PG(18:3(6Z,9Z,12Z)/20:3(5Z,8Z,11Z)) is a phosphatidylglycerol or glycerophospholipid (PG or GP). It is a glycerophospholipid in which a phosphoglycerol moiety occupies a glycerol substitution site. As is the case with diacylglycerols, phosphatidylglycerols 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. PG(18:3(6Z,9Z,12Z)/20:3(5Z,8Z,11Z)), in particular, consists of one chain of g-linolenic acid at the C-1 position and one chain of mead acid at the C-2 position. The g-linolenic acid moiety is derived from animal fats, while the mead acid moiety is derived from fish oils, liver and kidney. Phosphatidylglycerol is present at a level of 1-2% in most animal tissues, but it can be the second most abundant phospholipid in lung surfactant at up to 11% of the total. It is well established that the concentration of phosphatidylglycerol increases during fetal development. Phosphatidylglycerol may be present in animal tissues merely as a precursor for diphosphatidylglycerol (cardiolipin). Phosphatidylglycerol is formed from phosphatidic acid by a sequence of enzymatic reactions that proceeds via the intermediate, cytidine diphosphate diacylglycerol (CDP-diacylglycerol). Bioynthesis proceeds by condensation of phosphatidic acid and cytidine triphosphate with elimination of pyrophosphate via the action of phosphatidate cytidyltransferase (or CDP-synthase). CDP-diacylglycerol then reacts with glycerol-3-phosphate via phosphatidylglycerophosphate synthase to form 3-sn-phosphatidyl-1'-sn-glycerol 3'-phosphoric acid, with the release of cytidine monophosphate (CMP). Finally, phosphatidylglycerol is formed by the action of specific phosphatases. While most phospholipids have a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2 of the glycerol backbone, the fatty acid distribution at the C-1 and C-2 positions of glycerol within phospholipids is continually in flux, owing to phospholipid degradation and the continuous phospholipid remodeling that occurs while these molecules are in membranes. PGs have a net charge of -1 at physiological pH and are found in high concentration in mitochondrial membranes and as components of pulmonary surfactant. PG also serves as a precursor for the synthesis of cardiolipin. PG is synthesized from CDP-diacylglycerol and glycerol-3-phosphate.