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  • Title: 24-Oxo and 26,23-lactone metabolites of 1,25-dihydroxyvitamin D3 have direct bone-resorbing activity.
    Author: Stern PH, Rappaport MS, Mayer E, Norman AW.
    Journal: Arch Biochem Biophys; 1984 May 01; 230(2):424-9. PubMed ID: 6546849.
    Abstract:
    The biological activities of several 24-oxo and 26,23-lactone metabolites of vitamin D were determined in bone organ cultures. The 24-oxo metabolites were significantly more potent bone-resorbing agents than the lactones. 1,25-(OH)2-24-oxo-D3 had 0.18 X the bone-resorbing activity of 1,25-(OH)2D3 in fetal rat limb bones and was equipotent with 1,25-(OH)2D3 in neonatal mouse calvaria. In the limb bone system, 1,23,25-(OH)3-24-oxo-D3 had 0.08 X the activity of 1,25-(OH)2D3. 1,25-(OH)2D3 and 1,25-(OH)2-24-oxo-D3 had a similar time course of bone-resorbing effects in both bone culture systems. The most potent of the lactones, 1,25S-(OH)2D3-26,23R-lactone, had approximately 0.009 X the activity of 1,25-(OH)2D3 and approximately 500 times the activity of the 25S-OH-D3-26,23R-lactone. The 25S and 1,25S lactones were more potent than the 25R and 1,25R isomers. In experiments designed to determine whether either 1,25-(OH)2-24-oxo-D3 or 25R-OH-D3-26,23S-lactone could prevent the bone-resorbing activity of 1,25-(OH)2D3, no inhibitory effects were observed. The results suggest that conversion to the lactones represents a substantial inactivation step, whereas conversion to 24-oxo-derivatives results in less reduction in biological activity.
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