CA2+-INDUCED REVERSIBLE TRANSLOCATION OF PHOSPHOLIPASE-A2 BETWEEN THE CYTOSOL AND THE MEMBRANE-FRACTION OF RAT-LIVER MACROPHAGES

In cell-free extracts of rat liver macrophages (Kupffer cells) phospholipase A2 was found to be rapidly associated with the particulate fraction in a Ca2+-dependent manner at Ca2+ concentrations of 0.1 - 1.0-mu-M. This is also the range of the levels of intracellular Ca2+ reported for basal and various stimulated conditions. After translocation, phospholipase A2 could be released from the membranes in the presence of Ca2+ chelators, increasing the specific activity of phospholipase A2 in the supernatant fraction. These findings support the view that translocation is a regulatory mechanism of phospholipase A2 by bringing the enzyme to its substrate. Unlike the situation with protein kinase C, Mg2+ exerted little effect on phospholipase A2 translocation, indicating that this process is regulated in vivo mainly by fluctuations of the intracellular Ca2+ content.