An in vitro study of vitamin D2 hydroxylases in the chick.

An in vitro study of the liver 25-hydroxylation of vitamin D2 and the kidney 1- and 24-hydroxylations of 25-hydroxyvitamin D2 was undertaken in order to determine whether the discrimination against vitamin D2 seen in chicks in vivo is the result of a block of one or more of the steps in the activation of the vitamin D2 molecule. Vitamin D2 hydroxylation reactions in the chick are virtually identical with those observed with the vitamin D3 series. It is, therefore, concluded that the chick possesses the required enzymatic machinery to synthesize 1,25-dihydroxyvitamin D2 and that the discrimination must be because of some unknown metabolic reaction of the vitamin D2 compounds, to a defect in the transport of vitamin D2 metabolites, or to target organ discrimination.