DIACYLGLYCEROL IS THE PREFERRED SUBSTRATE IN HIGH-DENSITY-LIPOPROTEINS FOR HUMAN HEPATIC LIPASE

The hydrolysis of HDL phospholipids (PL) and glycerides by hepatic lip ase (HL) has been investigated in native and reconstituted HDL particl es (Lp2A-I). Fasting, normolipidemic HDL exhibit total lipid hydrolyti c rates of between 10 and 36 nM FA/h per mu M PL. Of the total fatty a cids liberated with HDL3 only 1% are from triolein (TG), while 49% are from diolein (DG) and 50% are from PL. A spherical reconstituted part icle containing 2 molecules of apoA-I, 120 molecules of PL, and 20 mol ecules of TG exhibits a total lipid hydrolytic rate of 18 nM FA/h per mu M PL and 93% of the fatty acids liberated are from FL. 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 specificit y, wherein 61% of the fatty acids are from DG and both TG and PL hydro lytic rates are significantly reduced. Increasing the amount of DG in the Lp2A-I particle further stimulates total lipid hydrolysis by raisi ng DG hydrolytic rates at the expense of PL and TG hydrolysis. A parti cle containing 10 molecules of TG and 40 molecules DG yields the faste st lipid hydrolytic late of 143 nM FA/h per mu M 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 th e HDL-DG content regulates the hydrolysis of both PL and TG by HL.