Role of Inositol Polyphosphates in Ca^<2+> Homeostasis

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A series of inositol 1.4.5-trisphosphate (IP_3) analogs, with a bulky substituent on the 2nd carbon of the inositol ring, has been synthesized and examined to explore the structure-activity relationships among IP_3-5-phosphatase. IP_3-3-kinase, and IP_3 binding protein. All analogs (racemic mixtures) inhibited the hydrolysis of [^3H] IP_3 catalyzed by erythrocyte ghosts. With a lower Ki value than seen with IP_3. The effective enantiomer in this process was found to be L-type. The analogs were capable of inhibiting the phosphorylation of [^3H] IP_3 to [^3H] IP_4 by brain cytosol. Although higher concentrations than IP_3 were required. By lowering free Ca^<2+>, the concentrations required for the inhibition became low. In this reaction. the D-enantiomer was effective. The D-enantiomer of these compoundsalso inhibited the binding of [^3H] IP_3 to cerebellum microsomes, With the same potency as seen with IP_3. And they acted as a full agonists in releasing Ca^<2+> from permeabilized macrophages. These results indicate that the 2nd position of IP_3 can be modified with only minor reduction in potency in several assay systems. Thus, the IP_3 analogs synthesized here may be linked to other molecules without loss of their biological activities. For example, the IP_3 analogs were coupled with Sepharose 4B to make IP_3 affinity columns, and the columns were proved to be useful for purifying the IP_3-recognizing proteins.