2D-NMR reveals different populations of exposed lysine residues in the apoB-100 protein of electronegative and electropositive fractions of LDL particles.

Several potentially atherogenic LDL subfractions present low affinity for the LDL receptor, which result in impaired plasma clearance. Electronegative LDL [LDL(−)] is one of these minor subfractions and the molecular basis for its reduced receptor affinity is not well understood. In the present study, high-resolution 2D-NMR spectroscopy has been employed to characterize the surface-exposed lysine residues of the apolipoprotein (apo)B-100 protein in both LDL(−) and LDL(+) subfractions. LDL(+) showed two populations of lysine residues, similar to those previously described in total LDL. “Normal” Lys have a pka of 10.4 whereas “active” Lys have a pka of 8.8 and have been suggested to be involved in receptor binding. In contrast to LDL(+), the LDL(−) subfraction presented a third type of Lys, named as “intermediate” Lys, with a different microenvironment and higher basicity (pka 10.7). These intermediate Lys cannot be reliably identified by 1D-NMR. Because the abundance of normal Lys is similar in LDL(+) and LDL(−), the intermediate Lys in the apoB-100 molcule of LDL(−) should come from a group of active Lys in LDL(+) particles that have a less basic microenvironment in the LDL(−) particle. These differences between LDL(+) and LDL(−) are indicative of a distinct conformation of apoB-100 that could be related to loss of affinity of LDL(−) for the LDL receptor.