Lipoprotein lipase enables triacylglycerol hydrolysis by perfused newborn rat liver

Fasted 1-day-old rat liver has high heparin-releasable (endothelial) lipoprotein lipase (LPL) activity, and its hepatocytes synthesize LPL protein. To test the physiological role of this LPL, we perfused the isolated organ with a 0.8 mM triacylglycerol (TAG) (Intralipid + glycerol tri[3H]oleate) 6.3% serum medium. Samples of the recirculated perfusate were taken at different times to determine 3H in TAG, free fatty acid (FFA), and water-soluble (WS) fractions. In the medium [3H]TAG disappeared and [3H]FFA and [3H]WS fractions appeared linearly with time. This TAG hydrolysis was 1) absent when medium was recirculated without liver, 2) not affected by chloroquine addition, 3) inhibited by anti-LPL immunoglobulins, 4) absent when serum was omitted from the medium, and 5) restituted when apolipoprotein CII was added to the medium without serum. Therefore, lysosomal lipase is not involved in this TAG hydrolysis, the features of which are characteristic of LPL, not of the so-called "hepatic endothelial lipase." Thus LPL activity enables the neonatal rat liver to hydrolyze and take up circulating TAG, i.e., has the same function as extrahepatic LPL.