Inositol 1,4,5-trisphosphate 3-kinase activity in frog skeletal muscle

Frog skeletal muscle contains a kinase activity that phosphorylates inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate. The inositol 1,4,5-trisphosphate 3-kinase activity was mainly recovered in the soluble fraction, where it presented a marked dependency on free calcium concentration in the physiological range in the presence of endogenous calmodulin. At pCa 5, where the activity was highest, the soluble 3-kinase activity displayed a Km for inositol 1,4,5-trisphosphate of 1.6 microM and a Vmax value of 25.1 pmol mg-1 min-1. The removal rates of inositol 1,4,5-trisphosphate by 3-kinase and 5-phosphatase activities of the total homogenate under physiological ionic conditions were very similar, suggesting that both routes are equally important in metabolizing inositol 1,4,5-trisphosphate in frog skeletal muscle.