Binding of aromatic donor molecules to lactoperoxidase: proton NMR and optical difference spectroscopic studies

The interaction of aromatic donor molecules with lactoperoxidase (LPO) was studied using 1H-NMR and optical difference spectroscopy techniques. pH dependence of substrate proton resonance line-widths indicated that the binding was facilitated by protonation of an amino acid residue (with pKa of 6.1) which is presumably a distal histidine. Dissociation constants evaluated from both optical difference spectroscopy and 1H-NMR relaxation measurements were found to be an order of magnitude larger than those for binding to horse radish peroxidase (HRP), indicating relatively weak binding of the donors to LPO. The dissociation constants evaluated in presence of excess of I- and SCN- showed a considerable increase in their values, indicating that the iodide and thiocyanate ions compete for binding at the same site. The dissociation constant of the substrate binding was, however, not affected by cyanide binding to the ferric centre of LPO. All these results indicate that the organic substrates bind to LPO away from the ferric center. Comparison of the dissociation constants between the different substrates suggested that hydrogen bonding of the donors with the distal histidine amino acid, and hydrophobic interaction between the donors and the active site contribute significantly towards the associating forces. Free energy, entropy and enthalpy changes associated with the LPO-substrate equilibrium have been evaluated. These thermodynamic parameters were found to be all negative and relatively low compared to those for binding to HRP. The distances of the substrate protons from the ferric center were found to be in the range 9.4-11.1 A which are 2-3 A larger than those reported for the HRP-substrate complexes. These structural informations suggest that the heme in LPO may be more deeply buried in the heme crevice than that in the HRP.     

Interrogation of heme pocket environment of mammalian peroxidases with diatomic ligands

Recent studies demonstrate that myeloperoxidase (MPO), eosinophil peroxidase (EPO), and lactoperoxidase (LPO), homologous members of the mammalian peroxidase superfamily, can all serve as catalysts for generating nitric oxide- (nitrogen monoxide, NO) derived oxidants. These enzymes contain heme prosthetic groups that are ligated through a histidine nitrogen and use H(2)O(2) as the electron acceptor in the catalysis of oxidative reactions. Here we show that heme reduction of these peroxidases results in distinct electronic and/or conformational changes in their heme pockets using a combination of rapid kinetics measurements, optical absorbance, and diatomic ligand binding studies. Addition of reducing agent to each peroxidase at ground state [Fe(III) state] causes immediate buildup of the corresponding Fe(II) complexes. Spectral changes indicate that two LPO-Fe(II) species are present in solution at equilibrium. Analyses of stopped-flow traces collected when EPO, MPO, or LPO solutions rapidly mixed with NO were accurately fit by single-exponential functions. Plots of the apparent rate constants as a function of NO concentration for all Fe(III) and Fe(II) forms were linear with positive intercepts, consistent with NO binding to each form in a simple reversible one-step mechanism. Fe(II) forms of MPO and LPO, but not EPO, displayed significantly lower affinity toward NO compared to Fe(III) forms, suggesting that heme reduction causes a dramatic change in the heme pocket electronic environment that alters the affinity and/or accessibility of heme iron toward NO. Optical absorbance spectra indicate that CO binds to the Fe(II) forms of both LPO and EPO, but not with MPO, and generates their respective low-spin six-coordinate complexes. Kinetic analyses indicate that the binding of CO to EPO is monophasic while CO binding to LPO is biphasic. Collectively, these results illustrate for the first time functional differences in the heme pocket environments of Fe(II) forms of EPO, LPO, and MPO toward binding of diatomic ligands. Our results suggest that, upon reduction, the heme pocket of MPO collapses, LPO adopts two spectroscopically and kinetically distinguishable forms (one partially open and the other relatively closed), and EPO remains open.     

