Abstract
1. 1. Uptake of 3H-25OHD 3, 3H-25OHD 3-DBP, 125I-holo-DBP and 125I-apo-DBP by Llc-Pk 1 cells was linearly related to the concentration of each in the culture media. The presence of DBP in the medium significantly reduced the amount of 3H-25OHD 3 taken up by cells. 2. 2. Free 25OHD 3 and 25OHD 3 bound to DBP were both metabolized by the cells to 24,25(OH) 2D 3 and an unidentified product of apparent lower polarity than 25OHD 3. 3. 3. A significant amount of DBP taken up by the Llc-Pk 1 cells was metabolized to a TCA-soluble form. 4. 4. Uptake of DBP was similar to horseradish peroxidase, but higher than inulin, indicative of a non-specific endocytic mechanism with an adsorptive component. 5. 5. It is suggested that both free circulating 25OHD 3 and that derived from lysosomal degradation of 25OHD 3-DBP are available for hydroxylation by the kidney.
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