Inhibition of phospholipase A₂ by protein I

The 36 kDa substrate of several tyrosine protein kinases has been shown to exist in monomeric and oligomeric (36₂10₂) forms. Partial sequence data has suggested that the oligomer, referred to as protein I, is homologous to a group of phospholipase A₂ inhibitory proteins, collectively called lipocortins. In the present communication we demonstrate that protein I inhibits bovine pancreas phospholipase A₂ with similar potency to that of lipocortin. Approximately 44 pmol protein I was required to produce 50% inhibition of 7.2 pmol of phospholipase A₂. Inhibition of phospholipase A₂ activity by calmodulin, S-100, calregulin, parvalbumin, troponin-C, or CAB-48 was not observed. These results indicate that protein I is a potent and specific inhibitor of phospholipase A₂ activity, and thus shares functional homology with the lipocortin proteins. We therefore propose that this protein be named lipocortin-85.+Postdoctoral fellows of the Alberta Heritage Foundation for Medical Research.