Melittin binds to secretory phospholipase A₂ and inhibits its enzymatic activity

Synthetic melittin inhibited the enzymatic activity of secretory phospholipase A₂ (PLA₂) from various sources, including bee and snake venoms, bovine pancreas, and synovial fluid from rheumatoid arthritis patients, irrespective of substrate (e,g., [¹⁴C]-phosphatidylcholine or phosphatidylethanolamine vesicles and [³H]-oleic acid-labeled E.coli). A Lineweaver-Burk analysis showed that melittin was a noncompetitive inhibitor of bee venom PLA₂, causing a change in V(max) from 200 to 50 units/min/mg of protein. The K(m) remained unchanged (0.75 nmole). Melittin inhibited approximately 50% of purified bee venom PLA₂ activity in a 30:1 molar ratio (melittin:enzyme). Because the enzyme kinetics indicated a PLA₂-melittin interaction, a melittin-sepharose affinity column was used to purify a PLA₂ from human serum. Further, an enzyme-linked assay was developed to quantitate PLA₂ activity in biological fluids using avidin-peroxidase and ELISA plates coated with biotinylated melittin. These observations may have potential therapeutic significance, as well as provide a convenient basis for the isolation and quantitation of PLA₂.