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M Tabak

Publications and source records attributed to M Tabak.

At least 19 recordsLinked to original sources
PubMed1998

Different Micellar Packing and Hydrophobicity of the Membrane Probes TEMPO and TEMPOL Influence Their Partition Between Aqueous and Micellar Phases Rather than Location in the Micelle Interior.

We report the investigation of the distribution of two stable nitroxyl radicals, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), between aqueous and micellar phases and in the micelle interior by quenching of fluorescence of dipyridamole (DIP) and by 1H NMR, ESR, and optical absorption spectroscopies. Cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS), zwitterionic N-hexadecyl-N,N-dimethyl 3-ammonio-1-propanesulfonate (HPS), and non-ionic octylphenoxypolyethoxyethanol (Triton X-l00) micelles were used. In all types of micelles the binding constants for more hydrophobic TEMPO appear higher than those for TEMPOL. At the same time the 1H NMR and optical absorption data show that the distribution of both radicals in the micelle interior is practically the same: The paramagnetic fragments are localized in the interface between the polar headgroup region and the hydrophobic interior of micelles. There is no clear correlation between Kb and the micelle charge. Indeed, Kb for TEMPO with SDS (2150 M-1) is lower than with CTAC (3250 M-1), but higher than with Triton X-l00 (1300 M-1) or HPS (1000 M-1). The Kb values correlate quite well with the packing of the monomers in the micelle, being higher for the loosely packed micelles (higher cmc). Besides, the micelle headgroup hydration water seems to decrease the binding constant. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

A molecular model for the d chain of the giant haemoglobin from Lumbricus terrestris and its implications for subunit assembly.

A structural model for the monomeric d chain of the giant haemoglobin from Lumbricus terrestris is described. Based on the crystal structures of other globins, the model provides evidence for the existence of a novel tryptophan-haem interaction. The observation that all three tryptophans are buried within the hydrophobic core is consistent with fluorescence data on the isolated monomer and the intact molecule. The model has also been used to predict the probable arrangement of the abcd tetramer as being similar to that observed in the clam Hb II structure. Such predictions allow the identification of four residues of particular importance in stabilising one of the subunit-subunit interfaces: Arg48, Arg97, His89 and Gln93. The latter two may be of special importance in the mediation of cooperative effects within the tetramer and indeed the intact molecule.

Amino Acid Sequence↗
PubMed1997

Binding of dipyridamole to phospholipid vesicles: a fluorescence study.

Binding and localization of the vasodilator and antitumor drug coactivator dipyridamole (DIP) and one of its derivatives, RA25, to phospholipid vesicles of DMPC (dimyristoylphosphatidylcholine) and DPPC (dipalmitoylphosphatidylcholine) was studied using fluorescence spectroscopy as well as quenching of fluorescence. The analysis of fluorescence data indicates that neutral dipyridamole binds to the phospholipids in their liquid crystalline phase with an association constant of 950 M(-1) and 1150 M(-1) to DMPC and DPPC, respectively. Protonation of DIP leads to a 3-fold reduction of the association constant. For the gel phospholipid phase, the binding is smaller (a factor of 2), independently of pH, suggesting that the more flexible lipid packing in the liquid crystalline phase facilitates the binding of the drug. The association constant of RA25 neutral form is considerably lower than for DIP, being around 295 M(-1). Fluorescence quenching with nitroxides TEMPO and stearic acid doxyl derivatives suggests the localization of DIP to be closer to the 5th carbon of alkyl chain. The quenching effect of 5-DSA below the lipid phase transition suggests that a strong static quenching may be operative. The quenching effect of 16-DSA is almost as great as that for 5-DSA below the phase transition, being even higher above the phase transition. This effect is probably due to the trans-gauche isomerization of the stearic acid nitroxide, making the encounter of its paramagnetic fragment with the DIP chromophore possible. Our data are consistent with DIP location close to the bilayer surface in the border of hydrophobic-polar heads interface which is similar to the data in micellar systems. In the case of RA25, the drug is in the outer part of the head group interface being much exposed to the aqueous phase and being significantly less accessible to the membrane nitroxide quenchers.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1997

Dipyridamole Interacts with the Polar Part of Cationic Reversed Micelles in Chloroform: 1H NMR and ESR Evidence

