Physiological Implications of the Contrasting Modulation of the Activities of the ε- and ζ-Isoforms of Diacylglycerol Kinase†
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We have shown that the requirement of the epsilon-isoform of diacylglycerol kinase for diacylglycerols containing arachidonic acid is specific for this substrate and cannot be replaced by the presence of an arachidonoyl group in other places in the membrane; rather, it has to be present on the substrate itself. In addition, we demonstrate that the increased activity shown toward 1-stearoyl-2-arachidonoylglycerol by the epsilon-isoform of diacylglycerol kinase is not a consequence of altered membrane physical properties but is rather a specific interaction with the arachidonoyl group. We have also compared the modulation of the activity of the epsilon-isoform of diacylglycerol kinase with that of the zeta-isoform with regard to some of the intermediates involved in phosphatidylinositol cycling. One of the products of the hydrolysis of phosphatidylinositol diphosphate is diacylglycerol enriched in arachidonic acid. The activity of the epsilon-isoform is known to be specific for this form of diacylglycerol. We show that in contrast, the activity of the zeta-isoform is lower against 1-stearoyl-2-arachidonoylglycerol compared with dioleoylglycerol. We demonstrate that addition of phosphatidylserine, as well as other anionic phospholipids including L-alpha-phosphatidylinositol 4,5-bisphosphate, strongly inhibits the epsilon-isoform, but these anionic lipids increase the activity of the zeta-isoform. Addition of Ca(2+), which is released from internal stores as a consequence of phosphatidylinositol cycling, promotes the activity of the epsilon-isoform of this enzyme but has little effect on the zeta-isoform. The contrasting conditions required for maximal activity of these two isoforms of diacylglycerol kinase, as well as their different substrate specificity, suggest that they have different physiological roles in signal transduction.
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