Preparation of pure lipid hydroperoxides

Increasing evidence of lipid peroxidation in food deterioration and pathophysiology of diseases have revealed the need for a pure lipid hydroperoxide (LOOH) reference as an authentic standard for quantification and as a compound for biological studies in this field. Generally, LOOH is prepared from photo- or enzymatically oxidized lipids; however, separating LOOH from other oxidation products and preparing pure LOOH is difficult. Early studies showed the usability of reaction between hydroperoxide and vinyl ether for preparation of pure LOOH. Because the reactivity of vinyl ether with LOOHs other than fatty acid hydroperoxides has never been reported, here, we employed the reaction for preparation of a wide variety of pure LOOHs. Phospholipid, cholesteryl ester, triacylglycerol, or fatty acid was photo- or enzymatically oxidized; the resultant crude sample containing hydroperoxide was allowed to react with a vinyl ether [2-methoxypropene (MxP)]. Liquid chromatography (LC) and mass spectrometry confirmed that MxP selectively reacts with LOOH, yielding a stable MxP adduct (perketal). The lipophilic perketal was eluted at a position away from that of intact LOOH and identified and isolated by LC. Upon treatment with acid, perketal released the original LOOH, which was finally purified by LC. Using our optimized purification procedures, for instance, we produced 75 mg of pure phosphatidylcholine hydroperoxide (>99%) from 100 mg of phosphatidylcholine. Our developed method expands the concept of the perketal method, which provides pure LOOH references. The LOOHs prepared by the perketal method would be used as "gold standards" in LOOH methodology.     

Structural insight into the specific interaction between murine SHPS-1/SIRP alpha and its ligand CD47

SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1 or SIRPα/BIT) is an immunoglobulin (Ig) superfamily transmembrane receptor and a member of the signal regulatory protein (SIRP) family involved in cell-cell interaction. SHPS-1 binds to its ligand CD47 to relay an inhibitory signal for cellular responses, whereas SIRPβ, an activating member of the same family, does not bind to CD47 despite sharing a highly homologous ligand-binding domain with SHPS-1. To address the molecular basis for specific CD47 recognition by SHPS-1, we present the crystal structure of the ligand-binding domain of murine SHPS-1 (mSHPS-1). Folding topology revealed that mSHPS-1 adopts an I2-set Ig fold, but its overall structure resembles IgV domains of antigen receptors, although it has an extended loop structure (C′E loop), which forms a dimer interface in the crystal. Site-directed mutagenesis studies of mSHPS-1 identified critical residues for CD47 binding including sites in the C′E loop and regions corresponding to complementarity-determining regions of antigen receptors. The structural and functional features of mSHPS-1 are consistent with the human SHPS-1 structure except that human SHPS-1 has an additional β-strand D. These results suggest that the variable complementarity-determining region-like loop structures in the binding surface of SHPS-1 are generally required for ligand recognition in a manner similar to that of antigen receptors, which may explain the diverse ligand-binding specificities of SIRP family receptors.     

Design of an active ultrastable single-chain insulin analog: synthesis, structure, and therapeutic implications

Single-chain insulin (SCI) analogs provide insight into the inter-relation of hormone structure, function, and dynamics. Although compatible with wild-type structure, short connecting segments (<3 residues) prevent induced fit upon receptor binding and so are essentially without biological activity. Substantial but incomplete activity can be regained with increasing linker length. Here, we describe the design, structure, and function of a single-chain insulin analog (SCI-57) containing a 6-residue linker (GGGPRR). Native receptor-binding affinity (130 +/- 8% relative to the wild type) is achieved as hindrance by the linker is offset by favorable substitutions in the insulin moiety. The thermodynamic stability of SCI-57 is markedly increased (DeltaDeltaG(u) = 0.7 +/- 0.1 kcal/mol relative to the corresponding two-chain analog and 1.9 +/- 0.1 kcal/mol relative to wild-type insulin). Analysis of inter-residue nuclear Overhauser effects demonstrates that a native-like fold is maintained in solution. Surprisingly, the glycine-rich connecting segment folds against the insulin moiety: its central Pro contacts Val(A3) at the edge of the hydrophobic core, whereas the final Arg extends the A1-A8 alpha-helix. Comparison between SCI-57 and its parent two-chain analog reveals striking enhancement of multiple native-like nuclear Overhauser effects within the tethered protein. These contacts are consistent with wild-type crystal structures but are ordinarily attenuated in NMR spectra of two-chain analogs, presumably due to conformational fluctuations. Linker-specific damping of fluctuations provides evidence for the intrinsic flexibility of an insulin monomer. In addition to their biophysical interest, ultrastable SCIs may enhance the safety and efficacy of insulin replacement therapy in the developing world.     

Characterization of a tobacco TPK-type K+ channel as a novel tonoplast K+ channel using yeast tonoplasts

The tonoplast K(+) membrane transport system plays a crucial role in maintaining K(+) homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K(+) channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased approximately 2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K(+) over Na(+). NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca(2+), but a cytosolic concentration of 45 microM Ca(2+) resulted in a 2-fold increase in the amplitude of the K(+) current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K(+) currents. These results show that NtTPK1 is a novel tonoplast K(+) channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.     