Lipid peroxidation in children with Duchenne's hereditary myopathy

Eighteen patients aged 4 to 13 years with Duchenne's progressive muscular dystrophy were examined for the content of lipid peroxidation (LPO) products (lowest volatile hydrocarbons) in the exhaled air. The level of LPO products was found to exceed normal 4-fold as compared to the control group including healthy children of the same age. The maximal increase was detected at the early stages of the disease with a rapid progressive degeneration. The role of LPO in the pathogenesis of Duchenne's muscular dystrophy and possible applications of free radical oxidation inhibitors as new effective medicinal tools are discussed.     

Steady-state kinetics of thiocyanate oxidation catalyzed by human salivary peroxidase

A steady-state kinetic analysis was made of thiocyanate (SCN-) oxidation catalyzed by human peroxidase (SPO) isolated from parotid saliva. For comparative purposes, bovine lactoperoxidase (LPO) was also studied. Both enzymes followed the classical Theorell-Chance mechanism under the initial conditions [H2O2] less than 0.2mM, [SCN-] less than 10mM, and pH greater than 6.0. The pH-independent rate constants (k1) for the formation of compound I were estimated to be 8 X 10(6) M-1 s-1 (SD = 1, n = 18) for LPO and 5 X 10(6) M-1 s-1 (SD = 1, n = 11) for SPO. The pH-independent second-order rate constants (k4) for the oxidation of thiocyanate by compound I were estimated to be 5 X 10(6) M-1 s-1 (SD = 1, n = 18) for LPO and 9 X 10(6) M-1 s-1 (SD = 2, n = 11) for SPO. Both enzymes were inhibited by SCN- at pH less than 6. The pH-independent equilibrium constant (Ki) for the formation of the inhibited enzyme-SCN- complex was estimated to be 24 M-1 (SD = 12, n = 8) for LPO and 44 M-1 (SD = 4, n = 10) for SPO. An apparent pH dependence of the estimated values for k4 and Ki for both LPO and SPO was consistent with a mechanism based on assumptions that protonation of compound I was necessary for the SCN- peroxidation step, that a second protonation of compound I gave an inactive form, and that the inhibited enzyme-SCN- complex could be further protonated to give another inactive form.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protein radical formation during lactoperoxidase-mediated oxidation of the suicide substrate glutathione: immunochemical detection of a lactoperoxidase radical-derived 5,5-dimethyl-1-pyrroline N-oxide nitrone adduct

A novel anti-5,5-dimethyl-1-pyrroline N-oxide (DMPO) polyclonal antiserum that specifically recognizes protein radical-derived DMPO nitrone adducts has been developed. In this study, we employed this new approach, which combines the specificity of spin trapping and the sensitivity of antigen-antibody interactions, to investigate protein radical formation from lactoperoxidase (LPO). When LPO reacted with GSH in the presence of DMPO, we detected an LPO radical-derived DMPO nitrone adduct using enzyme-linked immunosorbent assay and Western blotting. The formation of this nitrone adduct depended on the concentrations of GSH, LPO, and DMPO as well as pH values, and GSH could not be replaced by H(2)O(2). The level of this nitrone adduct was decreased significantly by azide, catalase, ascorbate, iodide, thiocyanate, phenol, or nitrite. However, its formation was unaffected by chemical modification of free cysteine, tyrosine, and tryptophan residues on LPO. ESR spectra showed that a glutathiyl radical was formed from the LPO/GSH/DMPO system, but no protein radical adduct could be detected by ESR. Its formation was decreased by azide, catalase, ascorbate, iodide, or thiocyanate, whereas phenol or nitrite increased it. GSH caused marked changes in the spectrum of compound II of LPO, indicating that GSH binds to the heme of compound II, whereas phenol or nitrite prevented these changes and reduced compound II back to the native enzyme. GSH also dose-dependently inhibited the peroxidase activity of LPO as determined by measuring 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) oxidation. Taken together, these results demonstrate that the GSH-dependent LPO radical formation is mediated by the glutathiyl radical, possibly via the reaction of the glutathiyl radical with the heme of compound II to form a heme-centered radical trapped by DMPO.     