The interaction of dipyridamole (DIP) with reversed micelles (RM) of cetyltrimethylammonium chloride (CTAC) in CDCl3 at different water contents was studied. The position and T1 relaxation of the water peak upon addition of extra water revealed three concentration ranges of CTAC: <10 mM (impurity water is mainly dispersed in CDCl3), >50-100 mM (water mainly inside the RM), and intermediate range. The resonances of CTAC protons in the polar layer broadened and displaced by up to 0.07 ppm as a function of CTAC concentration and extra water. At 10 mM CTAC, the addition of 40 mM DIP shifted the head group signals to high field by about 0.1 ppm. At high and intermediate CTAC concentrations, four nitroxide spin probes, hydrophobic 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), hydrophilic 4-amine-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPAMINE), and lipophilic 5- and 16-doxyl stearic acids (5- and 16-DSA), underwent partial immobilization. The rotational correlation time of TEMPAMINE (rather than TEMPO, 5-, and 16-DSA) in RM moderately increased upon addition of 1.5-2.0 mM DIP. At an excess of CTAC, only one DIP peak at 3.88 ppm remained measurable, and its selective T1 fell from 0.34 to 0.12 s. The association constant for DIP and CTAC was between 10 and 35 M-1. Thus, DIP incorporates into the polar region of RM influencing packing and dynamics of surfactant head groups. In contrast, in aqueous CTAC micelles, the preferential localization of DIP substituents is inside the nonpolar micelle core, and the binding constant is two orders of magnitude above that for RM.

Journal Article↗
PubMed1997

Aggregation phenomena in the complexes of iron tetraphenylporphine sulfonate with bovine serum albumin.

Binding of Fe(III) meso-tetrakis(p-sulfonatophenyl)-porphyrin (FeTPPS4) to bovine serum albumin (BSA) was studied by UV-VIS absorption, fluorescence quenching, circular dichroism, 1H NMR, and ESR. At excess of BSA, the bound form of FeTPPS4 is a high-spin monomer exhibiting a Soret band at 417 nm, a broad NMR peak at 10.3 ppm, an ESR signal at g = 5.7-5.9, and a strong enhancement of magnetic relaxation of water protons. In the intermediate concentration range, a formation of nonparamagnetic bound aggregates of FeTPPS4 occurs (up to 10-15 molecules at pH 6.0) with a Soret band at 414 nm and NMR peaks at 7.0, 8.1, and 12.7 ppm. In the physiologic pH range, BSA binds the monomeric form of FeTPPS4 with an association constant of about 10(8) M-1, the affinity to oxo-dimers in solution being much lower. BSA itself is also subject to aggregation with an average aggregation number of 4-8 in the physiological pH range. It is assumed that aggregation phenomena may play an important role, both in the relaxation efficiency of metalloporphyrins as MRI contrast agents and in the blood transport of porphyrin drugs by albumins.

Animals↗
PubMed1997

Inhibitory effect of dipyridamole and its derivatives on lipid peroxidation in mitochondria.

Dipyridamole (DIP), 2,6-bis(diethanolamino)-4,8-dipiperidino-[5,4-d] pyrimidine, is a coronary vasodilator widely used in clinics. It has also been reported to have coactivator activity for a number of antitumour drugs and antioxidant activity in membrane systems. In recent years we have been studying the spectroscopic properties of this drug and several of its derivatives as well as their interaction with charged micelles and phospholipid monolayers. A strong interaction of DIP and DIP derivatives with these model membrane systems and a dependence of the strength of the interaction upon the chemical structure of the DIP derivative was observed. Here, the antioxidant effect of DIP and the derivatives, RA14, RA47, and RA25, was compared. We observed that although it strongly inhibits the iron-induced lipoperoxidation on mitochondria (IC50 = 1 microM), it shows no protection against an organic oxidant, cumene hydroperoxide. The order of hydrophobicity of the DIP derivatives, DIP > RA14 > RA47 > RA25, correlates very well with both the values of the association constants of these derivatives to micelles, their localization in the micelles, and phospholipid films and their antioxidant effect on mitochondria. So, a very good correlation of the structure of the drug in regarded to the nature of its substituents with the biological activity is observed. Essentially the same result was observed either measuring the lipid peroxidation or the membrane fluidity by ESR, suggesting that the effect of DIP and DIP derivatives is probably associated to their binding to the lipid bilayer and not to interaction with membrane proteins.

