Mutational analysis of C-lobe ligands of human serum transferrin: insights into the mechanism of iron release

Each homologous lobe of human serum transferrin (hTF) has one Fe(3+) ion bound by an aspartic acid, a histidine, two tyrosine residues, and two oxygens from the synergistic anion, carbonate. Extensive characterization of these ligands in the N-terminal lobe has been carried out. Despite sharing the same set of ligands, there is a substantial amount of evidence that the N- and C-lobes are inequivalent. Studies of full-length hTF have shown that iron release from each lobe is kinetically distinguishable. To simplify the assessment of mutations in the C-lobe, we have created mutant hTF molecules in which the N-lobe binds iron with high affinity or not at all. Mutations targeting the C-lobe liganding residues have been introduced into these hTF constructs. UV-visible spectral, kinetic, and EPR studies have been undertaken to assess the effects of each mutation and to allow direct comparison to the N-lobe. As found for the N-lobe, the presence of Y517 in the C-lobe (equivalent to Y188 in the N-lobe) is absolutely essential for the binding of iron. Unlike the N-lobe, however, mutation of Y426 (equivalent to Y95) does not produce a stable complex with iron. For the mutants that retain the ability to bind iron (D392S and H585A), the rates of release are considerably slower than those measured for equivalent mutations in the N-lobe at both pH 7.4 and pH 5.6. Equilibrium binding experiments with HeLa S(3) cells indicate that recombinant hTF, in which Y426 or H585 is mutated, favor a closed or nearly closed conformation while those with mutations of the D392 or Y517 ligands appear to promote an open conformation. The differences in the effects of mutating the liganding residues in the two lobes and the subtle indications of cooperativity between lobes point to the importance of the transferrin receptor in effecting iron release from the C-lobe. Significantly, the equilibrium binding experiments also indicate that, regardless of which lobe contains the iron, the free energy of binding is equivalent and not additive; each monoferric hTF has a free energy of binding that is 82% of diferric hTF.     

State-dependent inhibition of L-type calcium channels: cell-based assay in high-throughput format

The current studies describe a new, robust cell-based functional assay useful to characterize L-type voltage-dependent calcium channels and their antagonists. The basis of this assay is measurement in plate format of Ca2+ influx through the L-type Ca2+ channel complex (alpha1C, alpha2delta, and beta2a subunits) in response to potassium-mediated depolarization; EC(50)=11 mM [K+](o). The Ca2+ influx was inhibited by the L-type Ca2+ channel antagonist, nimodipine; IC(50)=59 nM. These cells were also transfected with the Kir2.3 inward rectifier K(+) channel, which allows for changing the cell membrane potential by modulation of extracellular [K](o); -65 mV in physiological [K](o) and -28 mV in 30 mM [K](o) containing buffer. The conformational state of the voltage-sensitive Ca2+ channel is altered under these different conditions. Under the depolarized condition, nimodipine was a more potent antagonist, inhibiting Ca2+ influx with an IC(50) value of 3 nM. The results demonstrate that the interaction of nimodipine and other antagonists with the channel is modulated by changes in membrane potential and thus the state of the channel. Overall, this novel assay can be used to identify state-dependent calcium channel antagonists and should be useful for evaluating state-dependent inhibitory potency of a large number of samples.     

Application of 2-D free-flow electrophoresis/RP-HPLC for proteomic analysis of human plasma depleted of multi high-abundance proteins

Free-flow electrophoresis (FFE) and rapid (6 min) RP-HPLC was used to fractionate human citrate-treated plasma. Prior to analysis, the six most abundant proteins in plasma were removed by immunoaffinity chromatography; both depleted plasma and the fraction containing the six abundant proteins depleted were taken for MS-based analysis. Fractionated proteins were digested with trypsin and the generated peptides were subjected to MS-based peptide sequencing. To date, 78 plasma proteins have been unambiguously identified by manual validation from 16% (15/96 FFE total fractions) of the collected FFE pools; 55 identifications were based on > or = 2 tryptic peptides and 23 using single peptides. The molecular weight range of proteins and peptides isolated by this method ranged from approximately 190 K (e.g., Complement C3 and C4) to approximately 4-6 K (e.g., CRISPP and Apolipoprotein C1). This FFE/RP-HPLC approach reveals low-abundance proteins and peptides (e.g., L-Selectin approximately 17 ng/mL and the cancer-associated SCM-recognition, immunodefense suppression, and serine protease protection peptide (CRISPP) at approximately 0.5-1 ng/mL), where CRISPP was found in association with alpha-1-antitrypsin as a non-covalent complex, in the fraction containing the depleted high-abundance proteins. In contrast to shotgun proteomic approaches, the FFE/RP-HPLC method described here allows the identification of potentially interesting peptides to be traced back to their protein of origin, and for the first time, has confirmed the "protein sponge" hypothesis where the 35 residue CRISPP polypeptide is non-covalently complexed with the major circulating plasma protein alpha-1-antitrypsin.     

