Inhibition of receptor-mediated clearance of lysine and arginine- modified lipoproteins from the plasma of rats and monkeys (lipoprotein catabolism/protein modification/cell receptors/protein turnover)

Reductive methylation of at least 30% of the lysine residues or 1,2-cyclohexanedione modification of 45% of the arginine residues prevented low density lipoproteins (LDL) from binding to cell surface receptors of fibroblasts in vitro, without significantly altering other physical or chemical properties of the LDL. When rat or human LDL with more than 30% of the lysine residues methylated were injected intrave- nously into rats, the clearance of these lipoproteins from the plasma was slowed considerably. The half-life of the reductively methylated LDL was approximately twice that obtained for control (unmodified) LDL, and the value for the fractional catabolic rate was approximately half that of the control. Fur- thermore, when human LDL modified by reductive methylation were injected into rhesus monkeys, the rate of clearance was similarly retarded, and the value for the fractional catabolic rate was reduced by approximately 50% as compared with the value for control LDL. A dual isotope labeling technique (125I and 3I) was used to compare the disappearance of the control and modified LDL in the same animal. It was demonstrated that not only modification of lysine residues but also modification of the arginine residues with I,-cyclohexanedione retarded the plasma clearance of the rat LDL. However, the cyclohexane- dione modification was spontaneously reversible at 370C, whereas reductive methylation of the lysine residues was stable. It is concluded that the selective chemical modification of lysine or arginine residues of LDL interferes with the normal uptake of these lipoproteins in vivo as well as by fibroblasts in vitro. These data provide an estimation of the level of receptor-med- iated clearance of LDL from the plasma, a value that may be as high as 50% in rats and monkeys. The protein moieties of certain plasma lipoproteins have been shown to be involved in the control of various aspects of lipo- protein metabolism. It has been reported that the recognition site on lipoproteins responsible for their binding to the cell surface receptors of cultured fibroblasts resides with specific apoproteins (1-4). The B apoprotein of low-density lipoproteins (LDL) and the E apoprotein of certain high-density lipoproteins (HDL1, HDLC) react with the same receptors on the cell surface (1, 3). The modification of a limited number of arginine and lysine residues has been shown to prevent these lipoproteins from reacting with the apo-B,E receptor sites (5, 6). Arginine residues were selectively modified with 1,2-cy- clohexanedione by a mild procedure that did not otherwise significantly alter the physical or chemical properties of the LDL (5). The lysine residues were modified by acetoacetylation and reductive methylation (6). Both procedures were shown to be selective and mild. An important difference between the two procedures used for lysine modification was that acetoac- etylation neutralized the positive charge on the E-amino group, whereas reductive methylation did not alter the charge.