An O6-methylguanine-DNA methyltransferase-like protein from Thermus thermophilus interacts with a nucleotide excision repair protein

The major damage to DNA caused by alkylating agents involves the formation of O(6)-methylguanine (O(6)-meG). Almost all species possess O(6)-methylguanine-DNA-methyltransferase (Ogt) to repair such damage. Ogt repairs O(6)-meG lesions in DNA by stoichiometric transfer of the methyl group to a cysteine residue in its active site (PCHR). Thermus thermophilus HB8 has an Ogt homologue, TTHA1564, but in this case an alanine residue replaces cysteine in the putative active site. To reveal the possible function of TTHA1564 in processing O(6)-meG-containing DNA, we characterized the biochemical properties of TTHA1564. No methyltransferase activity for synthetic O(6)-meG-containing DNA could be detected, indicating TTHA1564 is an alkyltransferase-like protein. Nevertheless, gel shift assays showed that TTHA1564 can bind to DNA containing O(6)-meG with higher affinity (9-fold) than normal (unmethylated) DNA. Experiments using a fluorescent oligonucleotide suggested that TTHA1564 recognizes O(6)-meG in DNA using the same mechanism as other Ogts. We then investigated whether TTHA1564 functions as a damage sensor. Pull-down assays identified 20 proteins, including a nucleotide excision repair protein UvrA, which interacts with TTHA1564. Interaction of TTHA1564 with UvrA was confirmed using a surface plasmon resonance assay. These results suggest the possible involvement of TTHA1564 in DNA repair pathways.     

High-resolution crystals of the HU mutant K38N from Bacillus stearothermophilus

The DNA-binding protein HU is ubiquitous in the prokaryotic cell. It is a major protein component of isolated nucleoids and is believed to control the tertiary structure of prokaryotic DNA. The Bacillus stearothermophilus HU (BstHU) mutants obtained by mutagenesis have been investigated. Crystallization experiments of BstHU-K38N (Lys38 is substituted with Asn) resulted in two forms of crystals suitable for high-resolution x-ray analysis. The first form belongs to the monoclinic space group C2 with unit-cell dimensions of a = 90.1 A, b = 43.5 A, c = 63.7 A, and beta = 135.1 degrees, and it diffracts x rays to 1.5-A resolution. The second form belongs to the tetragonal space group I41 with a = b = 62.6 A and c = 43.3 A, and it diffracts up to 1.8-A resolution.     

Up-regulated L-type high voltage-gated calcium channels cause increase in diazepam binding inhibitor induced by sustained morphine exposure in mouse cerebrocortical neurons

Mechanisms of increase in diazepam binding inhibitor (DBI) mRNA expression in mouse cerebrocortical neurons after sustained morphine exposure were investigated. Increases in DBI and its mRNA expressions induced by sustained morphine (0.3 μM) exposure for 3 days were completely abolished by naloxone and nifedipine, but not by ω-agatoxin VIA and ω-conotoxin GIVA. Increase in [³H]diltiazem binding to the particulate fractions from the morphine-treated neurons was due to increased B_max value with no changes in K_d value. Western blot analysis on l-type high voltage-gated calcium channel (HVCC) subunits revealed the increased expressions of α1C, α1D, and α2/δ1 subunits and decreased of β4 subunit expression, whereas expression of N- and P/Q-type HVCC subunits was not changed. These results indicate that morphine-induced increase in DBI mRNA expression is mediated via increased Ca²⁺ entry through up-regulated l-type HVCCs.     

Preparation of C60-based active esters and coupling of C60 moiety to amines or alcohols

We report the syntheses of C(60)-based active esters and the coupling of their C(60) moiety to various amines or alcohols. Methano[60]fullerene carboxylic acid was activated by esterification with N-hydroxysuccinimide (NHS) or pentafluorophenol (PFP) and the active esters were isolated. Reactions of the active esters with amines or alcohols proceeded easily to give a variety of compounds having the C(60) moiety.     

Catalase gene disruptant of the human pathogenic yeast Candida albicans is defective in hyphal growth, and a catalase-specific inhibitor can suppress hyphal growth of wild-type cells

Although the catalase gene (CAT1) disruptant of the human pathogenic yeast Candida albicans was viable under ordinary growth conditions, we previously found that it could not grow on YPD (yeast extract/peptone/dextrose) containing SDS or at higher growth temperatures. To investigate the pleiotrophic nature of the disruptant, we examined the effect of the catalase inhibitor 3-AT on the growth of wild-type strains. Surprisingly, the addition of 3-AT and SDS caused the wild-type cells to be non-viable on YPD plates. We found an additional phenotype of the catalase gene disruptant: it did not produce normal hyphae on Spider medium. Hyphal growth was observed in a CAP1 (Candida AP-1-like protein gene) disruptant, a HOG1 (high-osmolarity glycerol signaling pathway gene) disruptant, and the double CAP1/HOG1 disruptant, suggesting that the defect in hyphal formation by the catalase disruptant was independent of these genes. Addition of 3-AT and SDS to hyphae-inducing media suppressed growth of normal hyphae in the wild-type strain. The potential necessity for catalase action upon exposure to hyphae-inducing conditions was confirmed by the immediate elevation of the catalase gene message. In spite of the requirement for catalase during hyphal growth, the catalase gene disruptant was capable of forming germ tubes in medium containing serum.