Effects of pH on the stability of the indium-113m blood protein complex and the selective binding of indium-113m to transferrin

Summary

Indium-113m (t _1/2 = 100min; gamma-emission of 393keV) in trace amounts was injected i.v. in rats. Blood was collected by heart puncture 15 min after the injection, and blood plasma was separated by centrifugation. Gel filtration of plasma on Sephadex G-25M equilibrated with glycine/HCl (pH 2.2–3.6), NaHCO₃/CO₂ (pH 4.0–11.0) glycine/NaOH (pH 8.6–10.6) or sodium acetate/acetic acid (pH 3.0–5.0) was used to separate free indium from indium bound to macromolecular proteins. Determination of radioactivity in eluted fractions showed that more than 85% of the plasma indium was bound to macromolecules at pH values between 5.0 and pH 10.6. However, dissociation of the indium plasma protein complexes occurred at pH values below 5.5, and more than 90% of the indium radioactivity was found in the low molecular weight fraction at pH 2.2.

Affinity chromatography using immobilized antibodies to rat transferrin was used to isolate transferrin at pH 7.4 and 5.5. Immunodiffusion and electrophoresis were used to identify the proteins in fractions obtained by affinity chromatography. It was found that the indium-113m activity was correlated with the content of transferrin and that 80%–90% of this activity was found in fractions that had affinity to antitransferrin. These fractions contained transferrin exclusively at pH 7.4, but additional protein fractions of albumin and alpha₁-globulin mobility at pH 5.5.

At pH 7.4 and 5.5, 10%–20% of the indium activity was detected in molecular fractions that had no affinity to antitransferrin. Immunologic analyses showed that these fractions contained transferrin. Why this transferrin did not bind to the antitransferrin remains unclear.

In conclusion, In-113m can be used as an indicator of plasma proteins between pH 5.0 and 10.6.