Synthesis of LHydroxy [6-3H]vitamin D, and Its Metabolism to lar,25-Dihydroxy [6-3H]vitamin D, in the Rat*

loc-Hydroxy[6-3H]vitamin D, has been synthesized with a specific activity of 4 Ci/mmol, and its metabolism in rats has been studied. It is rapidly converted to ln,25-dihydroxy[6-3H]vitamin D, in uiuo. Following an intravenous or oral dose, a maximal concentration of loc,25-dihydroxy[6-3H]vitamin D, is found 2 and 4 hours, respectively, before the maximal intestinal calcium transport response is observed. Similarly, la,25-dihydroxy[6-3H]vitamin D, accumulation in bone precedes the bone calcium mobilization response. It appears, therefore, that the biological activity of lcu-hydroxyvitamin D, is largely, if not exclusively, due to its conversion to la,25-dihydroxy[6-3H]vitamin D,. lu-Hydroxy[6-3H]vitamin D, and la,25-dihydroxy [6-3H]vitamin D, appear in intestine equally well after an oral or an intravenous dose of la-hydroxy[6-3H]vitamin D,. However, much less of both la-hydroxy[6-3H]vitamin D, and la,25dihydroxy[6-3H]vitamin D, appears in bone and blood after an oral than after an intravenous dose. A much reduced bone calcium mobilization response is also noted following an oral dose as compared to an intravenous dose of la-hydroxyvitamin D,, suggesting that oral lo-hydroxyvitamin D, is not utilized as well as intravenously administered material.