Inositol Trisphosphate Metabolism in Carrot (Daucus carota L.) Cells

The metabolism of exogenously added d-myo-1-³H]inositol 1,4,5-trisphosphate (IP₃) has been examined in microsomal membrane and soluble fractions of carrot (Daucus carota L.) cells grown in suspension culture. When ³H]IP₃ was added to a microsomal membrane fraction, ³H]IP₂ was the primary metabolite consisting of approximately 83% of the total recovered ³H] by paper electrophoresis. ³H]IP was only 6% of the ³H] recovered, and 10% of the ³H]IP₃ was not further metabolized. In contrast, when ³H]IP₃ was added to the soluble fraction, approximately equal amounts of ³H]IP₂ and ³H]IP were recovered. Ca²⁺ (100 micromolar) tended to enhance IP₃ dephosphorylation but inhibited the IP₂ dephosphorylation in the soluble fraction by about 20%. MoO₄²⁻ (1 millimolar) inhibited the dephosphorylation of IP₃ by the microsomal fraction and the dephosphorylation of IP₂ by the soluble fraction. MoO₄²⁻, however, did not inhibit the dephosphorylation of IP₃ by the soluble fraction. Li⁺ (10 and 50 millimolar) had no effect on IP₃ metabolism in either the soluble or membrane fraction; however, Li⁺ (50 millimolar) inhibited IP₂ dephosphorylation in the soluble fraction about 25%.