INOSITOL 1,4,5-TRISPHOSPHATE TURNOVER ENZYMES - ACTIVITIES AND SUBCELLULAR-DISTRIBUTION IN HEPATOCARCINOGENESIS

The metabolism of inositol 1,3,5-trisphosphate and inositol 1,3,4,5-te trakisphosphate in homogenates and sub-fractions from normal rat liver and premalignant liver nodules was investigated. The activities of 5- phosphatase, expressed as pmol converted substrate per minute and mg p rotein, were equal when using the two substrates, and did not differ b etween normal and nodular homogenates. Subcellular fractions were puri fied by sequential steps of differential centrifugation and density gr adient fractionation procedures. The total phosphatase activity was fo und to be distributed between cytosol (15%) and membraneous fractions (75%), with most of the enzyme activity residing in the plasma membran es. A doubling of phosphatase specific activity was seen in the nodula r low density membrane fraction, containing Golgi apparatus and endoso mes, as compared with normal liver. Inositol 1,4,5-trisphosphate 3-kin ase activity was found to be exclusively cytosolic. No difference in t his enzyme was seen between the two tissue types studied. Vasopressin (0.2 or 2 mu M) had no effect either on phosphatase or kinase activity . The compartmentalization of inositol polyphosphate 5-phosphatase act ivity presents a possible explanation of earlier findings that premali gnant liver tissue was able to respond with inositol 1,4,5-trisphospha te, but not inositol 1,3,4,5-tetrakisphosphate formation after agonist stimulation.