A heat-resistant phospholipase A2 has been detected in the secretory granules of the mast cell [Chock, Rhee, Tang and Schmauder-Chock (1991) Eur. J. Biochem. 195, 707-713]. By using ultrastructural immunocytochemical techniques, we have now localized this enzyme to the matrix of the secretory granule. Like the cyclo-oxygenase [Schmauder-Chock and Chock (1989) J. Histochem. Cytochem. 37, 1319-1328], this enzyme also adheres tightly to the ribbon-like granule matrix components. The results from Western-blot analysis suggest that it has a molecular mass of about 14 kDa. The localization of the phospholipase A2, 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 A2, together with its substrate and the components of the arachidonic acid cascade, in the secretory granule represents a physical arrangement by which the initiation of the cascade and the release of mediators can be directly linked to the stimulation of cell-surface receptors.

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Silica gel behaves as a weak acid ion exchanger. The negative surface sites are =Si-Oand possibly =Si(OH)2". Ad-sorbed cations are weakly bound to surface sites and are readily exchangeable. Labile coordination complexes, for example nickel ammine complexes, readily interchange ammonia and water according to the concentration of ammonia in solution. Adsorbed inert complex nations which contain an aquo ligand react with surface silanol groups to bind =Si-O-in the coordination sphere, and, in media of low dielectric constant, =Si-O'" can directly displace Cl-in adsorbed ¿ra»s-Co(en)2Cl2+. Species so bound are slowly extracted by dilute hydrochloric acid; nonbound inert complexes and labile complexes are extracted rapidly. We have determined the rates of hydrolysis of adsorbed cts-Co(en)2N02Cl+, troros-CoCen^C^"1", Pt(dien)-Br+, and Si(acac)3+. The pH independent hydrolysis goes about one-fifth as fast as in bulk solution; the second-order hydrolysis goes much more slowly. Rates are first order at constant pH.Representation of silica gel as a condensation polymer of silicic acid implies that the surface of silica gel is covered with -OH groups, silanol groups. Much evidence indicates that this is, indeed, the case. Various estimates of the number of such groups lie between 4.62a and 82b per 100 Á.2 345It is generally believed that the conventional silica xerogel is an agglomeration of spheroids, whose diameters are of the order of 100 A., cemented by interparticle siloxane (=Si-O-Sis) links. The porosity and large surface area of silica gel, hundreds of square meters per gram, reflects this structure.The surface silanol groups are strong hydrogen-bond donors,3-6 and they rather strongly adsorb molecules containing groups which function as hydrogen-bond receptors. They are also weak acids. The piTa of the first ionization of free silicic acid is 9.96 and one would expect that of a surface silanol group to be fairly close to this.It was observed long ago that cations could be adsorbed onto silica gel7-11 by two methods. One can expose silica gel to a basic solution containing the cation, for example a solution of sodium hydroxide,7 calcium hydroxide,10 11or an ammoniacal solution of copper hydroxide.8-11 In the last case, the coordination complex, Cu(NH3)4+2, which is adsorbed is not removed by washing with water,10 but it is largely removed by washing with dilute acid.12 _ In the second method, one treats the gel with a (1) (a) N.