LpPLA2, or platelet-activating factor acetylhydrolase, is a lipase enzyme found predominantly on the surface of LDL particles. Note that LpPLA2 is not an apolipoprotein. LpPLA2 is made by inflammatory cells (T cells, mast cells, macrophages) and is then integrated onto the surface of lipoprotein particles. The enzymatic function of LpPLA2 is to hydrolyze oxidized phospholipids in LDL.
LpPLA2 plays a corrective role in removing oxidized phospholipids. Thus, it might seem that having high levels of LpPLA2 would be good. However, although LpPLA2 has a positive role in removing oxidized lipids, it also generates inflammatory products in the process. So high levels of LpPLA2 are actually associated with increased cardiovascular risk. Researchers have identified high amounts of LpPLA2 in human atherosclerotic lesions. The LpPLA2 that accumulates in the vessel wall can come from LDL (which can carry LpPLA2 on its surface) or it can come from immune cells that have invaded the vessel wall. Since LpPLA2 is produced or localized in the plaque itself, it may be a more specific marker of cardiovascular function compared to systemic, more general inflammatory markers like hs-CRP.