Electron paramagnetic resonance spectroscopy of lactoperoxidase complexes: clarification of hyperfine splitting for the NO adduct of lactoperoxidase

Electron paramagnetic resonance (EPR) studies of the nitrosyl adduct of ferrous lactoperoxidase (LPO) confirm that the fifth axial ligand in LPO is bound to the iron via a nitrogen atom. Complete reduction of the ferric LPO sample is required in order to observe the nine-line hyperfine splitting in the ferrous LPO/NO EPR spectrum. The ferrous LPO/NO complex does not exhibit a pH or buffer system dependence when examined by EPR. Interconversion of the ferrous LPO/NO complex and the ferric LPO/NO2- complex is achieved by addition of the appropriate oxidizing or reducing agent. Characterization of the low-spin LPO/NO2- complex by EPR and visible spectroscopy is reported. The pH dependence of the EPR spectra of ferric LPO and ferric LPO/CN- suggests that a high-spin anisotropic LPO complex is formed at high pH and an acid-alkaline transition of the protein conformation near the heme site does occur in LPO/CN-. The effect of tris(hydroxymethyl)aminomethane buffer on the LPO EPR spectrum is also examined.     

H2O2-mediated cross-linking between lactoperoxidase and myoglobin: elucidation of protein-protein radical transfer reactions

The H(2)O(2)-dependent reaction of lactoperoxidase (LPO) with sperm whale myoglobin (SwMb) or horse myoglobin (HoMb) produces LPO-Mb cross-linked species, in addition to LPO and SwMb homodimers. The HoMb products are a LPO(HoMb) dimer and LPO(HoMb)(2) trimer. Dityrosine cross-links are shown by their fluorescence to be present in the oligomeric products. Addition of H(2)O(2) to myoglobin (Mb), followed by catalase to quench excess H(2)O(2) before the addition of LPO, still yields LPO cross-linked products. LPO oligomerization therefore requires radical transfer from Mb to LPO. In contrast to native LPO, recombinant LPO undergoes little self-dimerization in the absence of Mb but occurs normally in its presence. Simultaneous addition of 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and LPO to activated Mb produces a spin-trapped radical electron paramagnetic resonance signal located primarily on LPO, confirming the radical transfer. Mutation of Tyr-103 or Tyr-151 in SwMb decreased cross-linking with LPO, but mutation of Tyr-146, Trp-7, or Trp-14 did not. However, because DBNBS-trapped LPO radicals were observed with all the mutants, DBNBS traps LPO radicals other than those involved in protein oligomerization. The results clearly establish that radical transfer occurs from Mb to LPO and suggest that intermolecularly transferred radicals may reside on residues other than those that are generated by intramolecular reactions.     

Reaction of 3-amino-1:2:4-triazole with lactoperoxidase

Bovine lactoperoxidase (LPO) gradually lost its enzymatic activity when dialyzed against a solution of 3-amino-1:2:4-triazole (AT) and hydrogen peroxide at pH 7.0. Amino acid analysis of the completely inactive enzyme revealed the formation of a new ninhydrin-positive chromatographic peak. This peak which had been observed previously when catalases were similarly reacted with AT in the presence of hydrogen peroxide is attributed to the covalent reaction product of AT with a histidyl residue. Concurrently with the appearance of the new ninhydrin-positive peak, the histidine content of LPO decreased by approximately two residues. Four to five residues of tyrosine were also lost.     

