Isolation and Partial Characterization of a Heparin Fraction Specific for Binding of Bovine Milk Lipoprotein Lipase
Fletcher, Geralyn M.
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Lipoprotein lipase (LpL) is the enzyme responsible for the hydrolysis of the triglycerides of chylomicrons and very-low- density lipoproteins (VLDL). Heparin has been shown to release LpL from its normal site on the lumenal capillary endothelium, facilitating its hydrolytic activity. Heparin has also been found to affect LpL-catalyzed triglyceride hydrolysis in vitro; it is believed that it exerts this effect by binding to the LpL. In this study crude heparin from, porcine intestinal mucosa was fractionated according to its affinity for bovine milk LpL and that fraction which bound most tightly, designated "HRH" (highly reactive heparin), was isolated and partially characterized. It was found to precipitate low-density lipoprotein (LDL) as effectively as heparin isolated solely for this ability. LpL incubated with HRH retained up to 31.3% more of its catalytic activity in p-nitrophenyl butyrate hydrolysis than LpL incubated alone. Heparin with little affinity (URH, or unreactive heparin) for LpL had much less of an effect on Lpl activity. It was found that radioactively labelling HRH with 125 I left it unable to bind LpL in affinity chromatography and in a ligand-blotting assay. It is suggested that there exists a small fraction of crude heparin which is capable of interacting with lipoprotein lipase; further characterization of this fraction may prove useful in studies of lipoprotein metabolism.