fungi, oxylipins have been mostly studied in Aspergilli and shown to be involved in signalling asexual versus sexual development, mycotoxin production and interaction with the host for pathogenic species. Through targeted gene deletions of genes encoding oxylipin-producing enzymes and chemical analysis of oxylipins and volatile organic compounds, we show that in the distantly-related ascomycete Podospora anserina, isoprostanoids are likely produced enzymatically. We show the disappearance in the mutants lacking lipoxygenases and cyclooxygenases of the production of 10-hydroxy-octadecadienoic acid and that of 1-octen-3-ol, a common volatile compound. Importantly, this was correlated with the inability of the mutants to repel nematodes as efficiently as the wild type. Overall, our data show that in this fungus, oxylipins are not involved in signalling development but may rather be used directly or as precursors in the production of odors against potential agressors.

A Revised Mechanism for Human Cyclooxygenase-2

The mechanism of ω-6 polyunsaturated fatty acid oxidation by wild-type cyclooxygenase 2 and the Y334F variant, lacking a conserved hydrogen bond to the catalytic tyrosyl radical/tyrosine, was examined for the first time under physiologically relevant conditions. The enzymes show apparent bimolecular rate constants and deuterium kinetic isotope effects that increase in proportion to co-substrate concentrations before converging to limiting values. The trends exclude multiple dioxygenase mechanisms as well as the proposal that initial hydrogen atom abstraction from the fatty acid is the first irreversible step in catalysis. Temperature dependent kinetic studies reinforce the novel finding that hydrogen transfer from the reduced catalytic tyrosine to a terminal peroxyl radical is the first irreversible step that controls regio- and stereospecific product formation.     

Molecular oxygen dependent steps in fatty acid oxidation by cyclooxygenase-1

The mechanism by which cyclooxygenase-1 (COX-1), a heme- and tyrosyl radical-containing enzyme, catalyzes the regio- and stereospecific oxygenation of polyunsaturated fatty acids to prostaglandin or hydroperoxide products has not been understood. Steady-state kinetic studies conducted with the native substrate arachidonic acid and the slower substrate linoleic acid are described here. Second-order rate constants, kcat/KM for fatty acid and O2, are found to depend upon the concentration of the other cosubstrate. Competitive oxygen kinetic isotope effects (18O KIEs) kcat/KM(16,16O2)/kcat/KM(18,16O2) reveal that a peroxyl radical is formed in or before the first kinetically irreversible step. Together, the results indicate that the oxygenase reaction occurs by a sequential mechanism which most likely involves reversible abstraction of a hydrogen atom from the fatty acid prior to the trapping of the delocalized substrate radical by O2. The identity of the first kinetically irreversible step, subsequent to forming the peroxyl radical, is also discussed in the context of the magnitude of the oxygen kinetic isotope effects as well as the behavior of kcat/KM(O2) in response to changing solvent pH, pD, and viscosity.