PA(18:1(11Z)/18:1(9Z)) is a phosphatidic acid. It is a glycerophospholipid in which a phosphate moiety occupies a glycerol substitution site. As is the case with diacylglycerols, phosphatidic acids can have many different combinations of fatty acids of varying lengths and saturation attached at the C-1 and C-2 positions. Fatty acids containing 16, 18 and 20 carbons are the most common. PA(18:1(11Z)/18:1(9Z)), in particular, consists of one chain of vaccenic acid at the C-1 position and one chain of oleic acid at the C-2 position. The vaccenic acid moiety is derived from butter fat and animal fat, while the oleic acid moiety is derived from vegetable oils, especially olive and canola oil. Phosphatidic acids are quite rare but are extremely important as intermediates in the biosynthesis of triacylglycerols and phospholipids. Indeed, the concentration of phosphatidic acids is often over-estimated in tissues and biofluids as it can arise by inadvertent enzymatic hydrolysis during inappropriate storage or extraction conditions during analysis. The main biosynthetic route of phosphatidic acid in animal tissues involves sequential acylation of alpha-glycerophosphate by acyl-CoA derivatives of fatty acids. PAs are biologically active lipids that can stimulate a large range of responses in many different cell types, such as platelet aggregation, smooth muscle contraction, in vivo vasoactive effects, chemotaxis, expression of adhesion molecules, increased tight junction permeability of endothelial cells, induction of stress fibres, modulation of cardiac contractility, and many others. Diacylglycerols (DAGs) can be converted to PAs by DAG kinases and indirect evidence supports the notion that PAs alter the excitability of neurons. Phospholipase Ds (PLDs), which catalyze the conversion of glycerolphospholipids, particularly phosphatidylcholine, to PAs and the conversion of N-arachidonoyl-phosphatidylethanolamine (NAPE) to anandamide and PAs are activated by several inflammatory mediators including bradykinin, ATP and glutamate. PAs activate downstream signaling pathways such as PKCs and mitogen-activated protein kinases (MAPKs), which are linked to an increase in sensitivity of sensory neurons either during inflammation or in chronic pain models. Circumstantial evidence that PAs are converted to DAGs. (PMID: 12618218, 16185776).