Steroid hormonal regulation of calcium-binding protein regucalcin mRNA expression in the kidney cortex of rats

Ethylenediamine tetraacetate ( EDTA ) inhibits lactoperoxidase (LPO)-catalyzed rate of iodide oxidation in concentration and pH-dependent manner. A plot of log Ki_app values against various pH yields a sigmoidal curve from which an ionisable group of pK_a value 6.0 could be ascertained for controlling the inhibition of catalytically active LPO by EDTA. Kinetic studies indicate that EDTA competitively inhibits iodide oxidation by acting as an electron donor. EDTA al so reduces LPO-compound-11 to the native ferric state by one-electron transfer as evidenced by the spectral shift from 428 to 412 nm. Optical difference spectroscopic studies indicate that EDTA binds to LPO with the apparent equilibrium dissociation constant (KD) of 12 ± 2 mM at pH 6.5. A plot of log K_D values against various pH produces a sigmoidal curve from which an ionisable group of LPO having pk_a = 5.47 could be calculated, deprotonation of which favours EDTA binding. EDTA also binds to LPO-CN- complex indicating its binding site away from heme iron centre. The K_D of LPO-EDTA complex is significantly increased (62 ± 5 mM) by iodide suggesting that EDTA binds close to the iodide binding site. EDTA also increases the K_D value of LPO-hydroquinone complex from 62 ± 5 mM to 200 ± 21 mM indicating that EDTA and aromatic donor binding sites are also close. We suggest that EDTA inhibits iodide oxidation competitively as an electron donor by interacting at or near the iodide binding site and these sites are close to the aromatic donor binding site.     

Improved chromatographic method for purification of lactoperoxidase from different milk sources

Our previous studies showed that sulfanilamide is a new competitive inhibitor of and can be used in the purification of lactoperoxidase (LPO, EC1.11.1.7) from milk. However, this method has some disadvantages like a lower purification factor. The aim of the present study is to improve the purification process of milk LPO from different sources. For this purpose, 16 commercial sulfanilamide derivatives were selected for inhibition studies to determine the best inhibitor of bovine LPO by calculating kinetic parameters. A cyanogen bromide-activated Sepharose 4B affinity matrix was synthesized by coupling with each competitive inhibitor. Among the inhibitors, 5-amino-2-methylbenzenesulfonamide and 2-chloro-4-sulfamoylaniline were used as ligands for the purification of LPO from bovine, buffalo, cow, and goat milks with 1059.37, 509.09, 232.55, and 161.90, and 453.12-, 151.86-, 869.00-, and 447.57-fold, respectively. Our results show that 5-amino-2-methylbenzenesulfonamide, 2-chloro-4-sulfamoylaniline, and 5-amino-1-naphthalenesulfonamide are the best inhibitors for one-step purification of the enzyme.     

The inhibition stage of lipid peroxidation during stress

Stress is shown to induce at first the generalized inhibition of lipid peroxidation (LPO), and then the activation of LPO. In brain and blood serum of rats subjected to continuous footshock as well as to restraint stress LPO products decreased and superoxide scavenging activity increased during the initial period of stress, after 1 hour of footshock LPO indices nearly reached normal values, and after 2 hours of footshock the accumulation of LPO products and decrease of superoxide scavenging activity were seen. LPO inhibition was accompanied by accumulation of easy oxidizable brain phospholipids and by depletion of brain cholesterol, during LPO activation brain cholesterol content and cholesterol-phospholipid ratio increased. The content of LPO products--fluorescent Schiff bases in blood plasma of women suffering from algomenorrhea at first decreased (O-12 h) and then dramatically increased (12-24 h) after a onset of pain at the beginning of menstruation. The data suggest that the stage of LPO inhibition precedes its activation during stress.     

The effect of synthetic enkephalins on prostaglandin synthesis and lipid peroxidation in the isolated heart during the activation of free-radical processes]

The system Fe(2+)-ascorbate was used for lipid peroxidation (LPO) activation in isolated heart tissue. The stimulation of free radical processes in the myocardium caused a rise in the level of LPO products and a significant lowering of that of prostacyclin. Intravenous injection of synthetic analogues of enkephalins before LPO activation inhibited LPO and resulted in prostacyclin synthesis normalization. The addition of peptides under study into perfusion solution led to a decrease in LPO primary products level and in thromboxane synthesis.     

