Interaction between nephrocalcin and calcium oxalate monohydrate: a structural study

Study of crystals of calcium oxalate monohydrate grown from gels exposed to 0, 5.6x, 12.5x, 26.2x, 52.5x, 100x, 200 x 10(-7) M nephrocalcin indicate that this protein profoundly affects their habit, size, and crystal structure. By the time nephrocalcin concentration is 26.2 x 10(-7) M calcium oxalate monohydrate undergoes a phase change in its basic structure and both crystal size as well the resolution of its diffraction pattern are severely curtailed. These effects are magnified when the protein is 52.5 x 10(-7) M, since long-range disorder becomes extreme and, out of the entire diffraction pattern, only the 0k0's, h00's and a few other nonaxial reflections remain from the ordered part of the crystal structure. Finally, once the concentration of nephrocalcin is raised to 100 and 200 x 10(-7) M, growth is so inhibited that calcium oxalate monohydrate no longer grows as distinct individuals but rather as aggregates of very small crystallites. All of this is caused by the ability on the part of nephrocalcin to disturb the juxtaposition of the (101) layers along c by disrupting the organization of both the C(3)-C(4) oxalate groups and the water molecules. Such interaction is modulated by the efficiency with which nephrocalcin adsorbs upon the (101) planes; this process is stereospecific.