Inositol 1,4,5-trisphosphate (Ins-1,4,5-P3), a Ca2+-mobilizing messenger, can be phosphorylated by a cytoplasmic kinase, yielding inositol 1,3,4,5-tetrakisphosphate (Ins-1,3,4,5-P3). We observed that stimulation of the antigen receptor on a malignant human T cell line, Jurkat, led to substantial, sustained increases in Ins-1,4,5-P3 and InsP4. The Ins-1,4,5-P3 kinase partially purified from resting Jurkat cells had a maximum velocity (Vmax) of 0.09 nmol/min/mg protein and an apparent Michaelis constant (Km) of 0.2 microM. When the kinase was partially purified 10 min after stimulation of the antigen receptor or after the addition of phorbol myristate acetate, the Vmax was increased twofold. The activity of the Ins-1,4,5-P3 kinase obtained from either resting or stimulated Jurkat cells was enhanced in vitro by increasing the concentration of free Ca2+ from 0.1 to 0.5 microM. These results indicate that the activity of the Ins-1,4,5-P3 kinase is regulated as a consequence of stimulating the T cell antigen receptor.
J B Imboden, G Pattison