Melittin (free acid)

Hymenoptera are the large group of insects which includes honey-bees, bumble-bees, paper wasps, hornets, ants. Female hymenoptera possess specialized stinging apparatus with which they inject their venom into prey's or intruder's body. It could be life-threatening for people sensitive to the venom. The hymenoptera venom consists of mixture of biologically active substances, eg. enzymes (phospholipases, hialuronidase), peptides (melittin, apamin, mastoparans, bombolitins) and low-molecular-weight compounds (biogenic amines, acetylcholine, carbohydrates, lipids, free amino acids). Several types of reactions are possible to develop after stinging by hymenopteran insects: (1) non-allergic local reaction (pain, small oedema, redness at the site of the sting); allergic reactions: (2) large local reaction (extensive local swelling, exceeding 10 cm, persisting longer than 24 hours) and (3) anaphylaxis (generalized urticaria, bronchospasm, hypotension, cardiovascular collapse, loss of consciousness); (4) systemic toxic reaction (oedema, vomits, diarrhoea, headache, hypotension, seizures, altered mental status); (5) unusual reactions (cardiac ischaemia, encephalomyelitis et al.). Therapeutic management after stings includes removing of the stinger (bee stings), local remedies (ice-packs, topical steroids) and prevention and treatment of an anaphylactic shock (epinephrine, general steroids, beta-mimetics, fluid resuscitation, oxygen therapy). In the present review types of reaction after hymenoptera stings were described with special interest of anaphylactic and toxic reactions as well as therapeutic management after stings.

The effects of the phospholipase activator melittin on amino acid and free fatty acid release from the rat cerebral cortex were monitored and compared with those of a secretory PLA(2), using a cortical cup technique with topical application of these agents in artificial cerebrospinal fluid. Melittin (10 microg/ml; 3.5 microM) elicited a rapid increase in the levels of superfusate amino acids; aspartate, glutamate, GABA, glycine, taurine, glutamine, phosphoethanolamine, alanine, serine and the free fatty acids arachidonic, linoleic, palmitic and oleic acid. PLA(2) (25 microg/ml) also enhanced amino acid efflux but its effects were significantly slower to develop than those of melittin. The results confirm previous indications of an ability of phospholipases to augment extracellular levels of several amino acids, including the excitotoxins glutamate and aspartate, and further implicate phospholipase activation as a significant contributor to cerebral ischemic injury. Melittin has the potential to be a useful tool with which to evaluate the role of phospholipases in ischemia injury.

Melittin is known as a phospholipase A2 (PLA2) activator, but the selectivity of its effect on PLA2 is uncertain. We examined the selectivity of melittin effect on the release of free fatty acids (FFAs) from L1210 cells using various inhibitors. A systemic lipid analysis by HPLC and GLC revealed that melittin induced release of various FFAs including saturated, monounsaturated, and polyunsaturated FFAs. Various PLA2 inhibitors examined exerted only minimal effects on the melittin-induced arachidonic acid (AA) and palmitic acid (PAL) releases. Specific inhibitors of phosphatidylinositol-phospholipase C (U73122) and diacylglycerol lipase (RHC80267) exerted significant inhibitory effects on both AA and PAL releases. These results suggest that melittin-induced FFA release is most likely due to multiple participations of various types of lipases. Since BAPTA/AM, an intracellular Ca2+ chelator, did not influence the FFA release, the Ca2+ influxed by melittin appeared not to be a key factor for the FFA release. The mimicking of the melittin-induced FFA release by digitonin, a membrane-permeabilizing agent, implies that the membrane-perturbing action of melittin is likely the cause of the FFA release. Melittin also induced release of multiple FFAs from other cell lines including P388D1 and HL60. The rapid melittin-stimulated phospholipase D (PLD) observed in L1210 cells appeared not directly related to the steady release of FFA, as indicated by the fact that the PLD was not blocked by RHC80267. In view of melittin's multiple effects on the composition of cellular lipids, we conclude that melittin does neither exclusively release any single FFA nor selectively activate PLA2 in L1210 cells. The problem of using melittin as a PLA2 activator is discussed.