Mode of inhibitory action of melittin on Na+-K+-ATPase activity of the rat synaptic membrane

Antimicrobial peptides (AMPs): The quintessential 'offense and defense' molecules are more than antimicrobials

Antimicrobial peptides (AMPs) are cationic amphiphilic molecules with α-helix or β-sheet linear motifs and linear or cyclic configurations. For their role in 'defense and offense', they are present in all living organisms. AMPs are named so, as they inhibit a wide array of microbes by membrane pore formation and subsequent perturbation of mitochondrial membrane ionic balance. However, their functional repertoire is expanding with validated roles in cytotoxicity, wound healing, angiogenesis, apoptosis, and chemotaxis [1]. A number of endogenous AMPs have been characterized in human body such as defensins, cathelicidins, histatins etc. They mediate critical functions, but when homeostasis is broken, they turn hostile and initiate inflammatory diseases. This review discusses the sources of therapeutic AMPs; auto-immunity risks of endogenous AMPs, and their dermatological applications; normally overlooked risks of the peptides; and scopes ahead. This holistic work is expected to be a valuable reference for further research in this field.

Dual effects of extracellular Ca2+ on cardiotoxin-induced cytotoxicity and cytosolic Ca2+ changes in cultured single cells of rabbit aortic endothelium

The effects of extracellular Ca2+ on cytotoxicity induced by cardiotoxin (CTX), isolated from Chinese cobra venom, were investigated in cultured rabbit aortic endothelial cells (RAECs). In Hank's buffered saline solution (HBSS) containing 1.2 mM Ca2+, CTX (1-30 microM) caused cell necrosis and cell death in a concentration-dependent manner, as determined by trypan blue exclusion test performed after a 20-min CTX treatment. The concentration of CTX that caused 50% cell death was about 6.5 microM. CTX (10 microM)-induced RAEC damage was also evident but less prominent in Ca2+-free medium and almost completely prevented in medium containing 7-10 mM Ca2+. Therefore, Ca2+ appears to provoke CTX-induced injury at physiological concentrations, but protects against it at high concentrations. The protection of RAECs from CTX-induced injury could also be achieved by high concentrations of Ni2+ and Mg2+. Using the fura-2 fluorescence technique to measure the cytosolic free Ca2+ concentration ([Ca2+]i) of single RAEC, it was shown that in 1.2 mM Ca2+-containing HBSS, treatment of RAECs with 10 microM CTX for 7-35 min resulted in a tremendous and irreversible [Ca2+]i elevation, suggestive of cell membrane damage and extracellular Ca2+ entry. Ni2+ could also enter the cytosol of these damaged RAECs. However, there was no [Ca2+]i elevation or Ni2+ entry in RAECs that were preincubated in HBSS containing 7 mM Ca2+ or Ni2+ before CTX exposure. In RAECs protected with 7 mM Ca2+, the intracellular Ca2+ signals triggered by 100 microM extracellular ATP or 10 microM bradykinin in CTX-treated groups were similar to those in the untreated control groups. Taken together, the results indicate that high extracellular Ca2+ concentrations protected RAECs from CTX-induced injury, and preserved the ability of CTX-treated RAECs to generate Ca2+ signals in response to physiological stimuli.

Decrease in myocardial Na(+)-K(+)-ATPase activity and ouabain binding sites in hypercholesterolemic rabbits

OBJECTIVES

The purpose of this study was to explore the effect of high dietary cholesterol on the lipid composition, Na(+)-K(+)-ATPase activity and ouabain receptor property of the myocardial sarcolemma.

METHODS

Male New Zealand white rabbits were fed with standard chow or standard chow supplemented with 0.5% (w/w) cholesterol and 10% (w/w) coconut oil to induce hypercholesterolemia. After 8 weeks, the rabbits were sacrificed; a myocardial sarcolemma fraction was then prepared from the left ventricular myocardium and analyzed for lipid composition. Assay of Na(+)-K(+)-ATPase activity and 3H-ouabain binding studies were performed in the myocardial sarcolemma from the control and cholesterol-fed rabbits.

