Ternary complex formation between yttrium, phosphate and serum proteins

In order to investigate the transport form of yttrium(III) in physiological systems the binding of radioyttrium to human serum proteins in the presence of different endogenous anions was evaluated using gel filtration (GF) and ultrafiltration (UF) techniques. Among the most endogenous anions (hydroxide, sulfate, carbonate, citrate, fluoride et.) no influence on the binding of 88Y labeled Y(III) to human serum albumin (HSA) could be observed. The binding of Y(III) to HSA was as low as in pure aqueous isotonic solution (5% GF and 20% UF). The presence of phosphate anions in the protein-metal solution, however, significantly increased the binding of Y(III) to HSA (80% GF and 95% UF). Phosphate in turn was bound to serum proteins only in the presence of Y(III) as determined with 32P labeled PO43-. The simultaneous binding of phosphate and Y(III) to HSA occurred in a stoichiometric ratio of 1:1. A ternary complex formation between Y(III), phosphate and HSA was postulated. Information about the binding mechanism could be obtained by an analysis of the pH stability of the ternary complex which is stable in the neutral pH region of 5.5 to 8.5. An estimate of the complex formation constant of the ternary compound with HSA by competitive experiments yielded lgβ=10.9 (I=0.15 M; T=25 C). The same type of binding could be observed with transferrin and polyglutamic acid. Binding to the latter proved that carboxylate groups of the biomacromolecules are responsible for the simultaneous binding of Y(III) and phosphate. (orig.)