Effect of trypsin treatment on the heparin- and receptor-binding properties of human plasma low-density lipoproteins

The effect of trypsin treatment on the heparin- and receptor-binding properties of human plasma low-density lipoproteins (LDL) was examined. LDL were treated with trypsin (2% by weight) for 16 h at 37 degrees C, and the trypsinized core particles (T-LDL) were isolated by gel permeation chromatography on Sepharose CL-4B. Trypsin degraded the apolipoprotein B moiety (Mr = 550,000) of LDL into numerous peptides of Mr less than 110,000, resulting in the release of 25% +/- 5% (n = 6) of its surface-associated protein. Relative to LDL, T-LDL had an increased phospholipid/protein ratio, decreased flotation density and alpha-helical structure, and increased fluidity of the surface and core constituents. Compared to LDL, T-LDL showed a 60% decreased capacity to suppress [1-14C]acetate incorporation into cellular sterols consistent with decreased binding to the LDL receptor. In contrast, T-LDL showed an enhanced capacity to form soluble complexes with heparin in the absence and presence of 2 mM Ca2+. Between 5 and 25 mM Ca2+, both LDL and T-LDL were maximally precipitated by heparin; the stoichiometry of the insoluble complexes (uronic acid/phospholipid, w/w) was 0.054 +/- 0.004 and 0.055 +/- 0.005 (n = 18) for LDL and T-LDL, respectively. Thus, trypsin treatment significantly diminished the lipoprotein's interaction with cells but not with heparin. This finding suggests that proteolysis may decrease receptor-mediated uptake of LDL without diminishing the lipoprotein's reactivity with acellular components of the arterial wall.