Crystallization and preliminary x-ray analysis of a low density lipoprotein from human plasma

Single crystals of human plasma low density lipoprotein (LDL), the major transport vehicle for cholesterol in blood, have been produced with a view to analysis of the three-dimensional structure by x-ray crystallography. Crystals with dimensions of approximately 200 x 100 x 50 mu m have been reproducibly obtained from highly homogeneous LDL particle subspecies, isolated in the density ranges d = 1.0271-1.0297 g/ml and d = 1.0297-1.0327 g/ml. Electron microscopic imaging of ultrathin-sectioned preparations of the crystals confirmed the existence of a regular, quasihexagonal arrangement of spherical particles of approximately 18 nn in diameter, thereby resembling the dimensions characteristic of LDL after dehydration and fixation. X-ray diffraction with synchrotron radiation under cryogenic conditions revealed the presence of well resolved diffraction spots, to a resolution of about 29 Angstrom. The diffraction patterns are indexed in terms of a triclinic lattice with unit cell dimensions of a = 16.1 nm, b = 39.0 nm, c = 43.9 nm; alpha 96.2 degrees, beta = 92.1 degrees, gamma = 102 degrees, and with space group P1.