Acid phospholipase A1 requiring phospholipids or Triton X-100 in the cytosol of cultured cells.

A new type of soluble phospholipase A1 [EC 3.1.1.32] was found in the supernatant fraction (cytosol) obtained by centrifugation at 105,000 x g for 90 min of homogenates in 0.25 M sucrose of FL cells and several other cultured cells (L-132 cells, Vero cells, and BEK-1 cells). Phospholipase A1 in FL cell cytosol showed activity only in the presence of a non-ionic detergent, Triton X-100, or certain phospholipids such as phosphatidylinositol, cardiolipin, phosphatidylserine, phosphatidic acid, phosphatidylethanolamine, and lysophosphatidylcholine, among which phosphatidylinositol was the most active stimulator of the activity. This soluble enzyme in FL cells acted optimally at acidic pH like lysosomal acid phospholipases A1 and A2, which were recovered in the soluble subfraction of the lysosomal fraction. The acid phospholipase A1 in the cytosol was shown to be different from the lysosomal phospholipase A1 on the following counts: (1) the cytosol enzyme showed activity on the addition of phospholipids or Triton X-100, whereas lysosomal phospholipase A1 was present in the active form even in the absence of these activators; (2) no release of the lysosomal acid phosphatase and Triton X-100-independent phospholipase A1 or A2 into the cytosol occurred during cell fractionation; (3) the heat stabilities of these two enzymes as well as the effects of detergents on them were different. Similar Triton X-100-dependent enzyme activity was also found in the cytosol of several rat tissues.