Hydrolysis of low-density lipoprotein phospholipids in arterial smooth muscle cells

Abstract

The hydrolysis of phosphatidylcholine (PC) associated with low-density lipoprotein (LDL) by homogenates of smooth muscle cells from rabbit aorta was studied. 1-Palmitoyl-2-[ 14C]oleoylPC associated with LDL (LDL-P[ 14C]OPC) or 1-linoleoyl-2-[ 14C]linoleoylPC associated with LDL (LDL-L[ 14C]LPC) was used as the substrate. The optimum pH for the formation of [ 14C]oleoyllysoPC from LDL-P[ 14C]OPC and for the formation of [ 14C]linoleoyllysoPC from LDL-L[ 14C]LPC was pH 4.5, and pH 4.5 and 7.0, respectively. These activities were designated as phospholipase A 1 activities. The optimum pH values for the formation of [ 14C]oleate from LDL-L[ 14C]OPC and for the formation of [ 14C]linoleate from LDL-L[ 14C]LPC were pH 4.5 and 6.5, and pH 4.5, 6.5 and 8.5, respectively. These activities were designated as phospholipase A 2 activities. Ca 2+ did not affect acid phospholipase A 1 activity, but decreased acid phospholipase A 2 activity for the hydrolysis of LDL-L[ 14C]LPC. When smooth muscle cells were incubated with LDL, both phospholipase A 1 and phospholipase A 2 activities at pH 4.5 for the hydrolysis of LDL-L[ 14C]LPC increased significantly. These results indicate that phospholipases A 1 and A 2. which hydrolyze PC associated with LDL, exist in arterial smooth muscle cells and are involved in the metabolism of LDL incorporated into these cells.