LIPOPROTEIN LIPASE

lipase = lipid hydrolyzing enzyme lipoprotein = lipid and protein non-stoichiometric non-covalent aggregate in blood, in lymph = chyle (and bile) decreasing protein content (causing less density): nascent HDL, HDL, LDL, IDL, VLDL, chylomicron (Chy) High TAG containing lipoproteins: VLDL, Chy Localization of lipoprotein lipase = LPL  anchored to the inner wall of capillaries and artheries to glucoseaminoglycans= (GAG) = proteoglycans by ionic bonds  anchored to GAG to cells in interstitium  circulates in blood attached to lipoproteins  can be liberated by the addition of heparin

Place of synthesis not the endothel, but the parenchymal cells: adipocytes, myocytes, cardiocytes, macrophages, lung, spleen, mammary gland, pancreas, placenta, smooth muscle, steroid hormon producing glands LPL is not found in adult liver. Instead of LPL there is hepatic lipase there, anchored to sinusoids. Homologues of lipase family: lipoprotein lipase, hepatic lipase, pancreatic lipase, endothelial lipase. All can hydrolyse TAG, their active site contains Ser, His,Asp.

Function of LPL 1.) Hydrolyzes sn-1 and sn-3 from TAG of VLDL and chylomicron. The produced fatty acids and monoacyl-glycerol are a.) absorbed to cells and take part in beta-oxidation yielding ATP b.) absorbed to cells and TAG is resynthesized mainly in adipocytes and lactatingmammary gland c.) FA are liberated, joined to albumin, FFA gives enegy for neighburing cells (2.) Binds phospholipid surface of any of the lipoproteins and helps the uptake of the whole particle (CR, IDL, LDL). (3.) LPL is the ligand of different LP receptors, binds the LPs to cell surface, help the uptake of the particle. (4.) It helps the selective uptake of cholesterol ester from lipoproteins to cells, including steroid hormon producing cells.

Structure and binding sites of LPL It is a noncovalent homodimer. The monomer is unfunctional as hydrolase. N C phospho active siteGAG-bindingto other monomer lipid-bindingapo CII-siteLRP-binding lid (Ser, His,bindingsite Asp)(apo CII circulates with VLDL, Chy) Apoprotein CII activates LPL

Regulation of synthesis is specific for tissues The transcription, maturation, transport, activity are regulated by: hormons: insulin, estragen, adrenalin, prolactin metabolites: glucose, PUFA, cholesterol cytokines: TNFα, IL, IF, PG  In lactating mammary gland it is continuously very active to produce milk (milk production has priority over everything, in mild starvation as well).  In heart it is highly active ( and skeletal muscle) to yield energy especially in starvation, lower activity of LPL in well fed.  In adipocytes it is induced and activated in well fed state (to put on weight) and inhibited in fasting.  In bacterial infection it is inhibited in adipocytes to prevent FA uptake there. Instead,TAG hydrolysis ensures energy for other tissues.

Signs of LPL deficiency  In blood the degradation of chylomicron and VLDL are very slow, causing hyperchylomicronemia and hypertriglyceridemia (TAG >15 mM) blood plasm is like a red milk or tomato sauce  Cholic abdominal pain (even after 1st suckling)  Fat deposits everywhere, macrophages phagositose chylomicrones to cause hepatosplenomegaly, eruptive xanthomas, lipemia retinalis  Large chylomicrones plug the capillaries to prevent the traffic of materials, including oxigen. It causes dyspnoe in lung, memory loss, dementia in CNS, peripheral neuropathy.  pancreatitis  false laboratory values  dislike of fatty food, that caused symptoms Therapy avoidance of fatty food small amount of essential fatty acid containing see food, plant oils are allowed milk products are allowed (contain middle chain fatty acid absorbed from stomach)