An x-ray small-angle-scattering study of the structure of trypsin treated low density lipoprotein from human serum.

X-ray small-angle-scattering curves were obtained from human serum low-density lipoprotein (LDL) from which some 20 % of the protein moiety had been removed by limited tryptic digestion, and the results are compared to those from the undigested lipoprotein. The basic structural features of LDL, i.e. a quasi-spherical shape and internal lipid compartmentation, were essentially unaltered by tryptic treatment. The trypsin-treated material exhibited the same thermotropic transition as the control. Our experiments indicate that proteolysis affected the outermost electrondense shell in a uniform manner, without causing any significant alterations in the radial symmetry of the internal molecular architecture. Since none of the structural regularities observed in the real-space pair distance distribution are affected, in particular the 3.6 nm periodicity below 15°C, it is concluded that these regularities do not relate to the protein arrangement in LDL.