Subcellular localization and some properties of the enzymes hydrolysing inositol polyphosphates in rat liver

The hydrolysis of inositol [32P]trisphosphate (IP3) and inositol [32P]bisphosphate (IP2) has been examined in subcellular fractions of rat liver. IP3 was degraded by an enzyme located in the plasma membrane which did not degrade IP2. Cytosolic fractions were found to degrade both IP2 and IP3. The IP3 phosphatase activity of liver plasma membranes displayed a neutral pH optimum, was Mg2+ dependent and was not inhibited by high concentrations of Li+. Half-maximal activity of the enzymes hydrolysing IP3 and IP2 was obtained with substrate concentrations in the range 1-2 microM. The significance of these observations to the proposed Ca2+-mobilizing role of IP3 is discussed.