Diffusible Fraction of Serum Cholesterol and Atherogenesis

CHOLESTEROL is found in the blood as a structural component of lipoproteins concerned with the transport of other lipids¹. The high resolution nuclear magnetic resonance spectra of high density serum lipoproteins are similar to that observed when lipids are dissolved in organic solvents, or dispersed in water by bile salts or detergents, or in sonicated form. The lipid component in lipoproteins is therefore probably in an extremely fluid condition². If human serum is mixed with paraffin oil, some of the cholesterol diffuses into the oil without affecting the ultraviolet absorption spectrum of serum proteins. This procedure avoids any protein denaturing action used for cholesterol extraction^3–5. It therefore seems that serum cholesterol has two fractions, one strongly bound by lipoprotein structures and the other loosely bound and diffusible in an oil phase. In this article I designate the loosely bound fraction “diffusible”.     

Nesting Success of Costa Rican Lowland Rain Forest Birds in Response to Edge and Isolation Effects

Although open-cup nesting birds generally face increased risk of nest depredation from forest edge predators and brood parasites in fragmented temperate landscapes, little information exists to assess such risks in tropical birds. We compared nesting success of real birds' nests in large and small forest fragments to a control site in Caribbean lowland wet forest of Costa Rica. Pooling across species, nesting success was significantly greater in unfragmented forest than in either small, isolated fragments or the La Selva Biological Reserve, which is at the tip of a forest 'peninsula' embedded in a largely deforested landscape. Nesting success in isolated fragments did not vary according to distance from edge, suggesting that predators in fragments act throughout these forest patches. The case for increased nest predation as a plausible mechanism to explain the documented decline of forest interior bird populations in this fragmented tropical landscape is enhanced by a simple demographic model that suggests nesting success is likely too low to maintain populations at La Selva and in the fragments. The fact that the large (> 1000 ha) La Selva forest reserve is experiencing nest predation rates similar to those in much smaller fragments is cause for concern. Our results make a strong case for additional studies to document the identities of nest predators in both fragmented and unfragmented forests in such tropical forest landscapes.     

Extent of Intramolecular π-Stacks in Aqueous Solution in Mixed-Ligand Copper(II) Complexes Formed by Heteroaromatic Amines and Several 2-Aminopurine Derivatives of the Antivirally Active Nucleotide Analog 9-[2-(Phosphonomethoxy)ethyl]adenine (PMEA)

The acidity constants of twofold protonated, antivirally active, acyclic nucleoside phosphonates (ANPs), H(2) (PE)(±) , where PE(2-) =9-[2-(phosphonomethoxy)ethyl]adenine (PMEA(2-) ), 2-amino-9-[2-(phosphonomethoxy)ethyl]purine (PME2AP(2-) ), 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP(2-) ), or 2-amino-6-(dimethylamino)-9-[2-(phosphonomethoxy)ethyl]purine (PME(2A6DMAP)(2-) ), as well as the stability constants of the corresponding ternary Cu(Arm)(H;PE)(+) and Cu(Arm)(PE) complexes, where Arm=2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen), are compared. The constants for the systems containing PE(2-) =PMEDAP(2-) and PME(2A6DMAP)(2-) have been determined now by potentiometric pH titrations in aqueous solution at I=0.1M (NaNO(3) ) and 25°; the corresponding results for the other ANPs were taken from our earlier work. The basicity of the terminal phosphonate group is very similar for all the ANP(2-) species, whereas the addition of a second amino substituent at the pyrimidine ring of the purine moiety significantly increases the basicity of the N(1) site. Detailed stability-constant comparisons reveal that, in the monoprotonated ternary Cu(Arm)(H;PE)(+) complexes, the proton is at the phosphonate group, that the ether O-atom of the ?CH(2) ?O?CH(2) ?P(O)$\rm{{_{2}^{-}}}$(OH) residue participates, next to the P(O)$\rm{{_{2}^{-}}}$(OH) group, to some extent in Cu(Arm)(2+) coordination, and that π?π stacking between the aromatic rings of Cu(Arm)(2+) and the purine moiety is rather important, especially for the H?PMEDAP(-) and H?PME(2A6DMAP)(-) ligands. There are indications that ternary Cu(Arm)(2+) -bridged stacks as well as unbridged (binary) stacks are formed. The ternary Cu(Arm)(PE) complexes are considerably more stable than the corresponding Cu(Arm)(R?PO(3) ) species, where R?PO$\rm{{_{3}^{2-}}}$ represents a phosph(on)ate ligand with a group R that is unable to participate in any kind of intramolecular interaction within the complexes. The observed stability enhancements are mainly attributed to intramolecular-stack formation in the Cu(Arm)(PE) complexes and also, to a smaller extent, to the formation of five-membered chelates involving the ether O-atom present in the ?CH(2) ?O?CH(2) ?PO$\rm{{_{3}^{2-}}}$ residue of the PE(2-) species. The quantitative analysis of the intramolecular equilibria involving three structurally different Cu(Arm)(PE) isomers shows that, e.g., ca. 1.5% of the Cu(phen)(PMEDAP) system exist with Cu(phen)(2+) solely coordinated to the phosphonate group, 4.5% as a five-membered chelate involving the ether O-atom of the ?CH(2) ?O?CH(2) ?PO$\rm{{_{3}^{2-}}}$ residue, and 94% with an intramolecular π?π stack between the purine moiety of PMEDAP(2-) and the aromatic rings of phen. Comparison of the various formation degrees of the species formed reveals that, in the Cu(phen)(PE) complexes, intramolecular-stack formation is more pronounced than in the Cu(bpy)(PE) species. Within a given Cu(Arm)(2+) series the stacking intensity increases in the order PME2AP(2-) 相似文献   

