Inhibition of inositol 1,4,5-trisphosphate 3-kinase by heparin: basal and Ca2+/calmodulin-stimulated activity.

Inositol 1 ,4,5-trisphosphate (InsP3) 3-kinase is responsible for the synthesis of InsP4. The kinase in many tissues is regulated by Ca2+/calmodulin (CaM; see [1] for review). The activity of purified enzyme from bovine brain is stimulated up to 30-fold by Ca2+/CaM. Stimulation by CaM appears to result from an increase in enzyme maximal velocity without a substantial change of Km for InsP3 [2]. Similar changes in kinetics have been reported for another well-characterized CaM-regulated enzyme, the cyclic nucleotide CaM-sensitive phosphodiesterase [3]. Increases of activity of that enzyme are also 20-40-fold when assayed in the presence of Ca2` and saturated CaM