Metabolism of Inositol 1,4,5-Trisphosphate and Inositol 1,3,4,5-Tetrakisphosphate by the Oocytes of Xenopus laevis*
- From the Department of Physiology and Biophysics, University of California, Irvine, California 92697-4560
Abstract
The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured inXenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar 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+] (≥1 μm), IP3was metabolized predominantly by inositol 1,4,5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 μm or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both ≤400 nm), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was ∼300 μm suggesting 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 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 inXenopus oocytes was an order of magnitude or greater than that in small mammalian cells.
Footnotes
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↵* This work was supported by the Arnold and Mabel Beckman Foundation, the Searle Scholars Program, and National Institute of Mental Health Grant MH45324.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed. Tel.: 714-824-6493; Fax: 714-824-8540.
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↵1 The abbreviations used are: [Ca2+], free Ca2+ concentration; I-1-P1, inositol 1-phosphate; IP1, all isomers of inositol monophosphate; I-1,4-P2, inositol 1,4-bisphosphate; IP2, all isomers of inositol bisphosphate; IP3, inositol 1,4,5-trisphosphate; I-1,3,4-P3, inositol 1,3,4-trisphosphate; IP4, inositol 1,3,4,5-tetrakisphosphate; [IP3], concentration of IP3; [IP4], concentration of IP4; IP≤2, inositol, IP1, and IP2; [IP≤2], the sum of the concentration of inositol, IP1 and IP2; 5-phosphatase, inositol polyphosphate 5-phosphatase; 3-kinase, inositol 1,4,5-trisphosphate 3-kinase; HPLC, high pressure liquid chromatography; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid.
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- Received August 6, 1997.
- Revision received October 30, 1997.
- The American Society for Biochemistry and Molecular Biology, Inc.
