Balla T, Baukal A J, Guillemette G, Catt K J
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, Bethesda, Maryland 20892.
J Biol Chem. 1988 Mar 25;263(9):4083-91.
The production and metabolism of inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) and other inositol polyphosphates was studied in cultured bovine adrenal glomerulosa cells prelabeled for 24 h with [3H]inositol. During stimulation with angiotensin II, Ins-1,4,5-P3 increased to a peak of 15-fold above basal within 10 s, followed by a second phase of continuous increase over the next 30 min. Ins-1,4,5-P3 formed during agonist stimulation was rapidly metabolized by two distinct pathways. The more direct metabolic route was via degradation by sequential dephosphorylations to form inositol 1,4-bisphosphate and inositol 4-phosphate, and ultimately inositol. Lithium ions inhibited both the formation and dephosphorylation of inositol 4-monophosphate, which is a specific product of inositol polyphosphate metabolism. In addition, a cyclical metabolic sequence was initiated by the 3-phosphorylation of Ins-1,4,5-P3 to form inositol 1,3,4,5-tetrakisphosphate. The Ins-1,4,5-P3 3-kinase responsible for this reaction had a Km of 0.4 microM for Ins-1,4,5-P3 and a Vmax of 208 pmol/min/mg and was stimulated by increased Ca2+ concentrations in the micromolar range. Inositol 1,3,4,5-tetrakisphosphate was then dephosphorylated to inositol 1,3,4-trisphosphate, which in turn was either further degraded to inositol 3,4-bisphosphate or rephosphorylated to inositol 1,3,4,6-tetrakisphosphate. Lithium ions also inhibited the production of inositol 3,4-bisphosphate, explaining the large accumulation of inositol 1,3,4-trisphosphate in cells stimulated in the presence of lithium. Prolonged exposure to angiotensin II in the presence of Li+ caused a progressive decline in inositol polyphosphate formation without depletion of the lipid precursor, phosphatidyl-inositol 4,5-bisphosphate, suggesting that an accumulating product of polyphosphoinositide hydrolysis (possibly diacylglycerol) has an inhibitory effect on the phospholipase C-catalyzed breakdown process. These results indicate that, in addition to its breakdown by sequential dephosphorylations through Ins-1,4-P2 and Ins-4-P, Ins-1,4,5-P3 undergoes a complex series of phosphorylations and dephosphorylations to form at least two inositol tetrakisphosphates and their metabolites. These newly defined pathways may provide additional regulatory steps in the mechanism of cell activation by angiotensin II and other Ca2+-mobilizing hormones.
用[³H]肌醇预标记24小时的培养牛肾上腺球状带细胞中，研究了肌醇1,4,5-三磷酸（Ins-1,4,5-P3）和其他肌醇多磷酸的产生与代谢。在血管紧张素II刺激期间，Ins-1,4,5-P3在10秒内增加至基础水平以上15倍的峰值，随后在接下来的30分钟内持续增加。激动剂刺激期间形成的Ins-1,4,5-P3通过两条不同途径迅速代谢。更直接的代谢途径是通过依次去磷酸化降解形成肌醇1,4-二磷酸和肌醇4-磷酸，最终形成肌醇。锂离子抑制肌醇单磷酸的形成和去磷酸化，肌醇单磷酸是肌醇多磷酸代谢的特定产物。此外，Ins-1,4,5-P3的3-磷酸化引发了一个循环代谢序列，形成肌醇1,3,4,5-四磷酸。负责此反应的Ins-1,4,5-P3 3-激酶对Ins-1,4,5-P3的Km为0.4微摩尔，Vmax为208皮摩尔/分钟/毫克，并受到微摩尔范围内Ca²⁺浓度增加的刺激。肌醇1,3,4,5-四磷酸然后去磷酸化为肌醇1,3,4-三磷酸，肌醇1,3,4-三磷酸反过来要么进一步降解为肌醇3,4-二磷酸，要么重新磷酸化为肌醇1,3,4,6-四磷酸。锂离子也抑制肌醇3,4-二磷酸的产生，这解释了在锂存在下刺激的细胞中肌醇1,3,4-三磷酸的大量积累。在Li⁺存在下长时间暴露于血管紧张素II导致肌醇多磷酸形成逐渐下降，而脂质前体磷脂酰肌醇4,5-二磷酸未耗尽，这表明多磷酸肌醇水解的积累产物（可能是二酰基甘油）对磷脂酶C催化的分解过程有抑制作用。这些结果表明，除了通过Ins-1,4-P2和Ins-4-P依次去磷酸化分解外，Ins-1,4,5-P3还经历一系列复杂的磷酸化和去磷酸化，形成至少两种肌醇四磷酸及其代谢产物。这些新定义的途径可能在血管紧张素II和其他钙动员激素激活细胞的机制中提供额外的调节步骤。
