Any of various enzymes that cleave particular bonds in the polar phosphate ‘heads’ of glycerophospholipids (see phospholipid). For example, phospholipase C (PLC) cleaves the phosphate-glycerol bond and is important in liberating the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol in cell plasma membranes (see inositol). PLC plays a central role in signal transduction; it is activated by protein tyrosine kinases, which themselves are activated by ligand binding to cell surface receptors. PhospholipaseA2 (PLA2) is found in pancreatic juice and helps in the breakdown of ingested glycerophospholipids by cleaving the acyl residue (e.g. a fatty acid) from C2 of the glycerol.