The effect of the mass and initial chemical form of neptunium on its molecular associations in blood and liver.

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 237Np(V), 237Np(IV), 239Np(V) or 239Np(IV). 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 237Np compared to 239Np 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.

Identification of transferrin as the principal neptunium-binding protein in the blood serum of rats.

The binding of 239Np(V) to blood serum components of rats was examined in vivo and in vitro. After gel filtration of the serum using a Sephacryl S-300 column, 98% of the applied activity appeared with protein fractions representing coeluted albumins and transferrin. A separation of the albumin- and transferrin-proteins by ion-exchange chromatography using DEAE-cellulose showed the 239Np being entirely bound to the iron-carrier protein transferrin. The high elution yields from the ion-exchange columns, greater than 90%, suggest that the binding may be quite strong. The binding capacity of transferrin for neptunium in vivo was found to decline when the iron level in blood serum was increased. Precipitation experiments showed that 84 +/- 2% of the 239Np was precipitated with 10% (w/v) trichloracetic acid, 77 +/- 3% with 90% ethanol but only 6 +/- 1% with saturated ammonium sulphate at pH 7.4. The available data indicate that as for plutonium, thorium, americium and curium, the iron transport protein, transferrin, may be the main carrier protein for neptunium in mammalian blood serum.

[Mechanism of distribution of neptunium and plutonium in rats]. / K mekhanizmu raspredeleniia neptuniia i plutoniia v organizme krys.

Neptunium administered intravenously together with the high molecular weight protein fraction of blood serum was almost completely accumulated by the liver after 60 min. Then neptunium left the liver and was partially deposited in the skeleton. Plutonium, being a component of a nonprotein and a low molecular weight protein fractions, was mainly found in the skeleton where it had been accumulated throughout the entire period of observation (14 days, 52-58%); as to the nonprotein fraction, the value of accumulation was somewhat higher.