Animals↗
PubMed1997

Effect of ranitidine on the urea breath test: a controlled trial.

Because Helicobacter pylori is an acid-sensitive organism, an elevation of the gastric pH by H2 inhibitors might improve the intragastric conditions for the development of this organism. We tested this hypothesis in a prospective and controlled trial including 43 patients positive for H. pylori using the rapid urease test. Twenty-six patients received 150 mg ranitidine twice daily and 17 patients received no treatment. The 14C-urea breath test was performed in both groups at the beginning of the study and 2 weeks later. Radioactive 14C in exhaled carbon dioxide was significantly increased (p = 0.045) in the patients treated with ranitidine, compared with the patients in the control group. Administration of this drug to patients infected with H. pylori is associated with an increase in the bacterial load after 2 weeks of treatment. This phenomenon might be attributed to increased bacterial growth due to the H2 blocker.

Anti-Ulcer Agents↗
PubMed1996

Spectroscopic studies of the met form of the extracellular hemoglobin from Glossoscolex paulistus.

Sephadex G-200 chromatography of the extracellular hemoglobin from the giant earthworm G. paulistus in the met form presents a single peak at pH 7.0 and two peaks at pH 9.0 as a result of alkaline dissociation. SDS-PAGE shows that the polypeptide chains are very similar to those observed for the oxy form and the two peaks at pH 9.0 correspond to the trimer contaminated by linkers and monomers which seems to be quite pure. The aquomet acid form is stable as an oligomer of molecular mass 3.1 x 10(6) Da only in a narrow pH range around neutrality. Increasing the pH above 7.5 leads to an irreversible transition from aquomet to hemichrome I which is the low-spin bis-imidazole complex. At pHs above 9.5-10.0 a second reversible transition takes place from hemichrome I to hemichrome II, a high-spin complex which is associated with the weakening and possible disruption of the proximal Fe--N histidine bond. Thus, increase in pH above 8.0 induces changes in the heme pocket that involve both the distal and proximal sides of the heme. EPR measurements show a very sharp decrease of the aquomet high-spin signal in the range of pH 7.0-8.0 and a very small low-spin signal even at liquid helium temperatures. The transition to hemichrome I is also accompanied by the loss of heme optical activity monitored by CD, which is consistent with the weakening of heme--globin interaction. Hemichrome I in the presence of cyanide gives the typical cyanometHb derivative which has a transition to a hemichrome at much higher pHs. This observation suggests that the dissociation of the oligomer in alkaline medium as well as the stability of the heme on the proximal side, depend both upon the ligand present at the sixth coordination position on the distal side. Hence, we believe that hemi(hemo)chrome formation in G. paulistus Hb and other invertebrate hemoglobins is a common phenomenon, not associated with protein denaturation, which may provide a fine tuning mechanism to control subunit interactions through changes in the distal side of the heme pocket.

Animals↗
PubMed1996

An ESR and spectrophotometric study of the denitration of nitroheterocyclic drugs by liver homogenates and their metabolic consumption by liver microsomes from cytochrome P-450-induced mice.

This work extends a previous study on the mechanism of hepatic denitration of two nitroheterocyclic drugs (NHCD), quinifuryl and nitracrine, in which the release of nitric oxide (NO) from these compounds can be accompanied by the formation of a NO-heme iron complex. Pretreating mice with three inducers of cytochrome P-450 (phenobarbital, clophen A50 and butylated hydroxytoluene (BHT)) increased the yield of the nitrosyl complex which correlated with a rise in the cytochrome P-450 content of mouse liver microsomes. In contrast, treating the animals with beta-naphthoflavone decreased the complex yield while still increasing P-450 content. Treating the animals with any of the above inducers significantly increased the rate of NHCD metabolism in mouse liver microsomes. Based on these results, a possible mechanism for hepatic NHCD denitration is discussed.

Aerobiosis↗
PubMed1996

Interaction of dipyridamole with lipids in mixed Langmuir monolayers.

