Metabolism of inositol 1,4,5-trisphosphate to higher inositol phosphates in bovine adrenal cytosol.

The metabolism of inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate was studied in a cytosolic fraction prepared from the bovine adrenal cortex. The activity of the partially purified inositol 1,4,5-trisphosphate 3-kinase was dependent on Ca2+/calmodulin, Mg2+, and pH, and was inhibited by 2,3-bisphosphoglycerate. The enzyme exhibited Michaelis-Menten behavior toward its two substrates, inositol 1,4,5-trisphosphate and ATP, with Km values of 0.42 mumol/L and 0.4 mmol/L, respectively. The presence of other inositol-phosphate metabolizing enzymes in the cytosolic fraction was indicated by the appearance of additional inositol polyphosphates during prolonged incubation with inositol 1,4,5-trisphosphate. These included inositol 1,3,4-trisphosphate, inositol 1,3,4,6-tetrakisphosphate, and inositol pentakisphosphate. These findings are consistent with the rapid phosphorylation of inositol 1,4,5-trisphosphate to the 1,3,4,5-tetrakisphosphate by the calcium/calmodulin-dependent 3-kinase, and its subsequent conversion to inositol 1,3,4-trisphosphate and thence to inositol 1,3,4,6-tetrakisphosphate in angiotensin-stimulated bovine glomerulosa cells. The formation of inositol pentakisphosphate during prolonged incubations suggests that inositol 1,3,4,6-tetrakisphosphate is slowly phosphorylated and serves as a source of inositol pentakisphosphate in the adrenal. The metabolic conversion of inositol 1,4,5-trisphosphate to several higher inositol polyphosphates provides potential new messengers for intracellular regulation in agonist-stimulated target cells.