Ca2+ and calmodulin-sensitive inositol trisphosphate kinase from bovine parathyroid

A Ca2+ and calmodulin-activated inositol 1,4,5 trisphosphate kinase activity was detected in both soluble and membrane fractions from bovine parathyroid glands. Ca2+ activated the soluble enzyme in the concentration range 100 nM to 1 microM, which corresponds to the Ca2+ concentration range observed in the intact cell following maximal variation in extracellular Ca2+, the principal regulator of parathyroid hormone release. The Ca2+ sensitivity of the enzyme was absolutely dependent upon calmodulin. A similar activity was detected in the membranes but could be progressively removed by repeated washing at low ionic strength. This, together with data demonstrating binding of the enzyme to the hydrophobic matrix, Phenyl-Sepharose, suggests that the association of the enzyme with the membrane is likely to involve a significant hydrophobic component. The organic base, amiloride was identified as an inhibitor of the activity, the degree of inhibition being most marked in the presence of Ca2+ and calmodulin (K0.5 approx. 0.1 mM). The Ca2+ concentration dependence of the IP3 kinase suggests that inositol 1,3,4,5 tetrakisphosphate may be a messenger in the signal transduction pathway for the feedback inhibition of PTH secretion by extracellular Ca2+.