**2.4 Hepatic lipase**

As the name suggests, hepatic lipase (EC 3.1.1.3) is synthesized mostly by hepatocytes in the liver and found localized at the surface of liver sinusoidal capillaries (Perret & al., 2002). The human hepatic lipase presents four glycosylation sites, which are localized at positions 20, 56, 340, and 375, and a molecular mass around 65 kDa (Ben-Zeev & al., 1994 ; Wolle & al., 1993). Together with lipoprotein lipase (LPL), hepatic lipase (HL) could be considered as a lipase of the vascular compartment (Perret & al., 2002). Unlike pancreatic lipase, hepatic lipase does not require a cofactor for its activity; is stable at high salt concentrations and is inactivated by sodium dodecyl sulfate (Mukherjee, 2003). HL exerts both triglyceride lipase and phospholipase A1 activities, and is involved at different steps of lipoprotein metabolism (Santamarina-Fojo & al., 2004). The preferred physiological substrate of hepatic lipase is triglyceride of intermediate density lipoprotein (IDL) particle, which it hydrolyses to form triglyceride-poor and cholesterol-rich low-density lipoprotein (LDL). Hepatic lipase also converts post-prandial triglyceride rich high-density lipoprotein (HDL) particle (i.e.HDL2) to post-absorptive triglyceride poor HDL (i.e. HDL3) (Mukherjee, 2003).

#### **2.5 Lipoprotein lipase**

Lipoprotein lipase (EC 3.1.1.34) (LPL) is a non-covalent homodimeric protein produced mainly by the adipose, heart and muscle tissue and to some extent by macrophages (Camp & al., 1990). LPL is secreted from parenchymal cells as a glycosylated homodimer, after which it is translocated through the extracellular matrix and across endothelial cells to the capillary lumen. After secretion, however, the mechanism by which LPL travels across endothelial cells is still unknown (Braun & Severeson 1992; Mead & al., 2002). The glycosylation sites of LPL are Asn-43, Asn-257, and Asn-359 (Mead & al., 2002). Lipoprotein lipase has multiple functional domains including lipid-binding, the dimer formation, heparin binding, cofactor interaction and fatty acid-binding domains (Santamarina-Fojo & Dugi, 1994). Interaction of the enzyme with the lipoprotein substrate takes place in the lipidbinding domain. This results in a conformational change that leads to the movement of a short helical segment or 'lid' to expose the active site containing the Ser-Asp-His catalytic triad, where hydrolysis of triacylglycerol takes place (Emmerich & al., 1992). As a homodimer, LPL has the dual function of triglyceride hydrolase and ligand/bridging factor for receptor-mediated lipoprotein uptake. Through catalysis, triacylglycerol present in very low-density lipoprotein (VLDL) and chylomicron particles is converted to triglyceride-poor intermediate-density lipoprotein (IDL) and chylomicron remnants, respectively (Mukherjee, 2003). Apolipoprotein CII (ApoCII) present on VLDL particles is the co-factor required for activating the enzyme (Mukherjee, 2003).
