The binding of plutonium by a sialoprotein isolated from bovine cortical bone and by transferrin has been compared using a gel filtration procedure. The pH at which maximum binding of plutonium to bone sialoprotein was observed, pH 6, was different from that for transferrin, pH 7, and the magnitude of the binding was greater for bone sialoprotein. The effects of added iron and bicarbonate ions, and of removal of sialic acid on the binding of plutonium to these two glycoproteins, were also dissimilar. These results show that plutonium binds to the carboxyl groups of bone sialoprotein, and this may be responsible for the high stability of the plutoniumbone sialoprotein complex. For the plutonium-transferrin complex, the results obtained suggest that the plutonium is bound at the iron-binding sites, tyrosyl groups, and probably at other sites in the transferrin molecule.

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