RESULTS

The cholesterol content, but not the phospholipid content, of the sarcolemma was significantly greater in the cholesterol-fed group, thus, resulting in an increased cholesterol/phospholipid molar ratio in the cholesterol-fed group. In addition, a decrease in Na(+)-K(+)-ATPase activity was also found in this group. The decrease in Na(+)-K(+)-ATPase activity was selective, since the Mg(++)-ATPase and 5'-nucleotidase activities remained unchanged. In the 3H-ouabain binding study, a decrease in the number of maximum binding sites, but not the binding affinity, for 3H-ouabain was found in the cholesterol-fed group.

CONCLUSIONS

High dietary cholesterol induces higher levels of cholesterol not only in the plasma, but also in the myocardial sarcolemma. These changes result in decreased myocardial Na(+)-K(+)-ATPase activity mediated by a reduction in the maximum number of binding sites for ouabain but not a change in binding affinity.

Brain lesions induced by specific and non-specific inhibitors of sodium-potassium ATPase

The cytotoxicity in the brain of potent selective and non-selective inhibitors of Na+/K+ ATPase has been assessed. Following injection of cardiac glycosides into the dorsal hippocampus of rats, the extent of neuronal loss roughly paralleled their potencies as inhibitors of the enzyme. Dihydroouabain was less potent than ouabain as a cytotoxin by an order of magnitude, similar to their relative affinities for Na+/K+ ATPase. The non-specific inhibitors, melittin, erythrosin B and zinc (chloride) were less neurocytotoxic than the selective inhibitors having equivalent potencies. The toxicity of a low dose of ouabain appeared to be selective for neuronal perikarya as staining for acetylcholinesterase (present on the nerve terminals of the afferent cholinergic innervation) was unaffected. A higher dose of ouabain caused a non-specific necrosis including damage to the neuropil and a loss of cholinesterase staining. Concurrently, there was an invasion of the tissue by cells resembling foaming macrophages. Other inhibitors of the enzyme caused a mixed pathology with both types of responses evident. It is suggested that the pathological response may depend on the relative degrees to which the glial and neuronal activities of the enzyme are affected.

Possible mechanisms of action of cobra snake venom cardiotoxins and bee venom melittin

Cobra snake venom cardiotoxins and bee venom melittin share a number of pharmacological properties in intact tissues including hemolysis, cytolysis, contractures of muscle, membrane depolarization and activation of tissue phospholipase C and, to a far lesser extent, an arachidonic acid-associated phospholipase A2. The toxins have also been demonstrated to open the Ca2+ release channel (ryanodine receptor) and alter the activity of the Ca(2+)+Mg(2+)-ATPase in isolated sarcoplasmic reticulum preparations derived from cardiac or skeletal muscle. However, a relationship of these actions in isolated organelles to contracture induction has not yet been established. The toxins also bind to and, in some cases, alter the function of a number of other proteins in disrupted tissues. The most difficult tasks in understanding the mechanism of action of these toxins have been dissociating the primary from secondary effects and distinguishing between effects that only occur in disrupted tissues and those that occur in intact tissue. The use of cardiotoxin and melittin fractions contaminated with trace ('undetectable') amounts of venom-derived phospholipases A2 has continued to be common practice, despite the problems associated with the synergism between the toxins and enzymes and the availability of methods to overcome this problem. With adequate precautions taken with regard to methodology and interpretation of results, the cobra venom cardiotoxins and bee venom melittin may prove to be useful probes of a number of cell processes, including lipid metabolism and Ca2+ regulation in skeletal and cardiac muscle.

