Résumé : Inositol 1,4,5-trisphosphate (InsP3) 3-kinase catalyses the ATP-dependent phosphorylation of InsP3 to inositol 1,3,4,5-tetrakisphosphate (InsP4). A method is presented for the rapid purification of InsP3 3-kinase from bovine brain by calmodulin (CaM)-Sepharose affinity chromatography. Maximal activation of the purified InsP3 3-kinase by Ca2+/CaM was 6-7-fold as compared with the activity measured in the presence of EGTA (1 mM) and 10 microM-InsP3. At 10 microM-InsP3 and 0.1 mM free Ca2+, half-maximal activation required about 2 nM-CaM. The mechanism of activation by CaM appeared to be an increase in the maximal velocity of the enzyme without a substantial change in the Km for InsP3. Further purification was achieved by phosphocellulose chromatography eluted with ATP. Specific activity of the purified enzyme at 37 degrees C and 10 microM-InsP3 was 10-20 mumol/min per mg. The apparent Mr of the enzyme, determined by f.p.l.c.-gel filtration, was estimated as about 44,000. The purified InsP3 3-kinase was subjected to SDS/10%-polyacrylamide-gel electrophoresis. InsP3 3-kinase activity was associated with three silver-stained bands, which migrated with apparent Mr values of approx. 52,000, 38,000 and 35,000.