Inositol 1,4,5-trisphosphate turnover enzymes--activities and subcellular distribution in hepatocarcinogenesis

The metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in homogenates and sub-fractions from normal rat liverand premalignant liver nodules was investigated. The activities of5-phosphatase, expressed as pmol converted substrate per minute and mgprotein, were equal when using the two substrates, and did not differbetween normal and nodular homogenates. Subcellular fractions were purifiedby sequential steps of differential centrifugation and density gradientfractionation procedures. The total phosphatase activity was found to bedistributed between cytosol (15%) and membraneous fractions (75%), withmost of the enzyme activity residing in the plasma membranes. A doubling ofphosphatase specific activity was seen in the nodular low density membranefraction, containing Golgi apparatus and endosomes, as compared with normalliver. Inositol 1,4,5- trisphosphate 3-kinase activity was found to beexclusively cytosolic. No difference in this enzyme was seen between thetwo tissue types studied. Vasopressin (0.2 or 2 microM) had no effecteither on phosphatase or kinase activity. The compartmentalization ofinositol polyphosphate 5-phosphatase activity presents a possibleexplanation of earlier findings that premalignant liver tissue was able torespond with inositol 1,4,5-trisphosphate, but not inositol 1,3,4,5-tetrakisphosphate formation after agonist stimulation.