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 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 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%