Activation and inhibition of lipoprotein lipase. Studies with artificial lipoproteins.

Human plasma very low density apolipoproteins C-I, C-II and C-III were recombined in vitro with triolein. The lipid-protein complexes were analyzed by ultracentrifugal flotation, agarose gel electrophoresis, immunoelectrophoresis and electron microscopy. Maximal protein/triolein ratios for apoprotein C-I, C-II, C-III-1 and C-III-2 were 50, 45, 95 and 55 microgram/mg, respectively. Electron micrographs exhibited spherical particles with diameters ranging from 200--2000 A comparable to native VLDL and chylomicrons. On agarose gel electrophoresis these complexes showed alpha-mobility. Kinetics of triolein hydrolysis by purified human plasma lipoprotein lipase were studied using these artificial lipoprotein substrates with different apoprotein/triolein ratios. The reaction followed the Michaelis-Menten equation. With increasing amounts of apo C-II, the apparent Km decreased from 0.60 to 0.11 mM. Incubation of the substrate with either rabbit anti-apo C-II gamma-globulins or digestion with trypsin prior to hydrolysis reversed this lowering effect on apparent Km. V was not altered significantly. Increasing amounts of apo C-I, apo C-III-1 or apo C-III-2 without apo C-II caused inhibition of triolein hydrolysis. In the presence of apo C-II, however, similar kinetic parameters were obtained as described above.