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. Phospholipase A₂ (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.