Dipyridamole (DIP), a well known coronary vasodilator and coactivator of anti-tumor activity of a number of drugs, forms stable Langmuir monolayers with the zwitterionic lipid dipalmitoylphosphatidylcholine (DPPC) and the negatively charged dipalmitoylphosphatidylglycerol (DPPG) at an air/aqueous solution interface. The drug binds to the lipid molecules and change their packing density in the monolayer in the process of compression, the effect depending on the drug location in the monolayer, protonation of the drug and also on the charge state of the lipid. The incorporation of dipyridamole (DIP) into neutral DPPC monolayers causes them to be more expanded at low DIP concentrations but more condensed at high concentrations, resembling the effect of cholesterol. Maximum expansion occurs for a DIP concentration of 2 mol%. For slightly charged DPPG monolayers spread on ultra pure water, the monolayers become increasingly more expanded with increasing DIP concentrations. For the negatively charged DPPG monolayers spread on buffer solutions, the incorporation of DIP has similar effects to that observed for DPPC monolayers. This is probably due to the interaction between the charged DPPG molecules and the protonated DIP molecules. Also, introduction of protonated DIP brings an increase in surface potential of DPPG monolayers because the negative contribution from the double layer is decreased. The results indicated that DIP molecules are located deeper in the hydrophobic region of DPPC monolayers, whereas in DPPG ones they appear to be located very close to the polar head region. Due to the electrostatic interaction of protonated DIP with the charges on the polar heads of lipids it is inclined with respect to the plane of the monolayer.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1996

H NMR and electronic absorption spectroscopy of paramagnetic water-soluble meso-tetraarylsubstituted cationic and anionic metalloporphyrins.

The ionization, mu-oxo-dimerization and axial ligation equilibria of free bases, iron(III) and manganese(III) derivatives of meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS4) and meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin (TMPyP) in aqueous solution are studied by 1H NMR and electronic absorption spectroscopy. At physiological pH, Fe(III) complexes of TMPyP and TPPS4 exist predominantly as dimers and may undergo transition to low spin species upon binding to biomolecules, whereas Mn(III) complexes are essentially monomeric. Dicyano and bis-imidazole complexes of FeTMPyP and FeTPPS4 are low spin monomer adducts in the pH range 2.0 to 11.2. No low spin dimeric complexes were found. The low spin monocyano and high spin mono-imidazole complexes of FeTMPyP are formed in acidic and alkaline media, respectively. T1-relaxation enhancement of water protons at 200 MHz induced by FeTPPS4 falls dramatically in the sequence high spin >> dimeric > low spin form.

Chemical Phenomena↗
PubMed1996

Binding of manganese and iron tetraphenylporphine sulfonates to albumin is relevant to their contrast properties.

The interaction of Fe(III) and Mn(III) complexes of TPPS4 with bovine serum albumin (BSA) was studied by T1 relaxation measurements of water protons and high resolution 1H NMR of the porphyrin moieties. At excess of BSA, both metalloporphyrins bind to BSA as the high spin monomers. The relaxivity of bound MnTPPS4 is significantly higher as compared to the free form in solution. When metalloporphyrins are in excess, they aggregate at the BSA surface, up to two MnTPPS4, and up to 10-15 FeTPPS4 units per BSA globule. Bound aggregates are unable to enhance magnetic relaxation of water protons due to the antiferromagnetic coupling between metal ions in the aggregates. Therefore, the dose-effect dependences for metalloporphyrins in the range of metalloporphyrin/BSA ratio of 0 to 25 at the constant BSA concentration at pH 7.4 are characterized by a local maximum at about 2 for MnTPPS4, and a global maximum at about 3 for FeTPPS4, MnTPPS4 complex is more effective than FeTPPS4 in the whole concentration range. It is suggested that the difference in binding and aggregation properties of metalloporphyrins may be relevant to their relaxation efficiency in vivo, blood transport, and biodistribution.

Animals↗
PubMed1996

Water increases the fluidity of intercellular membranes of stratum corneum: correlation with water permeability, elastic, and electrical resistance properties.

We used the spin label electron spin resonance technique to monitor the hydration effect on the molecular dynamics of lipids at C-5, C-12, and C-16 positions of the alkyl chain. Increase in water content of neonatal rat SC leads to an increase in membrane fluidity, especially in the region near the membrane-water interface. The effect is less pronounced deeper inside the hydrophobic core. The reorientational correlation time at the C-16 position of hydrocarbon chains showed a higher change up to approximately 18% (w/w) of water content. This behavior was accompanied by an exponential decay both in elastic modulus and electrical resistance with water content. On the contrary, the segmental motion at C-5 and C-12 positions of the chain and the permeability constant increased in the range of around 18% w/w) up to the fully hydrated condition (58 +/- 7%). Our results give a better characterization of the fluidity of SC and show that it is the principal parameter involved in the mechanism of the permeability of different compounds through skin.

