Regulation of vitamin D metabolism: factors influencing the rate of formation of 1,25-dihydroxycholecalciferol by kidney homogenates (Short Communication)

Abstract

The formation of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol by chick kidney homogenates is inhibited by increasing concentrations of Ca²⁺. The apparent K_m for the hydroxylation reaction is 1×10^-7m, significantly lower than that reported for isolated mitochondria. Separated cytoplasmic and particulate fractions are inactive, but on recombination activity is restored, possibly because of the presence in the soluble fraction of a factor with a high affinity for 25-hydroxycholecalciferol.