Sequential colonization by river periphyton analysed by microscopy and molecular fingerprinting

1. An artificial glass substratum was incubated in the River Danube for a period of 28 days in order to detect the sequential colonization of microorganisms.
2. Light and fluorescent microscopy showed that microalgae and the picoalgal fraction on the slides increased rapidly over the first 2 weeks of colonization. Diatoms were numerically the most abundant component of the periphyton and their species richness and diversity increased rapidly in the early phase of colonization whereas diversity subsequently increased moderately.
3. Evenness of the diatom community was initially high, lower in the intermediate phase and again higher later on. Succession involving early, intermediate and late colonizer species was observed. Community composition during the first 5 days of colonization was very different from later stages whereas there were only minor changes subsequently.
4. Molecular community analysis by means of terminal restriction fragment length polymorphism analysis of PCR amplified 16S rRNA and 18S rRNA genes pointed to even larger differences between the composition of samples obtained early and late in the period.
5. The number of 18S rRNA and 16S rRNA terminal restriction fragments (T-RF-s) was variable over the colonization period and the fragment patterns of both the bacterial and eukaryotic portion of the microbial community were variable, with most T-RF-s unique to a single sample, suggesting a wide diversity and dynamic properties of periphytic organisms.     

Vibriobactin, a siderophore from Vibrio cholerae

A novel siderophore (microbial iron transport compound) has been isolated from low iron cultures of Vibrio cholerae. Belonging to the catecholamide family of chelators, it has been shown to contain three residues of 2,3-dihydroxybenzoic acid and two residues of threonine. Both threonine moieties are present in the form of oxazoline rings. Furthermore, the polyamine backbone of the molecule was proved to be not spermidine, but the rare N-(3-aminopropyl)-1,3-diaminopropane, norspermidine. The structure of the new siderophore has been determined to be N-[3-(2,3-dihydroxybenzamido)propyl]-1, 3-bis[2,3-dihydroxyphenyl)-trans-5-methyl-2-oxazoline-4-carboxamido]prop ane. The compound has been given the trivial name vibriobactin. Mutants defective in the synthesis and utilization of vibriobactin were isolated. In an iron-limited environment V. cholerae was found to respond more strongly to vibriobactin, agrobactin, and ferrichrome than to enterobactin.     

Comparison of the metal-ion-promoted dephosphorylation of the 5'-triphosphates of adenosine, inosine, guanosine and cytidine by Mn2+, Ni2+ and Zn2+ in binary and ternary complexes.

