LIPOPROTEIN-LIPASE IN HUMAN PLASMA IS MAINLY INACTIVE AND ASSOCIATED WITH CHOLESTEROL-RICH LIPOPROTEINS

This study was designed to further ascertain the presence in plasma of lipoprotein lipase (LPL) bound to circulating lipoproteins. Lipoprote in lipase mass and activity values in preheparin plasma from 20 volunt eers were 69.8 +/- 6.6 ng. ml-1 and 1.54 +/- 0.15 mU. ml-1, respective ly, and no significant correlation between mass and activity was obser ved. Fifteen min after heparin injection, LPL mass had increased to 53 6 +/- 60 ng . ml-1 and LPL activity to 261 +/- 34 mU . ml-1 and a high ly significant correlation between the increments in mass and activity was observed. The released material had a specific activity of 0.57 /- 0.03 mU . ng-1. The LPL mass in preheparin plasma eluted early from heparin-Sepharose, in the position expected for inactive LPL monomers . Western blot analysis showed that the eluted material had the size e xpected for the LPL subunit (55 kDa). The increment of mass and activi ty after heparin eluted later from heparin-Sepharose, in the position expected for active LPL dimers. It is concluded that preheparin plasma contains substantial amounts of inactive LPL protein, and that hepari n releases mainly active LPL into circulation. On gel filtration LPL a ctivity and mass in postheparin plasma eluted mainly in the positions of LDL and HDL. Electron microscopy of immunostained fractions showed reaction for LPL and apolipoprotein B, or apolipoprotein A-I, on the s ame particles. LPL mass in preheparin plasma eluted in a similar patte rn, associated with LDL and HDL. In postprandial plasma substantial am ounts of LPL protein eluted with the triglyceride-rich lipoproteins. W hen I-125-labeled bovine LPL was added to plasma or to ultracentrifuga lly isolated lipoproteins and then analyzed by gradient gel electropho resis, the labeled lipase moved with the lipoproteins. The presence of substantial amounts of inactive LPL protein associated with lipoprote ins in plasma may have important implications for the metabolism of th e particles in view of recent reports on avid binding of LPL-lipoprote in complexes to cell surfaces and receptors.