The inhibition of calcium phosphate precipitation by fetuin is accompanied by the formation of a fetuin-mineral complex

The present studies show that the previously reported ability of fetuin to inhibit the precipitation of hydroxyapatite from supersaturated solutions of calcium and phosphate in vitro is accompanied by the formation of the fetuin-mineral complex, a high molecular mass complex of calcium phosphate mineral and the proteins fetuin and matrix Gla protein that was initially discovered in the serum of rats treated with etidronate and that appears to play a critical role in inhibiting calcification in vivo. Rat serum potently inhibited the precipitation of calcium phosphate mineral when the concentration of calcium and phosphate were increased by 10 mm each, and the modified serum was incubated at 37 degrees C for 9 days; in the absence of serum, precipitation occurred in seconds. Large amounts of the fetuin-mineral complex were generated in the first 3 h of this incubation and remained throughout the 9-day incubation. Purified bovine fetuin inhibited the precipitation of mineral for over 14 days in a solution containing 5 mM calcium and phosphate at pH 7.4 at 22 degrees C, whereas precipitation occurred in minutes without fetuin. There was a biphasic drop in ionic calcium in the fetuin solution, however, from 5 to 3 mM in the first hour and from 3 to 0.9 mM between 20 and 24 h; these changes in ionic calcium are due to the formation of complexes of calcium, phosphate, and fetuin. The complex found at 24 h to 14 days is identical to the fetuin-mineral complex found in the serum of etidronate-treated rats, whereas the complex found between 1 and 20 h is less stable.