Glycation of apolipoprotein E impairs its binding to heparin: identification of the major glycation site

The increased glycation of plasma apolipoproteins represents a possible maj or factor for lipid disturbances and accelerated atherogenesis in diabetic patients. The glycation of apolipoprotein E (apoE), a key lipid-transport p rotein in plasma, was studied both in vivo and in vitro. ApoE was shown to be glycated in plasma very low density lipoproteins of both normal subjects and hyperglycemic, diabetic patients. However, diabetic patients with hype rglycemia showed a 2-3-fold increased level of apoE glycation. ApoE from di abetic plasma showed decreased binding to heparin compared to normal plasma apoE, The rate of Amadori product formation in apoE in vitro was similar t o that for albumin and apolipoproteins A-I and A-II. The glycation of apoE in vitro significantly decreased its ability to bind to heparin, a critical process in the sequestration and uptake of apoE-containing lipoproteins by cells. Diethylenetriaminepentaacetic acid, a transition metal chelator, ha d no effect on the loss of apoE heparin-binding activity, suggesting that g lycation rather than glycoxidation is responsible for this effect. In contr ast, glycation had no effect on the interaction of apoE with amyloid beta-p eptide. ApoE glycation was demonstrated to be isoform-specific. ApoE(2) sho wed a higher glycation rate and the following order was observed: apoE(2) > apoE(4) > apoE(3). The major glycated site of apoE was found to be Lys-75. These findings suggest that apoE is glycated in an isoform-specific manner and that the glycation, in turn, significantly decreases apoE heparin-bind ing activity, We propose that apoE glycation impairs lipoprotein-cell inter actions, which are mediated via heparan sulfate proteoglycans and may resul t in the enhancement of lipid abnormalities in hyperglycemic, diabetic pati ents. (C) 1999 Elsevier Science B.V. All rights reserved.