Activation of Lipoprotein Lipase: EFFECTS OF RAT SERUM LIPOPROTEIN FRACTIONS AND HEPARIN

Guinea pig serum is deficient in high density lipoproteins (HDL); post-heparin lipoprotein lipase (LPL) from it hydrolyzes a triglyceride (TG) emulsion very slowly. The rate is markedly increased by the addition of rat serum and even further by rat serum plus heparin. We have studied further the activation of LPL in this system. Rat serum lipoprotein fractions were isolated by ultracentrifugation and added to guinea pig postheparin serum in the presence or absence of heparin. When added in proportion to their original serum concentrations, HDL caused the greatest increase in TG hydrolysis. When each fraction was added at equal protein concentrations, purified low density lipoproteins had almost no effect; both very low density lipoproteins and HDL were very effective in increasing the rate of hydrolysis. Rat serum or rat HDL, added to the assay system in increasing amounts, appeared to increase the effective substrate concentration. In the absence of heparin, increasing the concentration of HDL increased the reaction rate which approached a limiting velocity (Vmax) and produced a hyperbolic curve which conformed to Michaelis-Menten kinetics. In the presence of heparin, increasing the concentration of HDL produced an S-shaped curve and increased the Vmax. These data conformed to the sigmoidal kinetics described by the Hill equation. Our results suggest that (1) heparin may function as a specific ligand which acts as an allosteric modifier of LPL and alters the kinetics of interaction of LPL with the effective substrate and (2) the rate of hydrolysis of the effective TG substrate is regulated by the concentration of this substrate.