Lignans from the roots of Echinops giganteus

Two new lignans, (+)-4-(3-methylbutanoyl)-2,6-di(3,4-dimethoxy)phenyl-3,7-dioxabicyclo[3.3.0]octane and (+)-4-hydroxy-2,6-di(3,4-dimethoxy)phenyl-3,7-dioxabicyclo[3.3.0]octane, together with the known lupeol and sitosteryl beta-D-glucopyranoside, have been isolated from the roots of Echinops giganteus var. lelyi C. D. Adams (Compositae). The structures were elucidated on the basis of spectral studies and comparison with published data.     

Consolidation and volumetric soil-water content of salt marsh soils following habitat modification for mosquito control

The runnelling form of habitat modification for mosquito control in saltmarsh increases tidal frequency, and may affect soil properties such as volumetric soil-water content and consolidation. The effects of habitat modification on soil properties are in turn likely to affect ecological processes. Runnels constructed mechanically to a depth of no more than 0.3 m with smooth, spoon shaped edges linked isolated mosquito-breeding pools in the high marsh to the tidal source at the saltmarsh/mangrove interface. The physical design of runnels may result in a significant increase in the frequency of flooding tidal events that flush isolated mosquito-breeding pools. Impacts of the runnelling technique were determined at three marshes using two sampling protocols: (a) comparisons between modified and unmodified shores and (b) comparisons with lateral distance from a runnel. At one marsh, volumetric water content was significantly higher at runnelled than at unrunnelled sites after tides that only partly inundated the marsh, but this pattern was not found at the other marshes. Soil consolidation was greater further from the shore, but was not different between runnelled and unrunnelled shores. Measurements at different lateral distances from runnels demonstrated higher water content levels and lower consolidation up to 5 m from runnels and no effect further away. The varied responses to runnelling at different marshes may reflect specific site characteristics such as slope and hydraulic tidal forces. Remedial strategies for similar mosquito control techniques, based on habitat modification, should include dynamic classifications of saltmarshes.     

Peptide length variants p2Ca and QL9 present distinct conformations to L(d)-specific T cells

Recent advances have provided insights into how the TCR interacts with MHC/peptide complexes and a rationale to predict optimal epitopes for MHC binding and T cell recognition. For example, peptides of nine residues are predicted to be optimal for binding to H2-L(d), although 8 mer epitopes have also been identified. It has been predicted that 8 mer and 9 mer length variant peptides bound to L(d) present identical epitopes to T cells. However, in contrast to this prediction, we demonstrate here that the 8 mer peptide p2Ca and its 9 mer length variant QL9, extended by an N-terminal glutamine, assume distinct conformations when bound to L(d). We generated self-L(d)-restricted CTL clones specific for p2Ca that recognize L(d)/QL9 poorly if at all. This result is in sharp contrast to what has been observed with L(d)-alloreactive T cells that possess a much higher affinity for L(d)/QL9 than for L(d)/p2Ca. Alanine substitutions of the N-terminal residues of the QL9 peptide rescue detection by these self-L(d)/p2Ca-specific T cells, but decrease recognition by the L(d)-alloreactive 2C T cell clone. In addition, 2C T cell recognition of the p2Ca peptide is affected by different alanine substitutions compared with 2C T cell recognition of the QL9 peptide. These data clearly demonstrate that the p2Ca and QL9 peptides assume distinct conformations when bound to L(d) and, furthermore, demonstrate that there is flexibility in peptide binding within the MHC class I cleft.     

Discovery and initial structure-activity relationships of trisubstituted ureas as thrombin receptor (PAR-1) antagonists

Thrombin is the most potent agonist of platelet activation, and its effects are predominantly mediated by platelet thrombin receptors. Therefore, antagonists of the thrombin receptor have potential utility for the treatment of thrombotic disorders. Screening of combinatorial libraries revealed 2 to be a potent antagonist of the thrombin receptor. Modifications of this structure produced 11k, which inhibits thrombin receptor stimulated secretion and aggregation of platelets.