The enzymatic formation of 1 alpha,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3 in the liver of fetal rats.

We have confirmed the existence of 25-hydroxyvitamin D3 (25-OH-D3)-1 alpha-hydroxylase in the liver of fetal rats, in addition to that in the kidney, by in vitro experiments. The findings are similar to those in the fish liver as reported previously (Takeuchi et al., Life Sci. 32, 275-282, 1991). When [3H]-25-OH-D3 was incubated with liver homogenates of vitamin D-deficient fetal rats, a peak corresponding to [3H]-1 alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) was observed in the profile of high-performance liquid chromatography (HPLC). The formation of the metabolite was confirmed by thermal isomerization into the pre-isomer, binding affinity to the receptor and mass fragmentography. Lineweaver-Burk plot analysis of mitochondrial 25-OH-D3-1 alpha-hydroxylase in the liver gave an apparent Km value of approximately 2 microM of 25-OH-D3 and a Vmax value of 0.2 pmol of 1,25(OH)2D3/30 min/mg protein. These findings suggest that the enzyme in the liver disappeared with the growth of the fetus and became predominant in the kidney of mature rats.