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

The metabolism of exogenously added d-myo-[1-(3)H]inositol 1,4,5-trisphosphate (IP(3)) has been examined in microsomal membrane and soluble fractions of carrot (Daucus carota L.) cells grown in suspension culture. When [(3)H]IP(3) was added to a microsomal membrane fraction, [(3)H]IP(2) was the primary metabolite consisting of approximately 83% of the total recovered [(3)H] by paper electrophoresis. [(3)H]IP was only 6% of the [(3)H] recovered, and 10% of the [(3)H]IP(3) was not further metabolized. In contrast, when [(3)H]IP(3) was added to the soluble fraction, approximately equal amounts of [(3)H]IP(2) and [(3)H]IP were recovered. Ca(2+) (100 micromolar) tended to enhance IP(3) dephosphorylation but inhibited the IP(2) dephosphorylation in the soluble fraction by about 20%. MoO(4) (2-) (1 millimolar) inhibited the dephosphorylation of IP(3) by the microsomal fraction and the dephosphorylation of IP(2) by the soluble fraction. MoO(4) (2-), however, did not inhibit the dephosphorylation of IP(3) by the soluble fraction. Li(+) (10 and 50 millimolar) had no effect on IP(3) metabolism in either the soluble or membrane fraction; however, Li(+) (50 millimolar) inhibited IP(2) dephosphorylation in the soluble fraction about 25%.