Animals↗
PubMed1996

In vitro inhibition of Helicobacter pylori by extracts of thyme.

Extracts of several plants were tested for inhibitory activity against Helicobacter pylori. Among these plants thyme (aqueous extract) and cinnamon (alcoholic extract) were the most effective. Since aqueous extract of thyme is easier to produce and consume, it was further investigated. Compared with several antibacterials, the thyme extract had a significant inhibitory effect on H. pylori, reducing both its growth and potent urease activity. From the results of this study, the aqueous extract of thyme possesses a therapeutic potential which merits validation by clinical studies.

Anti-Bacterial Agents↗
PubMed1995

Conformational changes upon binding of a receptor loop to lipid structures: possible role in signal transduction.

The mas oncogene codes for a seven transmembrane helix protein. The amino acid sequence 253-266, from the third extracellular loop and beginning of helix 7, was synthesized either blocked or carrying an amino acid spin label at the N-terminus. Peptide binding to bilayers and micelles was monitored by ESR, fluorescence and circular dichroism. Binding induced tighter lipid packing, and caused an increase of peptide secondary structure. While binding to bilayers occurred only when peptide and phospholipid bore opposite charges, in micelles the interaction took place irrespective of charge. The results suggest that changes in lipid packing could modulate conformational changes in receptor loops related to the triggering of signal transduction.

Amino Acid Sequence↗
PubMed1995

Localization of dipyridamole molecules in ionic micelles: effect of micelle and drug charges.

The localization of the coronary vasodilator dipyridamole (DIP) in cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS) and zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and lysophosphatidylcholine (HPS and LPC) micelles was investigated using fluorescence quenching by quenchers with known localization in the micelle (TEMPO and 5-doxyl and 12-doxyl stearic acids). The use of fluorescence quenching jointly with fluorescence and 1H-NMR spectral measurements shows that DIP molecules in both protonated and nonprotonated forms are localized in micelles near the region which separates their polar and nonpolar parts, the polarizable heteroaromatic cycle of DIP being close to the polar part and the nonpolar substituents penetrating the hydrophobic interior of the micelle. The electrostatic interaction between the protonated DIP molecules and micelle charges either moves DIP into the micelle interior (for cationic and zwitterionic micelles) or draws it closer to the micelle surface (for anionic ones). Our results could be relevant to the mechanism of DIP action since many data indicate the interaction of the drug with cell membranes. The ability of DIP to localize near the membrane surface with the substituents immersed into a hydrophobic moiety could be essential for the drug interaction with P-glycoprotein, which is responsible for mediation of the effects of several antitumour drugs.

Dipyridamole↗
PubMed1995

Effect of hydration upon the fluidity of intercellular membranes of stratum corneum: an EPR study.

The principal mechanisms controlling the molecular permeability through the skin are associated to the intercellular membranes of stratum corneum (SC), the outermost layer of mammalian skin. It is generally accepted that an increase in fluidity of these membranes leads to a reduction of the physical barrier exerted by SC with a consequent enhancement in permeation of different compounds. It is known that water diffusion in SC increases with the increase in the water content in SC. Using the spin labeling method we evaluate the effect of hydration on the fluidity of intercellular membranes at three depths of the alkyl chain. Increase in the water content in SC leads to a drastic increase in membrane fluidity especially in the region near the membrane/water interface; the effect decreases on going deeper inside the hydrophobic core. Analysis of electron paramagnetic resonance (EPR) parameters as a function of temperature showed that the rotational motion at depth of the 16th carbon atom of the chain experienced a phase transition at 45 and 60 degrees C. These phase transition temperatures were not altered by changes in the water content of SC. A phase transition between 28 and 48 degrees C was observed from the segmental motion in the region near the polar headgroup (up to 12th carbon in the chain) and was strongly dependent upon the hydration of SC. Our results give a better characterization of the fluidity of SC, the main parameter involved in the mechanisms that control the permeability of different compounds through skin.

Animals↗