Estrogen Hormones Reduce Lipid Peroxidation in Cells and Tissues of the Central Nervous System

Effects of estrogen hormones on lipid peroxidation (LPO) were examined in rat brain homogenates (RBHs), hippocampal HT 22 cells, rat primary neocortical cultures, and human brain homogenates (HBHs). Dose-response curves indicated half-maximal effective concentrations (EC50) of 5.5 and 5.6 mM for iron-induced LPO in RBHs and HT 22 homogenates. Incubation of living rat primary neocortical cultures with iron resulted in an EC50 of 0.5 mM, whereas culture homogenates showed an EC50 of 1.2 mM. Estrogen hormones reduced LPO in all systems: In RBHs, estrone inhibited iron-induced LPO to 74.1 +/- 5.8% of control levels (17beta-estradiol: 71.3 +/- 0.1%) at a concentration of 10 microM. In hippocampal HT 22 cell homogenates, levels of LPO were reduced to 74.8 +/- 5.5% by estrone and to 47.8 +/- 6.2% by 17beta-estradiol. In living neocortical cultures, 17beta-estradiol decreased iron-induced LPO to 79.2 +/- 4.8% and increased the survival of cultured neuronal cells. Of the other steroid compounds tested (corticosterone, progesterone, testosterone), only progesterone decreased LPO in HT 22 cell homogenates. In HBHs, LPO was dose-dependently increased by iron concentrations from 2.7 to 6.0 mM. Incubation with estrogens resulted in a dose-dependent inhibition of LPO to 53.8 +/- 8.6% with 10 microM 17beta-estradiol, whereas estrone failed to affect iron-induced LPO to a significant extent. Nonestrogenic steroids, including hydrocortisol, did not show significant effects on LPO in HBHs.     

Lipid peroxide formation in relation to membrane stability of fresh and frozen thawed stallion spermatozoa

In this study we used a new method to detect reactive oxygen species (ROS) induced damage at the level of the sperm plasma membrane in fresh and frozen-thawed stallion sperm. Lipid peroxidation (LPO) in sperm cells was assessed by a fluorescent assay involving the labeling of stallion sperm with the LPO reporter probe C11-BODIPY(581/591). The peroxidation dependent spectral emission shift of this membrane probe could be localized using inverted spectral confocal microscopy and quantified on living and deteriorated sperm cells using flow cytometry. Mass spectrometric analysis of the main endogenous lipid class, phosphatidylcholine (PC), was carried out to determine the formation of hydroxy- and hydroperoxyphosphatidylcholine in fresh sperm cells. Peroxidation as reported by the fluorescent probe corresponded with the presence of hydroxy- and hydroperoxyphosphatidylcholine in the sperm membranes, which are early stage products of LPO. This allowed us to correlate endogenous LPO with localization of this process in the living sperm cells. In absence of peroxidation inducers, only relatively little peroxidation was noted in fresh sperm cells whereas some mid-piece specific probe oxidation was noted for frozen-thawed sperm cells. After induction of peroxidation in fresh and frozen-thawed sperm cells with the 0.1 mM of lipid soluble ROS tert-butylhydrogen peroxide (t-BUT) intense probe oxidation was produced in the mid-piece, whereas the probe remained intact in the sperm head, demonstrating antioxidant activity in the head of fresh sperm cells. At higher levels of t-BUT, probe peroxidation was also noted for the sperm head followed by a loss of membranes there. Frozen-thawed sperm were more vulnerable to t-BUT than fresh sperm. The potential importance of the new assays for sperm assessments is discussed.     

