Central enzyme in the extracellular metabolism of plasma lipoproteins. Synthesized mainly in the liver and secreted into plasma where it converts cholesterol and phosphatidylcholines (lecithins) to cholesteryl esters and lysophosphatidylcholines on the surface of high and low density lipoproteins (HDLs and LDLs). The cholesterol ester is then transported back to the liver. Also produced in the brain by primary astrocytes, and esterifies free cholesterol on nascent APOE-containing lipoproteins secreted from glia and influences cerebral spinal fluid (CSF) APOE- and APOA1 levels. Together with APOE and the cholesterol transporter ABCA1, plays a key role in the maturation of glial-derived, nascent lipoproteins. Required for remodeling high-density lipoprotein particles into their spherical forms (By similarity). Has a preference for plasma 16:0-18:2 or 18:O-18:2 phosphatidylcholines (PubMed:8820107). a 1,2-diacyl-sn-glycero-3-phosphocholine + a sterol = a 1-acyl-sn-glycero-3-phosphocholine + a sterol ester APOA1 is the most potent activator in plasma. Also activated by APOE, APOC1 and APOA4 (By similarity). Secreted into blood plasma (PubMed:7592817, PubMed:8820107). Detected in blood plasma (at protein level) (PubMed:7592817, PubMed:8820107). Expressed in liver, brain and adrenal glands. Lower expression in testes. In laying hens, expressed higher in brain than in liver. In roosters, higher levels in liver than in brain (PubMed:7592817). Expressed in liver during embryogenesis. Levels higher in liver than in brain up to 15 weeks after hatching. In the brain, first detected at 11 weeks after hatching and increasing levels from 15 weeks to maturity (25 weeks). Expression in the liver is much lower throughout these later stages of hatching. Belongs to the AB hydrolase superfamily. Lipase family.