Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Onium Compounds”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 685 records · Page 38Linked to original sources
PubMed1978

Platelet 5-hydroxytryptamine transport, an electroneutral mechanism coupled to potassium.

Transport of 5-hydroxytryptamine into plasma membrane vesicles isolated from porcine blood platelets is stimulated when a potassium gradient (in greater than out) is imposed across the vesicle membrane. This stimulation occurs in the absence of measurable electrical potential across the membrane. Addition of valinomycin induces a membrane potential of approximately 50 mV (interior negative) as estimated by uptake of the lipophilic cation triphenylmethylphosphonium, but has surprisingly little effect on 5-hydroxytryptamine transport. Addition of 2,4-dinitrophenol dissipates the valinomycin-induced membrane potential. In the absence of valinomycin, 2,4-dinitrophenol has no effect on 5-hydroxytryptamine transport but valinomycin and 2,4-dinitrophenol together inhibit transport, probably by dissipation of the K+ gradient. These results are consistent with an electroneutral mechanism in which 5-hydroxytryptamine influx is directly coupled to potassium ion efflux and argue against an electrogenic mechanism in which there is a net influx of positive charge with 5-hydroxytryptamine.

Animals↗
PubMed2005

Substrate-induced tryptophan fluorescence changes in EmrE, the smallest ion-coupled multidrug transporter.

Tryptophan residues may play several roles in integral membrane proteins including direct interaction with substrates. In this work we studied the contribution of tryptophan residues to substrate binding in EmrE, a small multidrug transporter of Escherichia coli that extrudes various positively charged drugs across the plasma membrane in exchange with protons. Each of the four tryptophan residues was replaced by site-directed mutagenesis. The only single substitutions that affected the protein's activity were those in position 63. While cysteine and tyrosine replacements yielded a completely inactive protein, the replacement of Trp63 with phenylalanine brought about a protein that, although it could not confer any resistance against the toxicants tested, could bind substrate with an affinity 2 orders of magnitude lower than that of the wild-type protein. Double or multiple cysteine replacements at the other positions generate proteins that are inactive in vivo but regain their activity upon solubilization and reconstitution. The findings suggest a possible role of the tryptophan residues in folding and/or insertion. Substrate binding to the wild-type protein and to a mutant with a single tryptophan residue in position 63 induced a very substantial fluorescence quenching that is not observed in inactive mutants or chemically modified protein. The reaction is dependent on the concentration of the substrate and saturates at a concentration of 2.57 microM with the protein concentration of 5 microM supporting the contention that the functional unit is a dimer. These findings strongly suggest the existence of an interaction between Trp63 and substrate, and the nature of this interaction can now be studied in more detail with the tools developed in this work.

Amino Acid Sequence↗
PubMed2006

Iodination of proteins by IPy2BF4, a new tool in protein chemistry.

Iodination is a very useful method for protein characterization and labeling. However, derivatization chemistries used in most conventional iodination procedures may cause substantial alterations in protein structure and function. The IPy(2)BF(4) reagent [bis(pyridine)iodonium (I) tetrafluoroborate] has been shown to be an effective iodinating reagent for peptides. Herein we report the first application of IPy(2)BF(4) in protein iodination in an aqueous medium using three representative substrates: insulin, lysozyme, and the enzyme 1,3-1,4-beta-d-4-glucanohydrolase. Our results show that IPy(2)BF(4) has clear advantages over existing methods in that the reaction is quantitative, fast, and selective for the most accessible Tyr residues of a protein, and it preserves the functional integrity of the protein when moderate Tyr labeling levels are pursued.

Amino Acid Sequence↗
PubMed1996

2H NMR detection of transmembrane potential-driven tetraphenylphosphonium transbilayer redistribution.

2H NMR of specifically choline-deuterated phosphatidylcholine incorporated into giant unilamellar vesicles (GUVs), composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) plus 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) plus cholesterol (CHOL), was shown to detect a transmembrane potential-driven redistribution of the potential-sensitive, surface- binding dye tetraphenylphosphonium (TPP+) across the GUV lipid bilayer. The method is based on resolving differences in the surface charge at the inner versus the outer monolayer of the vesicle's bilayer using the so-called 2H NMR "molecular voltmeter" technique. A mathematical model to describe the 2H NMR results was derived by combining the Nernst, Boltzmann, Langmuir, and Gouy-Chapman equations with the established sensitivity of deuterium quadrupolar splittings from choline- deuterated POPC to surface electrostatic charge effects. This model identified experimental factors likely to yield enhanced sensitivity and resolution of the inner versus outer monolayer surface charges via 2H NMR. The predictions of the model were then confirmed experimentally. The improvement in resolution resulting from these studies removes a major hindrance to the general exploitation of 2H NMR for monitoring transbilayer surface charge asymmetrics.