The extraordinary antioxidant activity of vitamin E phosphate

The antioxidant activities of RRR-vitamin E (VE), all-rac-vitamin E (all-rac-VE), trolox, RRR-vitamin E acetate (VEA), all-rac-vitamin E phosphate (VEP) and RRR-vitamin E succinate (VES) were compared. In this study, the rank order in the inhibition of lipid peroxidation (LPO) of VE and its derivatives was trolox>VE approximately all-rac-VE>VEA>VES. VE and trolox inhibited LPO in non-heated and heated rat liver microsomes. It has generally been accepted that this is due to scavenging of free radicals by these antioxidants, and during this protection the antioxidants are oxidized. VEA and VES have to be converted into VE by esterases to obtain antioxidant activity against LPO. VEP, however, had a potent antioxidant effect of its own without conversion to VE. In contrast to VE, VEP is not consumed during this protection. Of the compounds tested, VEP is the most potent in induction of hemolysis of erythrocytes. EPR experiments using the spin label 16-doxylstearic acid showed that VEP reduces membrane fluidity, in contrast to VE. This indicates that VEP acts as a detergent and forms a barrier that might inhibit the transfer of radicals from one polyunsaturated fatty acid to another. This new mechanism may form the basis for a new class of antioxidants.     

Reaction of ferrous lactoperoxidase with hydrogen peroxide and dioxygen: an anaerobic stopped-flow study

Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions including milk, tears, and saliva and has physiological significance in antimicrobial defense which involves (pseudo-)halide oxidation. LPO compound III (a ferrous-dioxygen complex) is known to be formed rapidly by an excess of hydrogen peroxide and could participate in the observed catalase-like activity of LPO. The present anaerobic stopped-flow kinetic analysis was performed in order to elucidate the catalytic mechanism of LPO and the kinetics of compound III formation by probing the reactivity of ferrous LPO with hydrogen peroxide and molecular oxygen. It is shown that ferrous LPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl LPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (7.2+/-0.3) x 10(4) M(-1) s(-1) at pH 7.0 and 25 degrees C. The H2O2-mediated conversion of compound II to compound III follows also second-order kinetics (220 M(-1) s(-1) at pH 7.0 and 25 degrees C). Alternatively, compound III is also formed by dioxygen binding to ferrous LPO at an apparent bimolecular rate constant of (1.8+/-0.2) x 10(5) M(-1) s(-1). Dioxygen binding is reversible and at pH 7.0 the dissociation constant (K(D)) of the oxyferrous form is 6 microM. The rate constant of dioxygen dissociation from compound III is higher than conversion of compound III to ferric LPO, which is not affected by the oxygen concentration and follows a biphasic kinetics. A reaction cycle including the redox intermediates compound II, compound III, and ferrous LPO is proposed, which explains the observed (pseudo-)catalase activity of LPO in the absence of one-electron donors. The relevance of these findings in LPO catalysis is discussed.     

Upregulation of PBR mRNA expression in human neuroblastoma cells by flavonoids

To investigate the putative mediation of peripheral benzodiazepine receptor (PBR) in the cytotoxicity of flavonoids, in this study, modulatory effects of several flavonoids on the lipid peroxide (LPO) production and PBR mRNA expression of human neuroblastoma cells were observed. Elevated levels of peroxidated products in cancer cells may activate pro-apoptotic and anti-proliferative signaling pathways. Treatment of 10(-6) M 4'-chlorodiazepam and PK 11195 ligands of the PBR for 6 days enhanced the generation of LPO of the human neuroblastoma cells. Several flavonoids, well-known cytotoxic substances, potentiated the enhancement of LPO production by PBR ligands. Treatment of 10(-6) M flavonoids for 6 days elevated the expression of PBR mRNA in cells. These findings indicate that the potential of flavonoids to induce apoptosis in cancer cells is strongly associated with their PBR-inducing properties, thereby providing a new mechanism by which polyphenolic compounds may exert their cancer-preventive and anti-neoplastic effects.     

