2-acyl-sn-glycero-3-phosphocholines are a class of phospholipids that are intermediates in the metabolism of lipids. Because they result from the hydrolysis of an acyl group from the sn-1 position of phosphatidylcholine, they are also called 1-lysophosphatidylcholine (or 1-lysoPC, in short). The synthesis of phosphatidylcholines with specific fatty acids occurs through the synthesis of 1-lysoPC. The formation of various other lipids generates 1-lysoPC as a by-product.
Other synonyms for this class of compounds are 2-acylglycero-3-phosphocholine, 1-lyso-2-acyl-sn-glycero-3-phosphocholine, β-lysophosphatidylcholine, 2-acylglycerophosphocholine, L-1-lysolecithin and 1-lecithin.
Chemical properties
1-LysoPC can convert to the structurally similar 2-lysoPC. This happens by the migration of the acyl group from the sn-1 position of the glycerol backbone to the sn-2 position.
The lowest rate of isomerization is at pH 4-5. Starting from either 1-lysoPC or 2-lysoPC, an equilibrium mixture of 90% 2-lysoPC and 10% 1-lysoPC is obtained with a half-time of about 10 minutes under physiological conditions and about 1 hour under typical laboratory conditions. Thus, unless special care is taken to slow this isomerization reaction, characterization of either of these lipids using laboratory assays that take a significant amount of time is likely to produce results on a mixture of the two lipids.
In the fungus 'Penicillium notatum, an enzyme called lysolecithin acylmutase has been reported that can catalyze this isomerization reaction at low pHs at which the uncatalyzed reaction occurs very slowly.
Metabolic reactions
Because 1-lysoPC has a relatively short half-life (see above), it is primarily a metabolic intermediate or side-product in the formation or breakdown of other lipids.
As product of reactions
1-LysoPC can be produced from phosphatidylcholine by enzymes having phospholipase A1 activity, that is, enzymes hydrolyzing specifically at the sn-1 position of a phospholipid and releasing a fatty acid. Enzymes having phospholipase B activity hydrolyze both the sn-1 and sn-2 positions, so they produce both 1-lysoPC and 2-lysoPC. In humans, a number of enzymes with phospholipase A1 exist: they include two from the brain (with different optimal pHs), two from the liver (one cytosomal and one membrane-bound) and one from the mitochondrion.
1-LysoPC can be produced as a by-product of the reaction transferring an acyl group from a phosphatidylcholine to an acceptor molecule, resulting in the formation of an esterified acceptor molecule. In the human, an example of such a transacylation reaction is that catalyzed by the enzyme LRAT that converts an all-trans retinol to an all-trans-retinyl ester. The latter is a storage form of Vitamin A in various tissues, as well as a retinal precursor in the retina (see visual cycle).
1-LysoPC is also a by-product of the reaction in which N-arachidonoyl-1,2-diacyl-glycerol 3-phosphoethanolamine (NAPE) is produced. This is a rate-limiting step for the synthesis of anandamide and related signaling lipids. Two enzymes are known to catalyze this reaction. LRAT-like protein (RLP-1; a product of the gene HRASLS5), catalyzes transfer of an acyl group from phosphatidylcholine (PC) to phosphatidylethanol (PE), resulting in the formation of radioactive NAPE. RLP-1 does not show selectivity with respect to the sn-1 and sn-2 positions of PC as an acyl donor and therefore can generate N-arachidonoyl-PE (an anandamide precursor) from 2-arachidonoyl-PC and PE. There is also a Ca2+-dependent, membrane-associated enzyme (whose gene is currently not known) that is specific for the sn-1-acyl group of PC.
As substrate of reactions
1-lysoPC can be hydrolyzed further by lysophospholipases to lose the remaining fatty acid and yield L-1-glycero-3-phosphocholine. In humans, 1-lysoPC can be hydrolyzed by ten different enzymes: calcium-independent phospholipase A2-gamma (coded by the gene PNPLA8), neuropathy target esterase (PNPLA6), lysophospholipase (PLB1), eosinophil lysophospholipase (CLC), galactoside-binding soluble lectin 13 (LGALS13), Group XV phospholipase A2 (PLA2G15), acyl-protein thioesterase 1 (LYPLA1), lysophospholipase (PLA2G4A), acyl-protein thioesterase 2 (LYPLA2) and PNPLA7 (Some of these enzymes also have phospholipase A2 activity). Other organisms have different numbers of enzymes that catalyze the same chemical reaction. For example the bacterium E. coli has only the gene products of and , while the yeast S. cerevisiae has the gene products of NTE1, PLB2, PLB1 and PLB3.