1,25-Dihydroxyvitamin D synthesis in rat liver microsomes

Previous studies from our laboratory have shown 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] production by rat liver homogenates and a low affinity of the hepatic enzyme for 25-hydroxyvitamin D₃. Because the liver microsomal vitamin D-25-hydroxylase is the main source of systemic 25(OH)D₃, we examined the subcellular location and the kinetics of liver 1,25(OH)₂D₃ production. Unlike the renal 1α-hydroxylase activity which was assayed simultaneously, 1,25(OH)₂D₃ synthesis was undetectable in rat liver mitochondria, whereas in microsomes, 1,25(OH)₂D₃ production followed typical Michaelis Menten kinetics with a Km for 25(OH)D₃ of 13.4 μM and a V̇max of 109.8 pg/min per mg protein accounting for most of the 1,25(OH)₂D₃ synthesized by rat liver cytosol free homogenates. Thus, microsomes are the site for 1,25(OH)₂D₃ synthesis in the rat liver. This microsomal compartmentalization of the two major steps in the activation of vitamin D to 1,25(OH)₂D₃ suggests a role for the liver as an autocrine/paracrine organ for 1,25(OH)₂D₃.