The metabolism of inositol-1,4,5-trisphosphate was studied in the taste organ (barbel) of the channel catfish, Ictalurus punctatus. 2. Homogenates of epithelial barbel scrapings were incubated with [3H]-1,4,5-IP3, whose dephosphorylation or phosphorylation was assayed under first-order conditions by measuring the production of either [3H]-1,4-IP2 (representing the activity of IP3-5-phosphatase) or [3H]-1,3,4,5-IP4 (representing the activity of IP3-3-kinase). 3. Both enzymes were predominantly cytosolic, magnesium-dependent and maximally active at pH 6.4. For IP3-phosphatase, Km = 6 microM and Vmax = 10.5 nmol/min/mg. For IP3-kinase, Km = 0.23 microM and Vmax = 0.05 nmol/min/mg. 4. Neither enzyme was significantly affected by the presence of taste stimuli (amino acids), GTP gamma S, cAMP or phorbol esters. 5. In the presence of physiological levels of free calcium (0.05-12 microM) IP3-phosphatase was moderately activated whereas IP3-kinase was moderately inhibited. 6. IP3-phosphatase was moderately activated by Mn2+, unaffected by LiCl, and strongly inhibited by 2,3-diphosphoglycerate, Na-pyrophosphate, CdCl2, HgCl2, CuCl2, FeCl3 and ZnSO4 7. IP3-kinase was strongly activated by 2,3-diphosphoglycerate, Na-pyrophosphate, CdCl2, HgCl2, FeCl3 and LiCl and inhibited by ZnSO4 and Mn2+. 8. IP3-kinase was significantly activated in a calcium-dependent manner by exogenously-added phosphatidylcholine and sphingomyelin, and to a lesser extent by diacylglycerol. IP3-phosphatase was unaffected by exogenously-added lipids. 9. IP3-phosphatase may participate in taste transduction since calculations based on the first-order rate constant (6.9 sec-1) indicate that it is capable of dephosphorylating basal levels of IP3 with a half-life of 0.1 sec.