Coloring Agents↗
PubMed1998

Human protoporphyrinogen oxidase: relation between the herbicide binding site and the flavin cofactor.

Protoporphyrinogen IX oxidase (protox) catalyzes the oxidation of protoporphyrinogen IX to protoporphyrin IX in the penultimate step of heme and chlorophyll biosynthesis in animals and plants. Protox is the target of light-dependent peroxidizing herbicides and is inhibited at nanomolar levels by several chemical classes including tetrahydrophthalimides (discussed below) and diphenyl ethers (e.g., acifluorfen) usually with little selectivity between the mammalian and plant enzymes. The herbicide binding site is examined here with a photoaffinity radioligand optimized on the basis of structure-activity relationships. A radiosynthetic procedure is described for this new herbicidal probe, N-(5-azido-4-chloro-2-fluorophenyl)-3,4,5, 6-[3H]tetrahydrophthalimide ([3H]AzTHP), resulting in high specific activity (2.6 TBq/mmol). Human protox expressed in Escherichia coli and purified by affinity chromatography is used with [3H]AzTHP to characterize the herbicide/substrate binding site. Specific binding of [3H]AzTHP to human protox is rapid, completely reversible in the absence of light with a Kd of 93 nM, and competitively inhibited by the 5-propargyloxy analogue and by acifluorfen, which are known to bind at the substrate (protoporphyrinogen) site. The Bmax establishes one [3H]AzTHP binding site per FAD. Diphenyleneiodonium, proposed to inhibit protox by interaction with the FAD cofactor, inhibits enzyme activity by 48% at 100 micro M without affecting [3H]AzTHP binding in the presence or absence of substrate, suggesting that the herbicide binding site may not be proximal to FAD. The first step has been taken in photoaffinity labeling the herbicide/substrate site with [3H]AzTHP resulting in apparent covalent derivatization of 13% of the herbicide binding site.

Binding Sites↗
PubMed1999

The secondary multidrug transporter LmrP contains multiple drug interaction sites.

The secondary multidrug transporter LmrP of Lactococcus lactis mediates the efflux of Hoechst 33342 from the cytoplasmic leaflet of the membrane. Kinetic analysis of Hoechst 33342 transport in inside-out membrane vesicles of L. lactis showed that the LmrP-mediated H(+)/Hoechst 33342 antiport reaction obeyed Michaelis-Menten kinetics, with a low apparent affinity constant of 0.63 microM Hoechst 33342 (= 0.5 mmol Hoechst 33342/mol phospholipid). Several drugs significantly inhibited LmrP-mediated Hoechst 33342 transport through a direct interaction with the protein rather than through dissipation of the proton motive force or reduction of the membrane partitioning of Hoechst 33342. The characterization of the mechanism of inhibition of LmrP-mediated Hoechst 33342 transport indicated competitive inhibition by quinine and verapamil, noncompetitive inhibition by nicardipin and vinblastin, and uncompetitive inhibition by TPP(+). The three types of inhibition of LmrP-mediated Hoechst 33342 transport in inside-out membrane vesicles indicate for the first time the presence of multiple drug interaction sites in a secondary multidrug transporter.

Bacterial Proteins↗
PubMed2002

Alkynyliodonium salts in organic synthesis. Application to the total synthesis of (-)-agelastatin A and (-)-agelastatin B.

The asymmetric total syntheses of (-)-agelastatin A and (-)-agelastatin B were accomplished in 14 steps each from (R)-epichlorohydrin. The pivotal transformation in both sequences was a sulfinate-promoted cyclization of an alkynyliodonium salt to furnish a key functionalized cyclopentene intermediate. Selective bromination in the final step led to either agelastatin A or agelastatin B, depending upon conditions.

Alkaloids↗
PubMed2002

Alkynyliodonium salts in organic synthesis. Application to the total synthesis of the tropoloisoquinoline alkaloid pareitropone.

The synthesis of the tropoloisoquinoline alkaloid pareitropone has been accomplished in 14 steps from 2,3,4-trimethoxybenzoic acid. The key transformations include the generation of an alkylidenecarbene intermediate through intramolecular addition of a tosylamide anion to an alkynyliodonium salt, and the cycloaddition of that carbene to a peri positioned aromatic ring to afford a cycloheptatrienylidene product featuring the intact pareitropone skeleton.

Alkaloids↗
PubMed2003

Metal-stabilized phenoxonium cation.

An unprecedented metal-stabilized phenoxonium cation was prepared by a process involving dearomatization of a phenoxy complex. The unique eta2 C=O-metal (iridium) coordination mode leaves the positive charge delocalized within the former aromatic ring. The X-ray structure and conversion into eta2 C=O-coordinated metal-quinone complex are described.

Cations↗