Characterization of inositol 1,3,4‐trisphosphate phosphorylation in rat liver

Abstract

Liver homogenates phosphorylated Ins 1,3,4-P₃ to an InsP₄ isomer that was distinct from Ins 1,3,4,5-P₄. This InsP₄ isomer accumulated in vasopressin stimulated hepatocytes prelabeled with myo-[³H]inositol with a time course that lagged behind Ins 1,3,4-P₃ formation. The Ins 1,3,4-P₃ kinase responsible for its formation was partially purified from rat liver. The enzyme had a K _m for Ins 1,3,4-P₃ of 0.29 μM, a K _m for ATP of 141 μM and was not affected by changes in free Ca²⁺ in the physiological range. The relationship of this new InsP₄ isomer to the inositol phosphate signaling pathway is discussed.