The metabolism of human serum low-density lipoprotein (LDL) and its trypsin-treated counterpart have been compared in the guinea pig in vivo. Removal of surface-exposed protein from the lipoprotein particle in this way resulted in significant modification of its metabolism in guinea pigs in vivo. Limited trypsinisation of LDL permitted removal of 20 -25 % of its protein moiety ; trypsinised LDL was deficient in lysine and arginine residues (25 -30 of each removed). The modified particle retained its basic structural features, such as internal molecular architecture, but displayed an elevated net negative surface charge and diminished immunological reactivity. Following intravascular injection of iodinated LDL (13' I) and trypsinised LDL (1251) into the same animal, the two lipoproteins displayed biexponential decays; the rate constants for the plasma turnover of LDL and trypsinised LDL were significantly different ( P < 0.05), trypsinised LDL exhibiting a slower disappearance from the circulation. Density-gradient ultracentrifugation revealed marked elevation in the modal densities of both LDL and trypsinised LDL upon metabolism in vivo, although the rate of increase was greater for trypsinised LDL than LDL in each case (average increment 0.022 g/ml and 0.014 g/ml at 24 h respectively). The diminished plasma clearance of trypsinised LDL as compared to the native human and guinea pig LDL indicates that sites required for the cellular recognition and uptake of the LDL particle reside in its surface-exposed, trypsinaccessible protein. Furthermore, such protein appears to play a central role in regulating the intravascular .processes by which the lipid content of LDL is diminished, and by which it is transformed to a particle of higher density.Two processes may presently be distinguished as components of the metabolism of low-density lipoproteins (LDL) in vivo in the circulation of the guinea pig: (a) catabolism, involving the irreversible removal of LDL from the plasma compartment [l, 21 and its subsequent degradation within cells, and (b) intravascular transformation, this being modification of the physical properties and presumably chemical composition of the particle during its circulation [3].Recent investigations of the catabolism of LDL in other species have been focussed on the role of its protein moiety. Chemical modifications of the protein of LDL that abolish the positive charge of lysine residues increase its turnover in the circulation by virtue of its rapid clearance by kuppfer cells in vivo [4,5]; lipoproteins modified in this manner are also bound, internalised and degraded by macrophages in vizr-o [5,6]. In contrast, methylation of lysine residues, which preserves their positive charge, decreases the rate of turnover of LDL in vivo [7], and prevents lipoprotein binding and internalisation by cultured fibroblast in v i m [8]. Thus, specificity for the interaction of LDL with its cellular receptor involves lysine and arginine residues and residues with the protein and not the lipid moiety [9-111. In...