High sensitivity of glutamate uptake to extracellular free arachidonic acid levels in rat cortical synaptosomes and astrocytes

By using both synaptosomes and cultured astrocytes from rat cerebral cortex, we have investigated the inhibitory action of arachidonic acid on the high-affinity glutamate uptake systems, focusing on the possible physiological significance of this mechanism. Application of arachidonic acid (1-100 microM) to either preparation leads to fast (within 30 s) and largely reversible reduction in the uptake rate. When either melittin (0.2-1 microgram/ml), a phospholipase A2 activator, or thimerosal (50-200 microM), which inhibits fatty acid reacylation in phospholipids, is applied to astrocytes, both an enhancement in extracellular free arachidonate and a reduction in glutamate uptake are seen. The two effects display similar dose dependency and time course. In particular, 10% uptake inhibition correlates with 30% elevation in free arachidonate, whereas inhibition greater than or equal to 60% is paralleled by threefold stimulation of arachidonate release. In the presence of albumin (1-10 mg/ml), a free fatty acid-binding protein, inhibition by either melittin, thimerosal, or arachidonic acid is prevented and an enhancement of glutamate uptake above the control levels is observed. Our data show that neuronal and glial glutamate transport systems are highly sensitive to changes in extracellular free arachidonate levels and suggest that uptake inhibition may be a relevant mechanism in the action of arachidonic acid at glutamatergic synapses.

Differentiation of renal Na(+)-K(+)-ATPase in control and streptozotocin-induced diabetic mice by G-protein acting toxins and phorbol esters

The specific activity of Na(+)-K(+)-ATPase in the renal medulla and cortex of 50-day-old streptozotocin (STZ)-induced diabetic mice was increased 58% and 50%, respectively, as compared to controls. Km values of Na+ and K+ for this enzyme were unaltered, while that of ATP was decreased in diabetic mice. The Na(+)-K(+)-ATPase in control medulla and cortex was activated by both cholera and pertussis toxins, while this effect was abolished in diabetics. Since dibutyryl cAMP stimulates cortical Na(+)-K(+)-ATPase activity in control mice, the activation effect of cholera toxin on this enzyme might be due to its interaction with a Gs-protein and the persistent stimulation of adenylate cyclase activity, while the effect of pertussis toxin might be due to its masking of the inhibitory action of a Gi-protein on adenylate cyclase activity. However, the protein kinase C (PKC)-associated Na(+)-K(+)-ATPase might also be quiescent in diabetes, because the stimulating effect of phorbol 12,13-dibutyrate (PDBu) and phorbol 12-myristate 13-acetate (PMA) on this enzyme was abolished in diabetic cortex. In addition, nicardipine and ouabain were found to have differential effects on this enzyme derived from control and diabetic mice.

Interaction of melittin with the (Na+ + K+)ATPase: evidence for a melittin-induced conformational change

The (Na+ + K+)ATPase is inhibited by the bee venom polypeptide, melittin. KCl and NaCl protect the enzyme from melittin inhibition. Analysis of the K+ and Na+ protection against melittin inhibition suggested a kinetic model which was consistent with slowly reversible melittin binding, and mutually exclusive binding of melittin with K+ and Na+. Accordingly, in the absence of salt, the KI for melittin inhibition = 1.2 microM, and the protection by KCl occurs with a KA,KCl = 0.6 mM. The protection by NaCl occurs with a KA,NaCl = 15 mM. Melittin inhibition of enzyme activity is due to direct interactions with the (Na+ + K+)ATPase, as demonstrated by photolabeling with [125I]azidosalicylyl melittin, which labeled the alpha subunit, but not the beta subunit of the (Na+ + K+)ATPase. Melittin and KCl reduced the extent of labeling. In non-covalent binding studies using [125I]azidosalicylyl melittin, the stoichiometry of binding was 1.6 melittin per (Na+ + K+)ATPase. Ligand-induced conformational changes of FITC-labeled (Na+ + K+)ATPase were examined in the presence and absence of melittin. K+ alone or melittin alone caused a fluorescence intensity quenching consistent with formation of an E2 form of the enzyme. The NaCl-induced (E2----E1) fluorescence intensity changes were maximal when the enzyme was treated with K+. NaCl-induced fluorescence changes did not occur when the enzyme was treated with melittin in the absence of K+. However, when K+ was present before the addition of melittin, NaCl-induced fluorescence intensity increases were observed, which were dependent upon the concentration of K+ in the preincubation mixture. The results of the labeling and conformational studies support the kinetic model and suggest a mechanism for inhibition of ion pumps by (poly)peptides.

