Evaluation of the roles of lipoprotein lipase and hepatic lipase in lipoprotein metabolism: in vivo and in vitro studies in man

The roles of lipoprotein lipase (LPL) and hepatic lipase in very low density lipoprotein (VLDL) and VLDL remnant metabolism were investigated by (1) in vivo studies where the kinetics of VLDL-apo B removal were measured in patients with non-functioning lipoprotein lipase systems, and (2) in vitro studies where the relative capacities of hepatic lipase and LPL to hydrolyse the triglyceride (TG) of different lipoprotein substrates was measured. The results indicated that VLDL-apo B removal was not impaired in patients with non-functional LPL, nor ws there any apparent abnormality in the conversion of VLDL-apo B to intermediate- (IDL) and low (LDL) density lipoprotein-apo B. Post-heparin plasma hepatic lipase activity against VLDL was normal in these subjects. Purified normal hepatic lipase had a similar Km for VLDL-TG hydrolysis (1.57 mmol/l) to that of LPL (1.49 mmol/l). However, at equal lipoprotein TG concentration, hepatic lipase had increasing activity with lipoproteins of decreasing particle size, in the order chylomicrons much less than VLDL of Sf 100-400 less than VLDL of Sf 60-100 less than VLDL of Sf 20-60 less than IDL. The mean contribution of hepatic lipase to VLDL-TG hydrolysis by post-heparin plasma was 35% in normal controls, but the contribution to IDL-TG hydrolysis was significantly higher (mean - 58%). It is concluded that hepatic lipase plays a significant role in VLDL and, especially, IDL metabolism, at least in patients with non-functioning lipoprotein lipase.