Metabolism of inositol pentakisphosphate to inositol hexakisphosphate in Xenopus laevis oocytes.

The formation and metabolism of inositol pentakis-and hexakisphosphates (InsP5 and InsP6) were investigated in Xenopus laevis oocytes. After [3H]inositol injection, [3H]InsP5 and subsequently [3H]Insp6 increased progressively over 72 h. In intact oocytes, [3H]InsP5 was progressively converted to [3H]InsP6 from 6 to 72 h of incubation and was not metabolized to lower inositol phosphates. In contrast, [3H]InsP6 remained unmetabolized for up to 72 h. These data are consistent with the kinetics of the increases in [3H]InsP5 and [3H]InsP6 in [3H]inositol-labeled oocytes. The highly phosphorylated inositols showed significant changes during oogenesis and maturation. In oocytes incubated for 48 h after [3H]inositol injection, the radioactive incorporation into polyphosphoinositols increased progressively from stage 3 to stage 6, with 5- and 6-fold rises (cpm/mg protein) for [3H]InsP5 and [3H]InsP6, respectively. These developmental changes were associated with 5-fold increases in [3H]inositol tetrakisphosphate between stages 3 and 6 of oogenesis. Induction of oocyte maturation by progesterone (1 microM) during the last 12 of a 36-h incubation with [3H]inositol doubled the levels of [3H]InsP6 relative to [3H]InsP5, suggesting that the activity of inositol pentakisphosphate kinase increases during maturation. These results provide direct evidence for metabolic conversion of InsP5 to InsP6 in animal cells and show that the higher inositol polyphosphates, unlike the lower phosphoinositols, are extraordinarily stable. These species increase markedly during ovum development and may play a regulatory role in oogenesis and maturation.