Do cardiotoxins possess a functional site? Structural and chemical modification studies reveal the functional site of the cardiotoxin from Naja nigricollis

Examination of the literature has revealed that regarding the amino acid sequences, cardiotoxins constitute a family of homogeneous compounds. In contrast, cardiotoxins appear heterogeneous as far as their biological and spectroscopic properties are concerned. As a result, comparison between these molecules with a view to establishing structure-activity correlations is complicated. We have therefore reviewed recent works aiming at identifying the functional site of a defined cardiotoxin, ie toxin gamma from the venom of the spitting cobra Naja nigricollis. The biological and structural properties of toxin gamma are first described. In particular, a model depicting the 3-dimensional structure of the toxin studied by NMR spectroscopy is proposed. The toxin polypeptide chain is folded into 3 adjacent loops rich in beta-sheet structure connected to a small globular core containing the 4 disulfide bonds. A number of derivatives chemically modified at a single aromatic or amino group have been prepared. The structure of each derivative was probed by emission fluorescence, circular dichroism and NMR spectroscopy. Also tested was the ability of the derivatives to kill mice, depolarize excitable cell membranes and lyse epithelial cells. Modification of some residues in the first loop, in particular Lys-12 and at the base of the second loop substantially affected biological properties, with no sign of concomitant structural modifications other than local changes. Modifications in other regions much less affected the biological properties of the toxin. A plausible functional site for toxin gamma involving loop I and the base of loop II is presented. It is stressed that the functional site of other cardiotoxins may be different.

Novel peptide inhibitor (SPAI) of Na+, K+-ATPase from porcine intestine

Three unique inhibitors (SPAI-1, -+2, and -3) were first purified from porcine duodenal extract based on the Na+, K+-ATPase inhibitory activity. These peptide inhibitors had four disulfide bridges in common. The sequencing results of their S-carboxymethyl derivatives, lysilendopeptidase fragments, and chymotryptic peptides disclosed their entire primary structures. Both SPAI-2 and -3 consisted of 61 amino acids, respectively, and had almost the same sequences except for two amino acid substitutions, while SPAI-1 was found to lack the N-terminal twelve amino acid sequence of SPAI-2. The kinetics study revealed that SPAIs inhibited Na+, K+-ATPase by the competitive mode against Na+ and were uncompetitive with K+.

Stimulation by melittin of adrenocorticotropin and beta-endorphin release from rat adenohypophysis in vitro

The effect of melittin on the release of adrenocorticotropin (ACTH) and beta-endorphin from the corticotropic cells of the rat adenohypophysis was examined in vitro. Anterior pituitary quarters were perifused or incubated in vitro and ACTH- (ACTH-IR) or beta-endorphin-like immunoreactivity (beta-End-IR) in the medium was measured by radioimmunoassays. Melittin stimulated ACTH-IR and beta-End-IR release. This effect was rapid in onset, reversible, and concentration-related (50-5000 ng/ml) and depended on the presence of calcium ions in the incubation medium. Melittin also elevated the tissue content of unesterified 3H-arachidonic acid that had previously been incorporated into lipids. Purported phospholipase A2 inhibitors, mepacrine (up to 1 mM), dexamethasone (0.5 mg/kg in vivo, 50 nM in vitro), or p-bromophenacylbromide (100 microM), did not decrease the melittin (500 ng/ml) - induced beta-End-IR release, although mepacrine and dexamethasone may have inhibited phospholipase A2 activity as indicated by an inhibition of melittin-evoked prostaglandin E2 formation. After stimulation by melittin (500 ng/ml), beta-End-IR release was not affected by the cyclooxygenase inhibitor indomethacin (up to 140 microM), whereas nordihydroguaiaretic acid (100 microM), a lipoxygenase inhibitor, or BW755C (250 microM), an inhibitor of both cyclooxygenase and lipoxygenase, abolished melittin-induced hormone secretion. We conclude that melittin generates a signal in the corticotropic cells of the rat adenohypophysis which induces hormone secretion by exocytosis. This signal may be unrelated to the activation by melittin of phospholipase A2.