The biochemistry of epsilon-amino groups of lysine residues from apolipoprotein B of human low density lipoprotein.
The availability of epsilon-lysine residues of apolipoprotein B in LDL for chemical or enzymic modification was investigated. Amino acid analyses of detergent-solubilized apolipoprotein B, following cyanoethylation with acrylonitrile, revealed that 10% of the lysine in apolipoprotein B were unreactive. The unreactive residues were associated with the most hydrophobic subfraction of apolipoprotein B. Since apolipoprotein B has a high molecular weight a study was undertaken to determine whether lysine residues were crosslinked to glutamic acid via epsilon-(gamma-glutamyl)lysine as demonstrated for fibrin. Apolipoprotein B was digested exhaustively with proteases. The content of epsilon-(gamma-glutamyl) lysine was determined by chromatography and isotope dilution. In contrast to earlier reports for serum LDL the data showed that less than 0.01 moles of lysine/mole of LDL apolipoprotein B were present as epsilon-(gamma-glutamyl)lysine in plasma LDL. It was determined also that the crosslinks were not found in apolipoprotein B during clotting since LDL was not a substrate for clotting factor XIII which forms the bond in fibrin. Furthermore, the lipoprotein contained no inherent transglutaminase activity. It is concluded that the lysine residues in LDL, which are unreactive to cyanoethylation, can not be detected in the digests as epsilon-(gamma-glutamyl)lysine.
D000181:Acrylonitrile; D001055:Apolipoproteins B / Q000097:blood*; D004151:Dipeptides / Q000097:blood*; D005176:Factor XIII / Q000378:metabolism; D006801:Humans; D006949:Hyperlipidemias / Q000097:blood; D066298:In Vitro Techniques; D008077:Lipoproteins, LDL / Q000097:blood*
D001055:Apolipoproteins B; D004151:Dipeptides; D008077:Lipoproteins, LDL; C001299:epsilon-(gamma-glutamyl)-lysine; D005176:Factor XIII; D000181:Acrylonitrile