Tryptic Modification of Human Serum Low‐Density Lipoprotein Leads to Altered Rates of Turnover and Intravascular Transformation in the Guinea Pig

Abstract

The metabolism of human serum low-density lipoprotein (LDL) and its trypsin-treated counterpart were 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 trypsinization of LDL permitted removal of 20-25% of its protein moiety; trypsinized 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 (131I) and trypsinized LDL (125I) into the same animal, the 2 lipoproteins displayed biexponential decays; the rate constants for the plasma turnover of LDL and trypsinized LDL were significantly different (P < 0.05), trypsinized LDL exhibiting a slower disappearance from the circulation. Density-gradient ultracentrifugation revealed marked elevation in the modal densities of both LDL and trypsinized LDL upon metabolism in vivo, although the rate of increase was greater for trypsinized 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 trypsinized 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 tryspin-accessible protein. 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.