Increase in the density of lighter low density lipoprotein by hepatic triglyceride lipase

The role of HTGL in LDL metabolism was investigated. HTGL was separated from the postheparin plasma (PHP) by heparin-Sepharose affinity chromatography. 125I-LDL1 (1.019 less than d less than 1.045) was incubated in fasting plasma with or without HTGL at 37 degrees C for six hours. VLDL, IDL, LDL1, LDL2 (1.045 less than d less than 1.063), and HDL (d greater than 1.063) were harvested at 0, 180 and 360 minutes of incubation by ultracentrifugation, and the radioactivities in all lipoprotein fractions and the specific activities of apoprotein B in LDL1 and LDL2 were measured. Consistent small increments of the radioactivities were observed in all lipoprotein fractions except for the substrate, LDL1. When 125I-LDL1 was incubated with HTGL, LDL2 radioactivities increased significantly with the concomitant decrease in LDL1 radioactivities. The changes in VLDL, IDL and HDL radioactivities were similar to those with the incubation without HTGL. The specific activities of apoprotein B in LDL1 were constant throughout the incubation with and without HTGL. HTGL accelerated the increase in the specific activities of apoprotein B in LDL2. We concluded that HTGL removed the triglycerides and phospholipids from the lighter LDL fraction and increased the density.