Calcification plays an important role in the human body in maintaining homeostasis. In the human body, the presence of a high amount of oxidized low-density lipoprotein (ox-LDL) is a consistent feature of the local areas that are common sites of ectopic calcification, namely dental calculus, renal calculus, and the areas affected by arteriosclerosis. Hence, ox-LDL may have some effect on calcification. Scanning electron microscopy (SEM) observation revealed a high amount of amorphous calcium phosphate (ACP) when ox-LDL was included in the solution. In the in vitro experiment, the highest amount of precipitation of calcium phosphate was observed in the solution containing ox-LDL compared to the inclusion of other biomaterials and was 4.2 times higher than that of deionized water for 4.86 mM calcium and 2.71 mM phosphate. The morphology of calcium phosphate precipitates in the solution containing ox-LDL differed from that of the precipitates in solutions containing other biomaterials, as determined by transmission electron microscopy (TEM). Through the time course observation of the sediments using TEM, it was observed that the sediments changed from spherical or oval shape to a thin film shape. These results indicate that sediments acquired a long-range order array, and the phase transitioned from non-crystalline to crystalline with an increased time and density of ACP. Thus, it is concluded that ox-LDL promoted ACP precipitation and it plays an important role in ectopic calcification.
Keywords: amorphous calcium phosphate; calcification; crystallization; dentin; oxidized low-density lipoprotein.