Human serum transferrin cobalt complex: stability and cellular uptake of cobalt

Transferrin (Tf) receptor expression is up-regulated on tumour cells. The human serum iron transport protein transferrin (Tf) can bind to many metals including gallium and cobalt. Cobalt has a positron-emitting isotope with a half-life of 18 h and would thus be a useful isotope for imaging purposes. This study has examined the stability of the Co-Tf in the presence of serum and albumin and the uptake of radioactive Co from Co-Tf by tumour cells. Dialysis of 57Co-Tf with serum or with apo-Tf resulted in loss of most 57Co from the complex. The time course of Co uptake from cells incubated with Co-Tf showed an initial rapid association with cells, then a slower rate of accumulation, that is, a similar uptake profile to that of iron. Competition and displacement experiments showed that uptake specifically occurred by interaction with Tf receptors.