The dependence of the rate of dephosphorylation of ATP, ITP, GTP and CTP (= NTP), expressed as first-order rate constants (50 degrees C; I = 0.1 M, NaClO4), on pH (2 to 10), in the absence and presence of Mn2+, Ni2+, and Zn2+, was investigated. The reaction is accelerated by Zn2+ and passes through a pH optimum at about 8 for the system Zn2+-ATP or 9 for Zn2+-ITP and Zn2+-GTP; this is analogous to observations made earlier with the corresponding Cu2+ systems. By computing the pH dependence of the distribution of the several species present in these systems it is shown that the highest rates are observed in the pH regions where the concentration of Zn(ATP)2-, Zn(ITP-H)3-, or Zn(GTP-H)3- dominates. By evaluating the pH dependence evidence is given that the attacking nucleophile is OH- or H2O for Zn (ATP)2- and H2O for Zn (ITP-H)3- or Zn(GTP-H)3-. For all these complexes metal-ion/nucleic-base interactions are known, leading to the formation of macrochelates. These metal-ion/nucleic-base interactions are crucial for the observation of a metal-ion-promoted dephosphorylation; in agreement with this, and the small tendency of the cytosine moiety to coordinate, the CTP systems are rather stable towards dephosphorylation. It should be noted that these experimental results do not necessarily mean that the macrochelates usually described are the reactive complexes, but only that the active complex must be closely related to them (e.g. isomers, etc). Although for the Ni2+ systems with ATP, ITP, and GTP, and for the Mn2+-ATP system a metal-ion/nucleic-base interaction is also known, these systems are not very sensitive to hydrolytic cleavage of the terminal P-O-P bond. The only known significant structural difference between the Ni2+-NTP or the Mn2+-ATP complexes and those of Cu2+ or Zn2+ is that Ni2+ Mn2+ coordinate to all three phsophate groups, whereas Cu2+ and Zn2+ involve only the beta and gamma ones. This structure-reactivity relationship is rationalized by the suggestion that in the active species the metal ion should be coordinated to the alpha,beta-phosphate groups leaving the gamma-group open to nucleophilic attack. Obviously, an initial beta,gamma-coordination is suitable for a shift of the metal ion along the phosphate back-bone into the reactive alpha-beta-position, while for an alpha,beta,gamma-coordination only the less favorable removal of the coordinated gamma-group remains. The metal-ion/nucleic-base interaction is considered as being important for achieving this reactive structure. The connection between trans-phosphorylation in vitro and in vivo is discussed. It is also shown that the formation of mixed-ligand or ternary complexes inhibits the dephosphorylation process. This is on the one hand of interest with regard to the transport of hydrolysis-sensitive phosphates in nature, while on the other it casts doubts on conclusions based on experiments carried out in the presence of buffers, because these contain weak bases and hence potential ligands.     

Distinct differences in metal ion specificity of RNA and DNA G-quadruplexes

RNA G-quadruplexes, as their well-studied DNA analogs, require the presence of cations to fold and remain stable. This is the first comprehensive study on the interaction of RNA quadruplexes with metal ions. We investigated the formation and stability of two highly conserved and biologically relevant RNA quadruplex-forming sequences (24nt-TERRA and 18nt-NRAS) in the presence of several monovalent and divalent metal ions, namely Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄ ⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. Circular dichroism was used to probe the influence of these metal ions on the folded fraction of the parallel G-quadruplexes, and UV thermal melting experiments allowed to assess the relative stability of the structures in each cationic condition. Our results show that the RNA quadruplexes are more stable than their DNA counterparts under the same buffer conditions. We have observed that the addition of mainly Na⁺, K⁺, Rb⁺, NH₄ ⁺, as well as Sr²⁺ and Ba²⁺ in water, shifts the equilibrium to the folded quadruplex form, whereby the NRAS sequence responds stronger than TERRA. However, only K⁺ and Sr²⁺ lead to a significant increase in the stability of the folded structures, which is consistent with their coordination to the O6 atoms from the G-quartet guanosines. Compared to the respective DNA motives, dNRAS and htelo, the RNA sequences are not stabilized by Na⁺ ions. Finally, the difference in response between NRAS and TERRA, as well as to the corresponding DNA sequences with respect to different metal ions, could potentially be exploited for selective targeting purposes.     

Two neighboring residues of loop A of the alpha1 subunit point towards the benzodiazepine binding site of GABAA receptors

Benzodiazepines are widely used drugs exerting sedative, anxiolytic, muscle relaxant, and anticonvulsant effects by acting through specific high affinity binding sites on some GABA(A) receptors. It is important to understand how these ligands are positioned in this binding site. We are especially interested here in the conformation of loop A of the alpha(1)beta(2)gamma(2) GABA(A) receptor containing a key residue for the interaction of benzodiazepines: alpha(1)H101. We describe a direct interaction of alpha(1)N102 with a diazepam- and an imidazobenzodiazepine-derivative. Our observations help to better understand the conformation of this region of the benzodiazepine pocket in GABA(A) receptor.