When human blood serum that had been tagged in vitro with monomeric PuIV was subjected to gel filtration, most of the PuIV was found in two molecular-weight ranges, that of the low-molecular-weight serum proteins and that of small molecules and ions. The distribution of PuIV between the two peaks was variable. In addition, a small amount was eluted in the highest-molecular-weight fraction. By a combination of gel filtration, ion exchange, and electrophoresis the PuIV protein complex was isolated, and the protein was identified as transferrin, the protein that transports iron. PuIV appeared to be bound at the iron-binding sites, and the reaction between PuIV and transferrin was shown to be reversible. The stability constant of the complex is high but less than that of the very stable FeIII transferrin complex. The variation in distribution of PuIV between transferrin and the low-molecular-weight region thus is related to the variation in the amount of iron bound by the transferrin.

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