The ecological significance of toxic nectar

Although plant-herbivore and plant-pollinator interactions have traditionally been studied separately, many traits are simultaneously under selection by both herbivores and pollinators. For example, secondary compounds commonly associated with herbivore defense have been found in the nectar of many plant species, and many plants produce nectar that is toxic or repellent to some floral visitors. Although secondary compounds in nectar and toxic nectar are geographically and phylogenetically widespread, their ecological significance is poorly understood. Several hypotheses have been proposed for the possible functions of toxic nectar, including encouraging specialist pollinators, deterring nectar robbers, preventing microbial degradation of nectar, and altering pollinator behavior. All of these hypotheses rest on the assumption that the benefits of toxic nectar must outweigh possible costs; however, to date no study has demonstrated that toxic nectar provides fitness benefits for any plant. Therefore, in addition to these adaptive hypotheses, we should also consider the hypothesis that toxic nectar provides no benefits or is tolerably detrimental to plants, and occurs due to previous selection pressures or pleiotropic constraints. For example, secondary compounds may be transported into nectar as a consequence of their presence in phloem, rather than due to direct selection for toxic nectar. Experimental approaches are necessary to understand the role of toxic nectar in plant-animal interactions.     

Heme binding and polymerization by Plasmodium falciparum histidine rich protein II: influence of pH on activity and conformation.

The histidine rich protein II (HRPII) from Plasmodium falciparum has been implicated as a heme polymerase which detoxifies free heme by its polymerization to inactive hemozoin. Histidine-iron center coordination is the dominant mechanism of interaction between the amino acid and heme. The protein also contains aspartate allowing for ionic/coordination interactions between the carboxylate side chain and the heme metal center. The pH profile of heme binding and polymerization shows the possibility of these two types of binding sites being differentiated by pH. Circular dichroism studies of the protein show that pH and heme binding cause a change in conformation above pH 6 implying the involvement of His-His+ transitions. Heme binding at pHs above 6 perturbs HRPII conformation, causing an increase in helicity.     

Characterization of freeze-thaw induced ultrastructural damage to endothelial cells in vitro

Summary The pathophysiology of endothelial cells is important to a variety of vascular conditions including coagulation and hemostasis resulting from clinical frostbite. Use of an in vitro model system demonstrated that when bovine endothelial cells were frozen at 1°C or 20°C/min and thawed immediately (20°C/min), a variety of ultrastructural alterations occurred. Membraneous structures were most extensively damaged, with mitochondria the most sensitive organelle. Low amplitude mitochondrial swelling, first evident at 0°C, progressed to high amplitude swelling by −10°C (frozen). In addition, the rough endoplasmic reticulum was dilated and formed large vesicles with a homogeneous matrix. Nuclear changes first occurred at −15°C. These included separation and distortion of the nuclear membrane, changes in chromatin distribution, and disruption of the nucleolus. Scanning electron microscopy revealed perforated plasma membranes in some cells at −10°C (frozen) and in most cells by −20°C. Cultures frozen at 20°C/min revealed mostly the same ultrastructural damage noted at 1°C/min except a higher percentage of cells exhibited alterations. Data from the recovery index and lactic dehydrogenase (LDH) release correlated well with observed ultrastructural changes. Early swelling of mitochondria and dilation of rough endoplasmic reticulum was not lethal in the absence of freezing. Increased swelling in cytoplasmic organelles coupled with nuclear alterations at −15°C resulted in a decreased survival rate and release of significant quantities of LDH by −20°C. No unique morphological changes were temperature specific, but the total number of cells that displayed alterations increased as temperature decreased. The views, opinions or findings, or both, contained in this report are those of the authros and should not be construed as indicative of an official Department of the Army position, policy, or decision unless so designated by other official documentation.     

Evidence for two conformational forms of monhistone protein BA which differ in their affinity for DNA

Nonhistone protein BA_free was purified from the 0.075 M NaCl/0.025 M $\text{EDTA}\text{pH}$ 8 extract of whole rat liver nuclei while protein BA_bound was isolated from the 0.05 M Na₂HPO₄/8 M urea/1% β-mercaptoethanol/pH 7.6 extract of dehistonized rat liver chromatin. Chromatin associated protein BA_bound was able to bind 60% of the [³H] DNA in a nitrocellulose filter binding assay while nucleoplasmic protein BA_free showed essentially no DNA binding activity. Circular dichroism analysis of the two forms of protein BA revealed substantial differences in their conformations. Protein BA_free was found to have an α-helix content of 41% while protein BA_bound displayed a spectrum more typical of unordered or β-turn structures.