Hypochlorous acid-induced heme degradation from lactoperoxidase as a novel mechanism of free iron release and tissue injury in inflammatory diseases

Lactoperoxidase (LPO) is the major consumer of hydrogen peroxide (H(2)O(2)) in the airways through its ability to oxidize thiocyanate (SCN(-)) to produce hypothiocyanous acid, an antimicrobial agent. In nasal inflammatory diseases, such as cystic fibrosis, both LPO and myeloperoxidase (MPO), another mammalian peroxidase secreted by neutrophils, are known to co-localize. The aim of this study was to assess the interaction of LPO and hypochlorous acid (HOCl), the final product of MPO. Our rapid kinetic measurements revealed that HOCl binds rapidly and reversibly to LPO-Fe(III) to form the LPO-Fe(III)-OCl complex, which in turn decayed irreversibly to LPO Compound II through the formation of Compound I. The decay rate constant of Compound II decreased with increasing HOCl concentration with an inflection point at 100 μM HOCl, after which the decay rate increased. This point of inflection is the critical concentration of HOCl beyond which HOCl switches its role, from mediating destabilization of LPO Compound II to LPO heme destruction. Lactoperoxidase heme destruction was associated with protein aggregation, free iron release, and formation of a number of fluorescent heme degradation products. Similar results were obtained when LPO-Fe(II)-O(2), Compound III, was exposed to HOCl. Heme destruction can be partially or completely prevented in the presence of SCN(-). On the basis of the present results we concluded that a complex bi-directional relationship exists between LPO activity and HOCl levels at sites of inflammation; LPO serve as a catalytic sink for HOCl, while HOCl serves to modulate LPO catalytic activity, bioavailability, and function.     

Interaction between arginine vasopressin and angiotensin II receptors in the central regulation of sodium balance

We speculated that the influence of lateral preoptic area (LPO) in sodium balance, involves arginine8-vasopressin (AVP) and angiotensin (ANG II) on Na+ uptake in LPO. Therefore, the present study investigated the effects of central administration of specific AVP and ANG II antagonists (d(CH2)5-Tyr (Me)-AVP (AAVP) and [Adamanteanacetyl1, 0-ET-d-Tyr2, Val4, Aminobutyryl6, Arg(8,9)]-AVP (ATAVP) antagonists of V1 and V2 receptors of AVP. Also the effects of losartan and CGP42112A (selective ligands of the AT1 and AT2 angiotensin receptors, respectively), was investigated on Na+ uptake and renal fluid and electrolyte excretion. After an acclimatization period of 7 days, the animals were maintained under tribromoethanol (200 mg/kg body weight, intraperitonial) anesthesia and placed in a Kopf stereotaxic instrument. Stainless guide cannula was implanted into the LPO. AAVP and ATAVP injected into the LPO prior to AVP produced a reduction in the NaCl intake. Both the AT1 and AT2 ligands administered into the LPO elicited a decrease in the NaCl intake induced by AVP injected into the LPO. AVP injection into the LPO increased sodium renal excretion, but this was reduced by prior AAVP administration. The ATAVP produced a decreased in the natriuretic effect of AVP. The losartan injected into LPO previous to AVP decreased the sodium excretion and the CGP 421122A also decreased the natriuretic effect of AVP. The AVP produced an antidiuresis effect that was inhibited by prior administration into LPO of the ATAVP. The AAVP produced no change in the antidiuretic effect of AVP. These results suggest that LPO are implicated in sodium balance that is mediated by V1, V2, AT1 and AT2 receptors.     

Stabilizing effect of hydroxybenzimidazole and its derivatives on biological membranes during activation of lipid peroxidation

Inhibition of lipid peroxidation (LPO) by oxybenzimidazole (OBI) and its derivatives--alkyloxybenzimidazole (AOBI) and alkylethoxybenzimidazole (AEBI) was studied in liver microsomes and brain synaptosomes. It has been shown that both OBI and AOBI strongly inhibit LPO in microsomes and not synaptosomes. AEBI failed to inhibit LPO in microsomes. AOBI is more potent than OBI both in ascorbate- and NADPH-dependent LPO of microsomes. An antioxidant effect of both compounds is more marked in ascorbate-dependent LPO. The investigation of the possible use of AOBI for the protection of liver membranes in various pathological conditions associated with LPO activation seems promising.