Calcium Binding to Phosphoprotein in Estrogenized Rooster Plasma Studied by Density-Gradient Centrifugation ∗

Summary

Density gradient ultracentrifugation of plasma from estrogenized roosters injected with ⁴⁵Ca or ³²P shows that most of the calcium is complexed to a phosphoprotein of uniform ³²P specific activity and on a molar calcium to phosphorus basis very near 1:1. The bound calcium is also completely exchangeable. Since no phosphoprotein other than phosvitin has been identified in the plasma of laying hens or estrogen treated birds, it is suggested that the multiple serine-phosphates of this unique protein complex calcium directly and are primarily responsible for both the magnitude and the nonfiltrable nature of estrogen induced hypercalcemia.