The present investigation was aimed at establishing the distribution of neptunium in blood and liver cells as a function of the mass and chemical form of the radionuclide injected. Four groups of rats received intravenous injections of <tex-math>${}^{237}{\rm Np}({\rm V}),{}^{237}{\rm Np}({\rm IV}),{}^{239}{\rm Np}({\rm V})$</tex-math> or <tex-math>${}^{239}{\rm Np}({\rm IV})$</tex-math>. Twenty-four hours after injection of the radionuclide, subcellular structures of the liver were separated by ultracentrifugation and serum and liver cytosol were subjected to gel permeation chromatography. The intracellular distribution of neptunium in liver depends on the mass of the radionuclide injected; the relative specific activity for <tex-math>${}^{237}{\rm Np}$</tex-math> compared to <tex-math>${}^{239}{\rm Np}$</tex-math> was 2 in nuclei and 0.5-0.9 in cytosol. By contrast, the initial chemical form of the radionuclide has no significant effect on its intracellular distribution. In cytosol, neptunium was bound mainly by two proteins of molecular weight 450 and 200 kDa, respectively. The former was identified as ferritin, but the latter remains unidentified. In this compartment, no effect of mass or chemical form was seen. In blood, the bulk of the radionuclide was bound to transferrin whatever the mass and initial chemical form injected.

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