Abstract
Melittin, the predominant fraction of bee venom proteins, was studied in an experimental model of human neutrophil granulocytes to reveal its influence on eicosanoid release, metabolism and receptor function in relation to intracellular calcium metabolism. Melittin (2 μmol/l) was as potent as the calcium ionophore A23187 (10 μmol/l) for activation of 5-lipoxygenase, releasing arachidonate only from phosphatidyl-choline and phosphatidyl-ethanolamine of cellular membranes, as judged from the decreases in radioactivity by 15.4% and 30.5%, respectively. The mechanism responsible for the release of arachidonate from cellular membranes is closely coupled to cellular calcium metabolism, and melittin was found to promote calcium entry through receptor gated calcium channels, probably due to an activation of phospholipase A2. Furthermore, a down-regulation of leukotriene B4 receptors was seen. The maximal number of binding sites per cell was reduced from a median of 1520 to 950 with melittin (1 μmol/l). The study has revealed some factors important for the inflammatory mechanisms mediated by melittin.
Highlights
Melittin is a polypeptide toxin which constitutes more than 50% of bee venom proteins
The aims of the present work were: (1) to assess if melittin was a stimulator of endogenous arachidonic acid (AA) metabolism in purified human polymorphonuclear neutrophils (PMN); (2) to compare its potency with that of the calcium ionophore A23187; (3) to reveal where in the phospholipid pool AA was mobilized by melittin challenge; (4) to evaluate its influence on cellular calcium metabolism; and (5) to investigate its possible action on surface leukotriene B4 receptors
The data demonstrate that the bee venom polypeptide, melittin, is a potent stimulator of endogenous AA metabolism to mono- and di-hydroxy products, including LTB4, in human PMNs which are of importance for inflammatory reactions. 12’13 Its potency regarding phospholipases stimulation in human PMNs is about 2/3 that of
Summary
Melittin is a polypeptide toxin which constitutes more than 50% of bee venom proteins. It has been claimed to release endogenous arachidonic acid (AA) from cultured cells and to increase formation of prostanoids through activation of membrane bound enzymes.[2,3,4] Further, melittin was found to be able to stimulate exogenous non-incorporated AA metabolism in human polymorphonuclear neutrophils (PMN), and recent research has dealt with the interaction between melittin and cellular membranes. 6
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