Purification and characterization of inositol 1,4,5-trisphosphate 3-kinase from pig aortic smooth muscle

Inositol 1,4,5-trisphosphate (InsP3) 3-kinase, which phosphorylates InsP3 to form inositol 1,3,4,5-tetrakisphosphate, was purified to apparent homogeneity by (NH4)2SO4 fractionation and sequential chromatographic steps on DEAE-sepharose, calmodulin-Affi-Gel and DEAE-5PW h.p.l.c. The purified enzyme had a specific activity of 24.4 nmol of inositol tetrakisphosphate formed/min per mg of protein, which represented a purification of approx. 195-fold with a 0.29% recovery, compared with the cytosol fraction of the muscle. SDS/polyacrylamide-gel electrophoresis showed a single protein-staining band of Mr 93,000. Moreover, the major protein peak, of Mr 84,000, was detected by TSK gel G3000SW gel-permeation chromatography of the purified sample. As this value was approximately consistent with the Mr determined by SDS/polyacrylamide-gel-electrophoretic analysis, the InsP3 3-kinase might be a monomeric enzyme. The purified enzyme had a Km for InsP3 of 0.4 microM, with an optimum pH range of 5.8-7.7. The enzyme was maximally activated by calmodulin, with a stoichiometry of 1:1.