Role of Kidney tissue in the Production of 25-Hydroxyvitamin D3-26,23-lactone and 1α ,25-Dihydroxyvitamin D3-26,23-lactone

Labeled 25-hydroxyvitamin D3-26,23-lactone was isolated from the serum of vitamin D-repleted rats given [3α -3H]-25-hydroxyvitamin D324 hr prior to sacrifice. The metabolite was identified by cochromatography with the authentic lactone on straight-phase and reversed-phase high-performance liquid chromatography. Production of the lactone was abolished by nephrectomy indicating that the kidney is the site of synthesis. Homogenate of kidneys from chickens given large doses of vitamin D can carry out in vitro production of the lactone from 25-hydroxyvitamin D3. When 1α ,25-dihydroxyvitamin D3was used as substrate, this system produced only traces of a compound believed to be 1α ,25-dihydroxyvitamin D3-26,23-lactone. However, incubation of rachitic chicken kidney homogenates with 25-hydroxyvitamin D3-26,23-lactone produced substantial amounts of a compound that has been identified by mass spectrometry as 1α ,25-dihydroxyvitamin D3-26,23-lactone. Thus, the development of a functional group on C-26 and eventual lactone formation takes place in kidney by a system acting on 25-hydroxyvitamin D3.