TERNARY COMPLEX-FORMATION BETWEEN YTTRIUM, PHOSPHATE AND SERUM-PROTEINS

In order to investigate the transport form of yttrium(III) in physiolo gical systems the binding of radioyttrium to human serum proteins in t he presence of different endogenous anions was evaluated using gel fil tration (GF) and ultrafiltration (UF) techniques. Among the most endog enous anions (hydroxide, sulfate, carbonate, citrate, fluoride et.) no influence on the binding of Y-88 labeled Y(III) to human serum albumi n (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 P-32 labeled PO43-. The simultaneous binding of phosp hate and Y(III) to HSA occurred in a stoichiometric ratio of 1:1. A te rnary complex formation between Y(III), phosphate and HSA was postulat ed. Information about the binding mechanism could be obtained by an an alysis of the pH stability of the ternary complex which is stable in t he neutral pH region of 5.5 to 8.5. An estimate of the complex formati on constant of the ternary compound with HSA by competitive experiment s yielded 1g beta = 10.9 (I = 0.15 M; T = 25 degrees C). The same type of binding could be observed with transferrin and polyglutamic acid. Binding to the latter proved that carboxylate groups of the biomacromo lecules are responsible for the simultaneous binding of Y(III) and pho sphate.