METABOLISM OF INOSITOL 1,4,5-TRISPHOSPHATE AND INOSITOL 1,3,4,5-TETRAKISPHOSPHATE BY THE OOCYTES OF XENOPUS-LAEVIS

The pathway and kinetics of inositol 4,4,5-trisphosphate (IP3) metabol ism were measured in Xenopus laevis oocytes and cytoplasmic extracts o f oocytes, Degradation of microinjected IP,in intact oocytes was simil ar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3 ] and the intracellular free Ca2+ concentration ([Ca2+]), At low [IP3] (100 nM) and high [Ca2+] (greater than or equal to 1 mu M), IP3 was m etabolized predominantly by inositol 1,4,5-trisphosphate 3-kinase (3-k inase) with a half-life of 60 s, As the [IP](3) was increased, inosito l polyphosphate 5-phosphatase (5-phosphatase) degraded progressively m ore IP3. At a [IP3] of 8 mu M or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At l ow [IP3] and low [Ca2+] (both less than or equal to 400 nM), the activ ities of the 5-phosphatase and 5-kinase were comparable. The calculate d range of action of IP3 in the oocyte was similar to 300 mu M suggest ing that IP3 acts as a global messenger in oocytes. in contrast to IP3 inositol 1,3,4,5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 20 min and independent of the [Ca2]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was are order of magnitude or greater than that in small mammalian cells.