Blood, actinide elements

The observations described above have important implications with regard to actinide/lanthanide distributions within the body. It is apparent that in binding to transferrin, the f elements are participating in certain aspects of the iron transport pathways in vivo. However, no plutonium, e.g., is found within red blood cells following incorporation and there is no unequivocal evidence that plutonium and the other actinides or lanthanides are transported into cells via transferrin-receptor-mediated endocytosis (Duffield and Taylor 1986). This, too, is a puzzling aspect of f-element-transferrin chemistry and biochemistry which needs more study. 

The physiological result of the binding of the non-essential lanthanide and actinide metals to albumin, or transferrin, in the blood plasma may well be that the metals are held in a form in which they are virtually unable to penetrate the cell membrane in ionic form thus limiting cellular uptake and also the ability to cause harmful effects by interaction with essential enzymes or other proteins. Thus for the non-essential f elements, protein binding may be regarded as part of a protective mechanism against chemical toxicity,... 

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