Isolation from urine of pyrophosphate, a calcification inhibitor

Abstract

The minimum value of the product, Ca X P, necessary for calcium phosphate precipitation to occur, was determined in vitro. The addition of 2.2% urine to the solutions raised the mean product from 53 to 105 (mg/100 ml)2, which shows that the urine contains inhibitors to hydroxyapatite precipitation. One of these was isolated and purified. Its characteristics are those of inorganic pyrophosphate. The mean level of urinary pyrophosphate was 2.24 mg P/liter, sufficient to inhibit precipitation of hydroxyapatite. Pyrophosphate is one of the substances permitting urine to be supersaturated with calcium and phosphate; other urinary inhibitors are still to be identified. It is suggested that the phosphate compound present in the plasma which inhibits hydroxyapatite precipitation is pyrophosphate. In bone, pyrophosphatase would act by locally destroying this calcification inhibitor, thus allowing the apatite crystals to form.