Lipolytic Enzymes and HDL: Influence of Drugs and Hormones

Abstract

A large body of evidence indicates that both lipoprotein lipase (LPL) and hepatic lipase activities have an important role in the regulation of plasma HDL₂ and total HDL levels (1–4). During the lipolysis of triglyceride-rich particles by LPL-released surface components, i.e. phospholipids, apoproteins C’s and E and free cholesterol, are transferred to HDL as demonstrated first by Patsch and co-workers (1). It is likely that both HDL₂ and HDL₃ are potential acceptors for the released surface material but the major acceptor is probably HDL₃. During this process there is a redistribution of HDL particles: the average density of HDL₃ decreases and may even reach the HDL₂ density range and particle size. If the lipolytic rate is slow as occurs in the presence of low LPL activity and/or substrate concentration, HDL₃ will incorporate the released surface components with a rise of particle density to less dense particles, i.e. light HDL₃. High LPL activity enhances the flux of the surface components to HDL₃ and consequently HDL₃ reaches the particle size of HDL_2a and HDL_2b (5). The complete conversion of HDL₃ to HDL₂ requires active LCAT reaction and an extra apo A-I molecule (5). We suggest that the degree of redistribution between HDL₃ and HDL₂ particles during lipolysis depends on the activity of the LPL system, on the pre-existing HDL particle distribution and the availability of apo A-I. Hepatic lipase is primarily a phospholipase which binds selectively to HDL₂ and hydrolyzes its phospholipids (6–8). Consequently, hepatic lipase is considered to have its major function in the catabolism of HDL₂ particles (4, 9).

This paper is dedicated to Esko A. Nikkilä, deceased September 21, 1986