Phospholipase A 2 Activity of Fc γ2b Receptors of Thioglycollate‐Elicited Murine Peritoneal Macrophages

The detergent lysate of plastic adherent cell population of thioglycollate‐elicited peritoneal exudate cells from 100 individual Swiss mice was subjected to affinity chromatography on two different media, Sepharose coupled to heat‐ aggregated human IgG (IgG‐Sepharose) and Sepharose coupled to the phosphatidylcholine analog, rac‐1 ‐(9‐carboxyl)nonyl‐2‐hexadecy1glycero‐3‐phos‐ phorylcholine (PC‐Sepharose). Both IgG‐ and PC‐binding proteins were further purified by Sephadex G‐100 gel filtration and isoelectric focusing in the presence of 6M urea. Both IgG‐ and PC‐binding proteins thus purified appear to be homogeneous in size as well as charge properties. The IgG‐binding proteins of a molecular weight of 25,000 had an isoelectric point of 4.8, whereas the PC‐binding proteins of a molecular weight of 42,000 were more basic and had an isoelectric point of 6.0. Both materials retained their IgG‐binding capabilities as judged by their inhibitory capacity of murine EA γ rosetting systems. The subclass specificities of the IgG‐ and the PC‐binding proteins were for lgG 2a and lgG 2b , respectively. The PC‐binding proteins possessed a typical phospholipase A 2 activity, which was maximal at pH 9.5, depended on Ca ++ , and was specific for cleavage of fatty acid from the sn‐2 position of phosphatidylcholine. The bindiong of aggregated lgG 2b to the PC‐binding proteins caused the ninefold increase in noted enzymatic activity in the presence of, but not in the absence of, Ca + + (5mM). The IgG‐binding proteins on the other hand, lacked any detectable phospholipase A 2 activity. Thus, the biochemical and biological properties of the PC‐ and IgG‐binding proteins isolated from murine peritoneal macrophages are essentially identical to those homologous proteins previously isolated from P388D 1 cells [20].