The existence of 25-hydroxyvitamin D3-1 alpha-hydroxylase in the liver of carp and bastard halibut.

We have found that carp and bastard halibut contain 25-hydroxyvitamin D3 (25-D3)-1 alpha-hydroxylase in the liver besides in the kidney by the following in vivo and in vitro experiments. When [3H]-25-D3 was intraperitoneally injected to vitamin D(D)-deficient carp and normal bastard halibut (D-deficient bastard halibut could not be raised because they died during farming), the profiles of high-performance liquid chromatography (HPLC) of the plasma lipid extract showed the formation of a peak corresponding to [3H]-1 alpha,25-dihydroxyvitamin D3 (1,25-D3). When [3H]-25-D3 was incubated with liver homogenates of the fish, a peak corresponding to [3H]-1,25-D3 was also observed in the profile of HPLC. The formation of the metabolite was confirmed by the thermal isomerization into the pre-isomer and mass fragmentography. Although the 1 alpha-hydroxylase was also observed in the kidney, the activity of the enzyme was lower than that in the liver. The results suggest that 25-D3-1 alpha-hydroxylase exists in the liver of carp and bastard halibut and the 25-D3 formed from D3 in the liver is immediately metabolized into 1,25-D3 in the same tissue. The suggestion is supported by the fact that D3 is a major circulating compound with small amounts of 1,25-D3 in the fish while the plasma levels of 25-D3 are under the limit of detection.