Characterization, Subcellular Localization, and Partial Purification of a Heparin-released Triglyceride Lipase from Rat Liver

Post-heparin plasma in the rat contains triglyceride lipase activity of both hepatic and extrahepatic origin. The lipase released into rat hepatic perfusate by heparin has been characterized in an assay containing Triton X-100, albumin, and [14C]triolein. Fatty acid release was linear for 120 min over a wide range of enzyme concentrations at 27° and 37°. The pH optimum was 9.5. Enzymatic activity was g80% inhibited by prior incubation with 0.5 m NaCl or 750 µg of protamine per ml. The apparent Km was 1.28 mm. The enzyme was localized in rat liver cell fractions. Cell fractions were checked for purity by marker enzymes and electron microscopy. Activity inhibited by NaCl or protamine was found in plasma membranes, microsomes, and cytosol, having pH optima of 9.5, 9.0, and 8.0, respectively. A different triglyceride lipase not affected by NaCl or protamine and having a pH optimum at 4.4 was found in lysosomes. The plasma membrane-bound lipase had a specific activity 10 times that in liver homogenate. Addition of heparin to the plasma membrane fractions in a concentration as low as 1 unit per ml effected the release of the enzyme into the medium. The plasma membrane-released lipase had the same apparent Km. A 360-fold enhancement of plasma membrane lipase activity over that in whole liver homogenate was achieved by heparin affinity chromatography.