Abstract
Comparative studies on the possible origin of extremely high contents of vitamin D_3 in some kinds of fish liver were undertaken. When bastared halibuts and carps were farmed from fingerlings to adults with feedstuffs containing vitamin D_2 or D_3, significant amounts of the vitamins were accumulated in the fish liver. Significant amounts of vitamin D_2 and D_3 in phytoplankton and vitamin D_3 in zooplankton were detected. Therefore, we concluded that the most probable origin of vitamin D_3 in fish liver is a result of food chains from plankton. Vitamin D metabolism in fish was investigated. When [^3H]-25-OH-D_3 was incubated with liver homogenates of D-deficient carp, a peak corresponding to [^3H]-1,25 (OH)_2D_3 was observed in the profiles of HPLC. Although 25-OH-D_3-1α-hydroxylase was also observed in the kidney, the activity of the enzyme was lower than that in the liver. Therefore, we concluded that fish contain 25-OH-D_3-1α-hydroxylase in the liver besides in the kidney. Comparative studies on vitamin D metabolism in vertebrates were undertaken. The formation of 1,25 (OH)_2D_3 was detected in the liver of fish and amphibians but not reptiles and mammals, while 1,25(OH)_2D_3 was formed in all of the kidney. The existence of vitamin D-binding protein (DBP) specifically bound to 25-OH-D_3 was not found in the plasma of fish and amphibians while it was in those of reptiles and mammals. These results suggest that in development of a special transporting system of 25-OH-D_3 using plasma DBP, the main 25-OH-D_3 -1α-hydroxylase activity changes from the liver to the kidney. We could confirm the existence of 25-OH-D_3-1α-hydroxylase in the liver of fetal rats in addition to that in the kidney. This indicates "ontogeny is a recapitulation of phylogeny" with respect to vitamin D metabolism.
