THE LOCALIZATION OF PHOSPHOLIPASE A(2) IN THE SECRETORY GRANULE

A heat-resistant phospholipase A, has been detected in the secretory g ranules of the mast cell [Chock, Rhee, Tang and Schmauder-Chock (1991) fur. J. Biochem. 195, 707-713]. By using ultrastructural immunocytoch emical techniques, we have now localized this enzyme to the matrix of the secretory granule. Like the cyclo-oxygenase [Schmauder-Chock and C hock (1989) J. Histochem. Cytochem. 37, 1319-1328], this enzyme also a dheres tightly to the ribbon-like granule matrix components. The resul ts from Western-blot analysis suggest that it has a molecular mass of about 14 kDa. The localization of the phospholipase A(2), the presence of a phospholipid store with millimolar concentrations of calcium and the localization of the enzymes of the arachidonic acid cascade make the secretory granule a natural site for lipid-mediator synthesis. The packaging of phospholipase A(2), together with its substrate and the components of the arachidonic acid cascade, in the secretory granule r epresents a physical arrangement by which the initiation of the cascad e and the release of mediators can be directly linked to the stimulati on of cell-surface receptors.