THE ENZYMATIC REMOVAL OF A SURFACTANT COATING FROM QUARTZ AND KAOLIN BY P388D1 CELLS

The macrophage-like cell line, P388D1, was exposed to dipalmitoyl leci thin (DPL)-coated respirable quartz and kaolin, and the disappearance of the DPL was monitored for up to 9 days. The coating was removed rap idly at first (about 50% in the first 3 days) and then more slowly ove r the remaining 6 days, until about 30% remained on day 9. The rate of DPL digestion was independent of the type of dust and the amount of c oated dust within the cell, indicating the existence of an extracellul ar phospholipase activity. This extracellular phospholipase activity w as partially characterized. It was sensitive to temperatures above 56 degrees C, the presence of EDTA, the action of the proteases trypsin a nd proteinase K, and pH, being active at pH 7 but not at pH 5. This is consistent with reports in the literature of the existence of an extr alysosomal phospholipase which is active at pH 7 and dependent on the presence of divalent metal ions. There was a dust-dependent difference in the extracellular rate of DPL digestion from quartz and kaolin The coating was removed more slowly from the kaolin than it was from quar tz. The removal of the DPL coating seen in the presence of cells was p resumably due to both an intracellular and an extracellular phospholip ase.