[Studies of phosphorylation in rat mast cells (second report)--phosphatidylinositol kinase in rat mast cell granule membranes].

Granules were isolated from sonicated purified rat mast cells on a Percoll gradient. The granules were shown to contain a highly active phosphatidylinositol kinase which catalyzes the formation of diphosphoinositide (DPI) from endogenous phosphatidylinositol in the granule membrane. The enzyme requires ATP and Mg2+ or Mn2+ for activity. DPI formation is almost completely dependent on MgCl2 or MnCl2, and maximal response was observed at 20 mM or 2 mM, respectively. The effects of the divalent cations are competitive. Ca2+, fluoride and cyclic AMP are inhibitory. The Km for ATP is 25 microM. The initial reaction is rapid, but the response ceases within a few min. The maximal response is seen at 28 degrees C.