A COMPARISON OF STRUCTURE AND THERMAL-BEHAVIOR IN HUMAN PLASMA LIPOPROTEIN(A) AND LOW-DENSITY-LIPOPROTEIN - CALORIMETRY AND SMALL-ANGLE X-RAY-SCATTERING

Differential scanning calorimetry (DSC) and small-angle X-ray scatteri ng (SAXS) studies have been performed to investigate the structural pr operties of lipoprotein(a) [Lp(a)] and low-density Lipoprotein (LDL) o btained from the same donor. In addition, a comparison was made betwee n autologous LDL and the remnant particle Lp(a-) obtained by removal o f apo(a) through chemical reduction. With Lp(a), three distinct therma l transitions have been observed: the first one around 20 degrees C, a rising from the core-located apolar lipids, similar to LDL but with a significantly lower melting temperature as compared to ,LDL of the sam e donor. The second one, at 55.7 +/- 0.25 degrees C, can be attributed to apo(a), since it was found to be absent in Lp(a-) and LDL, whereas isolated apo(a) in aqueous solution exhibited a similar transition. T he third transition, at 80.4 +/- 0.9 degrees C, corresponds to apo-B10 0 protein unfolding. The low melting temperature of the core lipids in Lp(a) is preserved in Lp(a-); this suggests that the apolar lipid int eractions are unaffected by apo(a) binding, and that the difference in the core melting behavior between Lp(a) and LDL is due to a different stabilization through interaction between the apolar core and the sur face monolayer lipids. SAXS curves exhibited qualitatively the same ch aracteristic features for LDL, Lp(a), and Lp(a-). Thus, the SAXS resul ts showed that no major deviations from spherical particle shape occur with Lp(a), indicating that apo(a) wraps around the particle surface without major globular protrusions into the aqueous surrounding. Overa ll, the calorimetric and X-ray results show that LDL and Lp(a-) are si milar but not identical in structure and thermal stability, which may be of metabolical interest.