To determine the nature of binding of transcobalamin II (TC-II) to liver cells, we covalently coupled purified holo-TC-II to submicron latex minibeads using glutaraldehyde. Incubation of the probe with liver cell suspensions at 4 degrees C led to its binding by endothelial cells but not by hepatocytes or Kupffer cells, as visualized by scanning electron microscopy. At 37 degrees C, the probe was internalized by the endothelium through a system of coated pits and vesicles as shown by transmission electron microscopy. Inhibition studies by pre-incubation with excess native TC-II demonstrated the specificity of binding. Fractionation of these cell suspensions on metrizamide gradients yielded large cell (hepatocyte-rich) and small cell (endothelium-rich) fractions. The binding of the minibead probe occurred again exclusively on endothelial cells in the small cell fraction. 125I-labeled holo-TC-II also bound to the small cell but not to the large cell fraction. Binding was saturable (Ka, 0.225 X 10(9) mol/L-1) and receptor number was calculated to be 1.33 X 10(3) per cell. Time-dependent incubation of 125I-labeled TC-II with the endothelium-rich fraction led to its uptake, reaching a steady-state plateau at 4 degrees C. At 37 degrees C, however, the initial uptake was followed by gradual release of the label into the medium. We conclude that in the liver, holo-TC-II binds initially to endothelium, where it is internalized and is subsequently released probably to the interstitial space. Thus, the endothelium may play a fundamental role in the regulation of the uptake of TC-II by the liver.

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