title: Diacylglycerol is the preferred substrate in high density lipoproteins for human hepatic lipase.
abstract: The hydrolysis of HDL phospholipids (PL) and glycerides by hepatic lipase (HL) has been investigated in native and reconstituted HDL particles (Lp2A-I). Fasting, normolipidemic HDL exhibit total lipid hydrolytic rates of between 10 and 36 nM FA/h per microM PL. Of the total fatty acids liberated with HDL3 only 1% are from triolein (TG), while 49% are from diolein (DG) and 50% are from PL. A spherical reconstituted particle containing 2 molecules of apoA-I, 120 molecules of PL, and 20 molecules of TG exhibits a total lipid hydrolytic rate of 18 nM FA/h per microM PL and 93% of the fatty acids liberated are from PL. Inclusion of 40 molecules of TG into the Lp2A-I particle doubles the rate of fatty acid hydrolysis by HL through a stimulation of TG hydrolysis. Further addition of 10 molecules of DG to the Lp2A-I complex has no effect on the overall rates of hydrolysis, but changes the substrate specificity, wherein 61% of the fatty acids are from DG and both TG and PL hydrolytic rates are significantly reduced. Increasing the amount of DG in the Lp2A-I particle further stimulates total lipid hydrolysis by raising DG hydrolytic rates at the expense of PL and TG hydrolysis. A particle containing 10 molecules of TG and 40 molecules DG yields the fastest lipid hydrolytic rate of 143 nM FA/h per microM PL, which constitutes 96% DG hydrolysis, 3% TG hydrolysis, and 1% PC hydrolysis. These data indicate that hepatic lipase acts primarily as a surface lipid lipase with HDL particles. DG is the preferred substrate of HL in HDL and the HDL-DG content regulates the hydrolysis of both PL and TG by HL.
authors.affiliation: University of Ottawa Heart Institute, and Department of Biochemistry, University of Ottawa, Ontario, Canada.
authors.forename: C R
authors.initials: CR
authors.lastname: Coffill
authors.forename: T A
authors.initials: TA
authors.lastname: Ramsamy
authors.forename: D M
authors.initials: DM
authors.lastname: Hutt
authors.forename: J R
authors.initials: JR
authors.lastname: Schultz
authors.forename: D L
authors.initials: DL
authors.lastname: Sparks
mesh_terms: D002250:Carbon Radioisotopes; D004075:Diglycerides / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D006801:Humans; D007700:Kinetics; D008049:Lipase / Q000378:metabolism*; D008075:Lipoproteins, HDL / Q000737:chemistry / Q000378:metabolism*; D008099:Liver / Q000201:enzymology*; D010316:Particle Size; D010713:Phosphatidylcholines / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism; D013379:Substrate Specificity; D014304:Triolein / Q000378:metabolism; D014316:Tritium
chemical_list: D002250:Carbon Radioisotopes; D004075:Diglycerides; D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; D010743:Phospholipids; D014316:Tritium; D014304:Triolein; D008049:Lipase; C028694:1-palmitoyl-2-oleoylphosphatidylcholine; C013965:diolein