THE INFLUENCE OF APOLIPOPROTEINS ON THE HEPATIC LIPASE-MEDIATED HYDROLYSIS OF HIGH-DENSITY-LIPOPROTEIN PHOSPHOLIPID AND TRIACYLGLYCEROL

This study describes the influence of apolipoproteins on the hepatic l ipase (HL)-mediated hydrolysis of phospholipids and triacylglycerol in high density lipoproteins (HDL), HL-mediated hydrolysis was assessed in well characterized, homogeneous preparations of spherical reconstit uted high density lipoproteins (rHDL), The rHDL were comparable in siz e and lipid composition and contained either apoA-I ((A-I)rHDL) or apo A-II ((AII)rHDL) as their sole apolipoprotein constituent. Preparation s of rHDL containing only cholesteryl esters (CE) in their core, (A-I/ CE)rHDL and (A-II/CE)rHDL, were used to assess phospholipid hydrolysis , Preparations of rHDL that contained triacylglycerol as their predomi nant core lipid, (A-I/TG)rHDL and (A-II/TG)rHDL, were used to assess b oth triacylglycerol and phospholipid hydrolysis, The rHDL contained tr ace amounts of either radiolabeled phospholipid or radiolabeled triacy lglycerol, Hydrolysis was measured as the release of radiolabeled none sterified fatty acids (NEFA) from the rHDL, Kinetic analysis showed th at HL had a greater affinity for the phospholipids in (A-II/CE)rHDL (K -m(app) = 0.2 mM) than in (A-I/CE)rHDL (K-m(app) = 3.1 mM). This was a lso evident when hydrolysis was measured directly by quantitating NEFA mass. HL also had a greater affinity for the phospholipids and triacy lglycerol in (A-II/TG)rHDL than in (A-I/TG)rHDL. The V-max for phospho lipid hydrolysis was, by contrast, greater for (A-I/CE)rHDL than for ( A-II/CE)rHDL: 309.3 versus 49.1 nmol of NEFA formed/ml of HL/h. Compar able V-max values were obtained for the hydrolysis of the phospholipid s in (A-II/TG)rHDL and (A-I/TG)rHDL. In the case of triacylglycerol hy drolysis, the respective V-max values for (A-I/TG)rHDL and (A-II/TG)rH DL were 1154.8 and 240.2 nmol of NEFA formed/ml of HL/h, These results show that apolipoproteins have a major influence on the kinetics of H L-mediated phospholipid and triacylglycerol hydrolysis in rHDL.