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The metabolism of dihydrotachysterols: Renal side chain and non-renal nuclear hydroxylations in vivo and in vitro
Authors:F Qaw  NJ Schroeder  MJ Calverley  DJH Trafford  HLJ Makin  G Jones
Institution:

1 Department of Biochemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6

2 Department of Chemical Pathology, The London Hospital Medical College, London E1 2AD, England

c Chemical Research Department, Leo Pharmaceutical Products, 2750, Ballerup, Denmark

Abstract:The metabolism of dihydrotachysterol (DHT), a hydrogenated analogue of vitamin D, has been studied in vivo using man and rat and in vitro using the perfused rat kidney, and hepatoma (3B) and osteosarcoma (UMR-106) cell lines. In vivo a large number of metabolites appeared in the plasma of rats given DHT2 and DHT3. Of particular interest was a compound more polar than 25-hydroxy-DHT, which has been designated compound H. Further study of this compound showed that it was composed of two components, one (Ha) being in much lower concentration than the other (Hb). The production of T2/H (peak H from DHT2) was demonstrated in human plasma after administration of oral DHT2. Comparison of the metabolites formed in vivo with those isolated from the rat kidney perfused with 25-hydroxy-DHT3 in vitro showed that 25-hydroxy-DHT3 was metabolized along two metabolic pathways previously described for vitamin D, culminating in the production of 25-hydroxy-DHT3-23,26-lactone and 23,25-dihydroxy-24-oxo-DHT3. The osteosarcoma cell line metabolized 25-OH-DHT3 in vitro along the same two metabolic pathways already demonstrated in the perfused rat kidney. More polar metabolites than compound H seen in rat plasma in vivo were shown to be metabolites of compound H and similar metabolites were also produced in the osteosarcoma cell line from chemically synthesized 1greek small letter alpha,25-dihydroxy-DHT3. The hepatoma cell line 25-hydroxylated DHT and no feed-back inhibition was observed. Use of the hepatoma cell to 25-hydroxylate a number of chemically synthesized 1-hydroxy-DHTs indicated that compound Ha was indistinguishable from 1greek small letter alpha,25-dihydroxy-DHT whereas compound Hb is possibly 1β,25-dihydroxy-DHT. Studies with the VDR in both chick gut and calf thymus indicated that 1greek small letter alpha,25-dihydroxy-DHT is very effective in displacing radiolabelled 1greek small letter alpha,25-dihydroxyvitamin-D3 and is thus most likely to be the calcaemic metabolite of DHT.
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