Biochimica et Siophysica Acta, 665 (1981) 519-530 Elsevier/North-Holland Biomedical Press 519 BBA 57860 REDUCED IMMUNOREGULATORY POTENCY OF LOW DENSITY LIPOPROTEINS WITH SELECTIVELY MODIFIED ARGININE AND LYSINE RESIDUES OF APOLIPOPROTEIN B J. GREG NOEL, DAVID Y. HUI *, DALE T. BLANKENSHIP and JUDITH A.K. HARMONY ** Department zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Biological Chemistry, University zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED of CincinnatiCollege zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON of Medicine, 231 Bethesda Avenue, Cincinnati, OH 45267 (lJ.S.A.j (Received April 29th, 1981) &Y words: ~rnrnunore~lat~~ potency; LDL; Ar&ine residue; Lysine residue; ApoIipoprotein B Human plasma low density lipoproteins (LDL) suppress lymphocyte activation invitro by i~ibiting the early, membrane-associated events such as phytohemagglutininenhanced Ca2+accumulation and phosphatidylmositol turnover. Chemical modification of the arginhte residues of the protein constituent of LDL by 1,2-cyclohexane- dione/borate or of the lysme residues by reductive methylation substantially decreases the immunosuppressive potency of LDL. The decrease in inhibitory capability of LDL correlates with a reduction in the ability of the derivatized LDL to compete with ‘2sI-labeied LDL for lymphocyte membrane receptors. This correlation indi- cates that immunore~ation by LDL is the direct result of the binding of LDL to specific receptors at the cell surface. The receptor recognition site of LDL may consist of a high content of basic amino acid residues such that chemical modification of the LDL apolipoprotein reduces the LDLlymphocyte interaction by specifical& altering the charge and/or steric properties of the receptor recognition site. Alternatively, chemical modification of arginine or lysine may cause a conformational change of the lipoprotein which is transmitted to the receptor recognition site. Derivatization of lysine or arginine residues does not elicit gross structnral alteration of LDL, as evidenced by chemical analysis and by fluorescence quenching analysis. The intrinsic fluorescence intensity of LDL is, however, decreased by chemical modi~cation, indicative of a minor but perhaps biologically significant ) structural alteration. Introduction Interaction of plasma lipoproteins with cell surface receptors and the consequences thereof have been the subjects of intensive research in recent years. Binding of low density lipoproteins to specific receptors, designated cholesterol transport receptors, on the sur- * Present address: Gladstone Foundation Laboratories, University of California Medical Center, San Francisco, DA 94140, U.S.A. * * To whom correspondence should be addressed. Abbreviations: LDL, low density lipoproteins (d 1.03-1.05 g/ml) ; CHD-LDL, LDL modified with 1,2~yclohexanedione/ borate; Me-LDL, alkylated (lysine) LDL prepared with for- maldehydejNaBH4 ; HDL,, high density lipoproteins isolated from cholesterol-fed canines; SDS, sodium dodecyl sulfate. face of mammalian cells is a prerequisite for the trans- port of cholesterol to extrahepatic tissues (for reviews, see Refs. l-3). LDL bind to cholesterol tram- ’ port receptors with high affinity, and binding initiates a sequence of events resulting in the intem~zation of the LDL by endocytosis with concomitant delivery of cholesterol to the cell. Internalized LDL-choles- terol regulates a number of intracellular processes including cholesterol biosynthesis, cholesterol esterifi- cation and biosynthesis of the cholesterol transport receptors. Receptor binding is mediated by the lipo- protein constituent apolipoprotein B [4]. Interaction of LDL at the surface of cells also per- turbs biochemical events which occur at the cell membrane. The LDL-thrombocyte interaction results in an increase of cellular cyclic AMP [5], and mem- OOOS-2760/81/0000-OOOO~$OZ.SO0 1981 Elsevier/North-Holland Biomedical Press