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 K-m for inositol 1,4,5-trisphosphate of 1.6 mu M and a V-max 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.